Glossary Flashcards
1
Q
Abiotic
A
A non-living aspect of a system; e.g. geological or meteorological factors.
2
Q
Ablation
A
The loss of mass from a glacier (e.g. by melting).
3
Q
Abrasion
A
Physical wearing and grinding of a surface.
4
Q
Abyssal plain
A
The flat part of the ocean floor that lies between about 4 and 6 km below the sea surface.
5
Q
Accumulation
A
The gain of mass on a glacier (e.g. by snowfall).
6
Q
Acidic cations
A
A positively charged ion that is able to interact with its surrounding water molecules to release one or more protons into solution, thus acting as an acid. An example is the Al3+ ion which is hydrated with six molecules of water, as [Al(H2O)6]3+, and can, depending on the solution pH, sequentially release protons, e.g.
[Al(H2O)6]3+ ⇌ [Al(H2O)5OH]2+ + H+
7
Q
Acid rain
A
Liquid precipitation (rain) that is acidic; usually defined as having a pH of less than 5, to avoid confusion with natural acid rain. Emissions such as SO2 and NOx are oxidised in the atmosphere, and the oxidised products dissolve in water vapour, forming sulfuric and nitric acids. These acids increase the acidity of rainfall.
8
Q
Actual evapotranspiration
A
The rate at which evapotranspiration occurs. It is dependent on a number of factors, including soil moisture content, vegetation type, rainfall and air temperature and movement.
9
Q
adenosine triphosphate (ATP)
A
An energy-rich molecule which all cells use as a temporary store of energy. It is produced by respiration and used in a very wide array of biochemical processes.
10
Q
Advection
A
In meteorology, the transport of air from one location to another. The term is generally applied to horizontal movement of air but vertical advection is also possible. Advection necessarily transfers atmospheric properties, such as humidity or cloudiness.
11
Q
Aeolian
A
Pertaining to the wind.
12
Q
Aerenchyma
A
Plant tissue type composed of hollow tubes. It is often abundant in the stems and roots of wetland species. It permits internal diffusion of gases within plants.
13
Q
aerodynamic resistance
A
A measure of the effectiveness of air movement in aiding evaporation, as a function of the ‘roughness’ of the surface over which it blows.
14
Q
A horizon
A
The surface horizon of a mineral soil consisting of organic matter mixed with mineral matter. The A horizon generally has maximum organic matter accumulation, maximum biological activity, and/or eluviation of materials such as iron and aluminium oxides and silicate clays.
15
Q
Air
A
Approximately 80% nitrogen and 20% oxygen, plus a number of other gases present in trace quantities.
16
Q
Air filled porosity
A
The proportion of a soil’s volume that is occupied by air.
17
Q
Albedo
A
The reflection coefficient of a surface - the fraction of the amount of incoming radiation that is reflected from a surface.
18
Q
algorithm
A
A calculation or formula that solves a recurrent problem.
19
Q
aliquot
A
A small portion of a total amount of a solution.
20
Q
alluvium
A
Loose (unconsolidated) sediments deposited by flowing water.
21
Q
alteration
A
Change brought about by geological processes after initial rock formation, especially through the action of hydrothermal fluids.
22
Q
aluminium buffer range
A
The soil solution pH (around pH 4.2–3.2) at which changes in pH are buffered by the dissolution of aluminium from clay minerals through the overall reaction:
AlOOH(s) + 3H+(aq) ⇌ Al3+(aq) + 2H2O(l)
Aluminium toxicity can be a major problem for soil biota and plant roots within this pH buffer range.
23
Q
aluminosilicate
A
Minerals containing aluminium and silicon. In aluminosilicate minerals, aluminium (3+) replaces some silicon (4+), so one extra positive metal ion is needed for every silicon replaced.
24
Q
aluminosilicate composition
A
Clay minerals containing aluminium and silicon. In aluminosilicate minerals, aluminium (3+) replaces some silicon (4+), so one extra positive metal ion is needed for every silicon replaced
25
ammonia volatilisation
The loss of NH3 from terrestrial ecosystems to the atmosphere.
26
Anabatic
Moving uphill, as in an anabatic wind, which is a warm wind that blows up a slope.
27
anaerobic
Oxygen-free conditions, typically found in waterlogged soils (bogs, swamps, marshes, etc.) and in the intestines of animals. The decomposition of organic matter (biomass) by bacteria adapted to these conditions (anaerobes) is the main natural source of atmospheric methane.
28
andesite
A fine-grained, intermediate, extrusive rock, with composition similar to diorite.
29
anemometer
An instrument for measuring wind speed.
30
aneroid barometer
An instrument for measuring barometric pressure.
31
angiosperms
A plant that produces flowers and whose seeds are covered by a protective layer of maternal tissue (e.g. fruit).
32
anion exchange capacity
The sum total of exchangeable anions that a soil can adsorb. Usually expressed as centimoles of charge per kilogram (cmolc kg-1) of soil (or of other adsorbing material, such as clay).
33
anions
Negatively charged ions.
34
anisotropy
The condition under which one or more of the hydraulic properties of an aquifer vary according to the direction of flow.
35
annual
A plant that completes its life cycle within a single year.
36
Anoxic
Oxygen-poor conditions, which can develop in soils and sediments if the supply of free oxygen is limited. The respiration of microbes adapted to these conditions releases trace gases containing nitrogen (N2, N2O and NO) and sulfur (H2S).
37
Antarctic
The region south of 66.6° S latitude, the Antarctic Circle.
38
Antarctic Bottom Water (AABW)
Antarctic Bottom Water (AABW)
Cold, dense bottom water mass that forms around the Antarctic continent (especially in the Ross and Weddell Seas) and spreads northwards in all three ocean basins.
39
anthropogenic
Produced as a result of human activity.
40
anticyclone
An extensive feature on a mean-sea-level pressure weather map around which air spirals out clockwise in the Northern Hemisphere (and anticlockwise in the Southern Hemisphere). Also referred to as a 'high'.
41
aquifer
A porous or permeable, subterranean water-bearing body of rock through which water can flow easily to wells and springs. See also confined aquifer; perched aquifer; unconfined aquifer.
42
aragonite
A mineral with chemical composition CaCO3, but rarer than the other form of calcium carbonate, calcite.
43
Arctic
The region north of 66.6° N latitude, the Arctic Circle.
44
arête
A bedrock ridge formed by two glaciers eroding away on opposite sides.
45
artesian well
A well located within a confined aquifer.
46
assimilated
Describes material that has been incorporated into an organism’s tissues, either by photosynthesis or the uptake of food.
47
associates
A plant species that is neither constant within nor particularly characteristic of a particular community in which it occurs.
48
Atom
The smallest component of a chemical element.
49
atomic number
The number of protons in the nucleus of an atom.
50
attrition
Wearing down of grains of rock by the impact of one against another.
51
autopippettes
Mechanical or battery operated calibrated tubes that are designed to measure and deliver precise volumes of solution. An autopipette is constructed from metal and plastic and uses disposable tips.
52
autotrophs
An organism able to build its tissues from simple molecules such as CO2 and H2O.
53
available water
Water that is held in a soil between field capacity and permanent wilting point. Plants are able to access this water.
54
available water capacity (AWC)
The proportion of a soil’s volume occupied by water that is available for plant uptake, i.e. at water potentials between field capacity (water potential = -5 kPa) and wilting point (water potential = -1500 kPa.)
55
axis
An imaginary line that indicates the orientation of a rotating object, such as the Earth.
56
background atmosphere
Air with a composition typical of that found in regions that are geographically remote from major anthropogenic sources of primary air pollutants.
57
backing
An anticlockwise change in wind direction at one place over time, or above one place at the same time.
58
ball-and-stick model
A molecular model used to display molecules in 3D by representing the atoms by spheres. Atoms are connected by cylindrical rods which represent the bonds.
59
ball clay
A valuable commercial clay deposit consisting mostly of kaolinite, found especially in the vicinity of Newton Abbot, in the Teign catchment.
60
barchan dune
A crescent-shaped sand dune with horns pointing downwind.
61
barchanoid ridges
A sand dune type that consists of several joined barchan dunes forming a connected ridged row – these occur when the sand supply is greater than in the conditions that create a barchan dune.
62
barrier island
A long, narrow, offshore deposit of sand or sediments that parallels the coastline. Saltmarshes develop on the side of the island that is protected from the force of waves.
63
basalt
A fine-grained, mafic, igneous rock. It is usually formed by cooling of lava erupted at the Earth's surface, but may also be found in vertical or horizontal sheets of lava, which cooled very quickly. On cooling, it may develop characteristic polygonal joint patterns.
64
baseflow
The flow rate of a river in situations where there has been no recent precipitation or snowmelt.
65
base level
The fixed lowest level of a river where it has its outlet into a lake or ocean. It is therefore the limiting level below which a stream cannot erode the land. For many streams, this hypothetical elevation is sea level.
66
base saturation
The percentage of basic cations (Ca2+, Mg2+, K+, and Na+) that comprise the total exchangeable cations in soil.
67
bathymetry
The study of underwater depth of lake or ocean floor
68
bauxite
The end-product of weathering in the tropics through extreme weathering of silicate minerals, and kaolinite. Bauxite is composed of variable proportions of gibbsite and other aluminium hydroxides, as well as some iron hydroxides, kaolinite and resistant heavy metals. It is an important raw material for aluminium production.
69
bay barrier
A ridge of sand or gravel that completely blocks the mouth of a bay.
70
beach drift
The lateral movement of sediments on a beach when the angles of swash and backwash differ
71
beaufort scale
A practical scale that relates the force of the wind (as a number from 0 to 12) to disturbance of the sea surface, or features across a land surface.
72
B horizon
A soil horizon, usually beneath the A horizon, characterized by a concentration of silicate clays, iron and aluminium oxides, and humus, alone or in combination, and often a higher bulk density than the A horizon. Coatings of iron and aluminium oxides may give the B horizon a darker, stronger, or redder colour than the other mineral soil horizons.
73
bioaccumulate
When an organism absorbs a toxic substance at a rate greater than that at which the substance is eliminated.
74
biochemical oxygen demand (BOD)
The capacity of a water sample to take up oxygen as a result of its constituent organic matter being oxidised by microbial respiration. It is measured using a standard protocol in which the sample of water is incubated for five days at 20 °C. (The term is used interchangeably with 'biological oxygen demand')
75
biodiversity
A general term that encompasses all aspects of biological diversity, from genetic variability to ecosystem complexity.
76
biogenic elements
An element that is essential to the biosphere.
77
biogeochemical cycling
A cycle of a biogenic element, between living organisms (the biosphere) and the non-living environment (the atmosphere, geosphere and hydrosphere).
78
biologically active nitrogen
Nitrogen that can be used by organisms.
79
biological oxygen demand (BOD)
The capacity of a water sample to take up oxygen as a result of its constituent organic matter being oxidized by microbial respiration. It is measured using a standard protocol in which the sample of water is incubated for five days at 20 °C.
80
biological pump
The transfer of CO2 from the surface waters to the deep ocean as a consequence of photosynthesis in shallow waters, the settling of organic matter, and organic decomposition in deep waters.
81
biomass
The total mass of living organisms within an ecosystem (or other defined area) at a point in time. Because the chemical composition of all organisms can be approximated by the formula for carbohydrate, (CH2O)n (where n is a whole number), the term is also used to describe the dead organic matter (i.e. food) that is oxidized during aerobic respiration.
82
biomes
A region of the Earth’s terrestrial surface that has a characteristic climate, vegetation type and fauna. Examples include tundra, desert and tropical forest.
83
biosphere
The total range of environments on Earth that support living organisms. It includes much of the atmosphere, hydrosphere and the Earth's crust.
84
biotic
Pertaining to a living organism within a system.
85
biotic index
A scoring system used for indicating the quality of an environment by taking into account the types of organisms present in it.
86
bioweathering
The fragmentation and decomposition of rocks as a result of biological (usually plant) activity. Caused by root development and the formation of plant acids, or through respiration and decay. It can be either chemical or physical. Examples include hydrogen ion pumping by plant roots, CO2 released by microbes, or roots breaking apart rocks.
87
black body
An ideal absorber of electromagnetic radiation that would absorb all the radiation that was incident upon it. Such a body would also be an ideal emitter, and would emit electromagnetic radiation with a spectrum that depended only on the temperature of the body.
88
blow out
A deflation hollow created when wind removes loose sediments.
89
bottom-up controls
Regulation of an organism’s population size by the availability of a resource, such as food supply or nesting sites.
90
boulder train
A deposit formed by large numbers of erratics that fan out from their source in areas that have been covered by ice sheets.
91
boundary layer
The layer of fluid (e.g. water or air) close to a solid surface, whose movement is constrained by frictional forces.
92
box model
A conceptual representation of a cycle (e.g. a hydrological cycle, or a geochemical cycle) in which reservoirs of a substance are depicted as boxes and transport between reservoirs is shown by arrows. No details about the processes that occur within the boxes are provided.
93
BP
Years before present.
94
braided channel
Shallow stream channel that is subdivided into a number of continually shifting smaller channels that are separated by bar deposits.
95
breccia
A coarse- or very coarse-grained fragmental (sedimentary) rock in which individual fragments are angular
96
brown earth
A soil type formed under temperate forest, in which nutrients (especially cations) are recycled by the vegetation and podzolization is avoided. The soils are usually deep and even in colour, with rather indistinct horizons.
97
brown soils
Well drained soils with brownish to reddish-brown subsurface horizons denoting iron oxide formation. One of the six categories of the current classification of British Soils (Avery, 1990).
98
bryophytes
A plant without specialized conducting (vascular) tissue, such as xylem vessels or phloem sieve tubes. All mosses and liverworts are bryophytes.
99
buds
The growing point of a plant, which survives hostile periods, such as winter or the dry season, in a dormant state.
100
buffered
Protected against fluctuations in pH. Buffering is a feedback mechanism in which a small change in pH initiates a chemical reaction to counteract that change; e.g. increasing acidity in a carbonate-rich soil solution results in the formation of hydrogen carbonate ions and a reduction of the acidity.
101
buffer strips
A strip of land that has permanent vegetation that helps to control environmental problems. It works by trapping sediment; this slows down runoff that could enter the local surface waters, which enhances filtration of nutrient and pesticides.
102
bulbs
A perennating organ, usually below the soil surface, which is composed of thickened leaf bases protecting a dormant bud. (e.g. daffodil or onion).
103
Butterfly Monitoring Scheme
Method of monitoring the relative abundance of British butterflies, undertaken at a large number of sites throughout the year.
104
C2 cycle
A metabolic process occurring in leaves receiving excess light, which results in the loss of previously fixed carbon. It is based on the oxygenase activity of the enzyme rubisco and was formerly referred to as photorespiration.
105
calcareous
Description of rocks or soils that have a high pH and abundant calcium. They generally contain large amounts of calcium carbonate (e.g. chalk, limestone).
106
calcicoles
A plant adapted to grow on calcareous soils (e.g. the rock-rose, Helianthemum nummularium).
107
calcifuges
A plant adapted to grow on soils of low pH, where calcium is in short supply (e.g. the cross-leaved heath, Erica tetralix).
108
calcite
A mineral with the composition calcium carbonate (CaCO3).
109
calving
The breaking off of icebergs from a glacier terminating in tidewater.
110
campos
Type of natural, temperate grassland found in Uruguay and Brazil.
111
canopy
The vegetation layer that intercepts direct sunlight.
112
canopy resistance
The resistance to evaporation of the overall vegetation canopy.
113
capillary retention
A process whereby water clings to the walls of narrow openings.
114
carbonate buffer range
In soils dominated by carbonate rock, the soil solution pH (around pH 8–6) at which changes in pH are buffered by the dissolution of calcium carbonate:
CaCO3(s) + H+(aq) = Ca2+(aq) + HCO3-(aq)
Within this buffer range, soils and soil organisms generally are little affected by acid deposition.
115
carbonate compensation depth (CCD)
The level below which the rate of carbonate dissolution exceeds the rate of its introduction.
116
carboxylic acid group
A functional group (–COOH) having one oxygen bonded to carbon by a double bond and the oxygen of an –OH group bonded by a single bond. Carboxylic acid groups are abundant in the constituents of humus.
117
carnivores
An organism that consumes animals as a major part of its diet.
118
carotenoids
Yellow or orange pigments that have a role in light absorption and in protecting plant cells from light-induced damage.
119
carrying capacity
The maximum density of mature individuals that can be supported by a given level of resource.
120
catalyst
A substance that speeds up a chemical reaction without being consumed itself.
121
catalytic cycle
Pictorial way of representing a catalytic reaction mechanism sequence where the catalyst appears at the top of the cycle and the reactions are connected in a cyclic manner. The catalyst always remains within the cycle pathway whereas reactants join the cycle at given points and products leave at other points.
122
catchment
An area from which any precipitation either evaporates or appears in a flow out of the catchment. Typically, a catchment is a valley or series of valleys containing a river system.
123
cation exchange capacity
The total amount of exchangeable cations that a soil can absorb; usually expressed as millimoles charge or centimoles charge per kilogram.
124
cations
Positively charged ions.
125
cellulose
A structural molecule composed of sugar subunits, which is the main component of plant cell walls. It is a complex carbohydrate that is very difficult for animals to digest.
126
cement
Mineral crystals that grow within the pore spaces of a sedimentary rock, holding together the fragmentary grains. Usually calcite or silica in composition.
127
chalk
A very fine-grained limestone which is often quite soft and usually white or cream in colour. Chalk is usually composed of the minute skeletal plates (known as coccoliths) of tiny marine algae or foraminifera.
128
chamaephytes
A woody plant that holds its buds above the ground, but is usually not more than 25 cm high; one of Raunkaier’s plant life-forms.
129
channelised flow
The flow of water in channels. Water flowing in this manner can become turbulent, thus scouring the soil surface and accelerating erosion.
130
channel precipitation
(Abbreviation: Qp) Precipitation that falls directly into drainage channels.
131
Chapman mechanism
A set of reactions representing the formation and destruction of ozone in the stratosphere, first proposed in 1930 by the British scientist Sydney Chapman.
132
charge density
The ratio between the charge of an exchange surface and its area in direct contact with the soil solution or soil air; usually measured as cmol charge m-2.
133
chelating agent
An organic compound that donates or shares one or more of its electrons through covalent bonds with a metal atom or ion.
134
chemical weathering
The gradual decomposition of rocks and minerals as the result of chemical reactions with dilute aqueous acid (especially solutions of CO2) and dissolved oxygen, in rainwater or groundwater.
135
chemoautotrophs
An organism able to build its tissues from simple molecules (autotroph), using energy from chemical transformations such as the oxidation of sulfur compounds.
136
chlorophyll
A green photosynthetic pigment able to absorb light energy and convert it into chemical energy
137
chloroplast
An organelle within a cell that contains chlorophyll and is the site of photosynthesis.
138
C horizon
Deepest soil horizon, composed of unweathered, or only slightly weathered, substrate. The C horizon is relatively unaffected by biological activity and soil development and is lacking properties diagnostic of an A and B horizon. The A and B may or may not be formed from material weathered from the C horizon.
139
cirques
(also called corries) These are bowl-shaped depressions on mountainsides which are produced by erosion at the head of a glacier. They start to develop beneath a snowfield where frost shattering eats away at the rock and meltwater removes the resulting debris. As the snowfield becomes a glacier, plucking and abrasion enlarge the depression.
140
cirrus
A high cloud composed of ice crystals with a wispy, fibrous appearance.
141
class
A level of classification intermediate between order and phylum.
142
clay mineral
A sedimentary aluminosilicate mineral formed by the chemical decomposition of other aluminosilicate minerals such as feldspars, pyroxenes and amphiboles, with sheet structures related to that of the mica, biotite.
143
climax
The end point of an initial succession when the most productive vegetation has established itself. This is rarely a stable position however.
144
climax community
The vegetation type produced by successional processes that is regarded as being stable and self-perpetuating.
145
climbers
A plant that relies on other species for support (e.g. honeysuckle, Lonicera periclymenum).
146
clods
Soil aggregates formed by artificial processes, such as ploughing or digging.
147
closed
(ecosystem) Used to describe ecosystems that do not exchange material with the wider environment.
148
cloud base
The lowest level at which cloud droplets or ice crystals are observed, often corresponding to the level at which condensation occurs.
149
Course fish
Fresh water fish other than game, for example, pike, carp and bream.
150
cold front
The leading edge of cold air within a frontal depression.
151
community
A group of populations living together in the same habitat.
152
competitive exclusion
This term refers to the interaction between two species that require identical resources from the environment, when one or more of those resources is limiting. The result is that one species may outcompete and replace the other.
153
competitors
Plant species whose survival strategy is based on maximizing its capture of resources from the environment at the expense of its neighbours.
154
complexation
In soil science, the formation of a chemical bond between an ion in solution and the surface of a soil particle. Anions that form complexes include phosphate, silicate, and probably sulfate and choride. Metal ions can also form complexes with organic molecules.
155
conductivity
The measure of the amount of electrical current a material can carry; expressed in μS cm−1.
156
cone of depression
cone of depression
The fall of the water table around a well from which water is being pumped. The shape and extent of the cone of depression depend on the hydraulic conductivity of the rock, the rate of pumping, and the duration of pumping.
157
confined aquifers
Aquifers that are overlain by an impermeable bed.
158
conglomerate
A coarse- or very coarse-grained sedimentary rock in which individual fragments are rounded.
159
constancy of composition
The principle for seawater that, although the concentration of dissolved salts can vary from place to place, the relative proportions of the ions remains virtually constant.
160
constants
Plant species that occurs at a consistently high frequency through all variants of a plant community type.
161
consumers
Any heterotrophic organism. The term can be applied to herbivores, carnivores or detritivores.
162
contact metamorphism
Metamorphism resulting from the intrusion of hot magma into colder, surrounding rocks, usually limited to the area immediately surrounding an igneous intrusion (the metamorphic aureole).
163
contamination
Introduction of material or compounds into the environment or water supply at levels that may or may not harm the living organisms that live in or consume it.
164
continental
(Of an air mass) An air mass that forms over a surface that is predominantly dry land.
165
continental crust
The crust from which the Earth's continental landmasses and surrounding continental shelves are composed. On average, it is of intermediate chemical composition, and is thus richer in silica than the oceanic crust.
166
continental shelf
The part of the ocean floor bordering the continents at a depth of 200 m or less below the sea surface.
167
continental slope
The part of the ocean floor extending from the edge of the continental shelf to the start of the continental rise. The continental slope has an average gradient of around 4deg.
168
convection
convection
Heat transfer process in which air is heated at a hot surface and rises, then cools and sinks again in a circulatory fashion.
169
core
The dense, iron-rich distinct central part of the Earth. The Earth's core has an inner solid part of iron and nickel, and an outer liquid part of molten iron diluted by a less dense element (probably O, S or K).
170
Coriolis effect
The influence of the Earth's rotation on the motion of air across its surface which produces an apparent deflection of air to the right of the direction of motion in the Northern Hemisphere, and to the left of the direction of motion in the Southern Hemisphere.
171
Coriolis force
An apparent force invented to explain the deflection of bodies moving over the surface of the Earth without being frictionally bound to it. It acts 90° to the right of the direction of motion in the Northern Hemisphere, and 90° to the left in the Southern Hemisphere.
172
corms
A short, swollen underground stem with one or more dormant buds, that lies dormant during unfavourable periods and contains a food store for rapid growth when conditions improve (e.g. Crocus spp.).
173
covalent bond
A chemical bond formed by sharing a pair of electrons, like the silicon-oxygen bond in the silicate etrahedral of silicate minerals.
174
crag and tail
A resistant rock knob with glacial deposits sited in its downstream shadow region.
175
Crassulacean acid metabolism
Crassulacean acid metabolism (CAM)
A photosynthetic pathway that allows plants in arid environments to harvest CO2 from the atmosphere during the night and store it for use during the day, thereby reducing water loss (e.g. many cacti and succulents).
176
crescentic dunes
These are crescent-shaped mounds or segments of sand ridges. Each segment is wider than it is long and is bounded on its concave side by a slip face. Such dunes are found in areas with a single wind direction
177
crevasses
Fractures in the surface of a glacier arising from the flow of the ice.
178
crust
The compositionally distinct layer overlying the mantle. In the terrestrial planets, the crust is richer in silica than the mantle. The Earth has two kinds of crust; oceanic crust and continental crust.
179
cryosphere
Those portions of the Earth’s surface where water is in solid form.
180
cryoturbation
The mixing of materials between various soil horizons affected by freeze-thaw processes.
181
crystalline rocks
Rocks composed of an interlocking mosaic of crystals, produced in igneous rocks by crystallisation from a magma, and in metamorphic rocks by the recrystallisation and growth of new minerals without melting.
182
cumulonimbus
A low cloud with a horizontal base and often an anvil-like, icy upper surface, which reaches to great heights. It produces showers of rain, snow, hail or sleet, or thunder and lightning.
183
cumulus
A type of low cloud with a flat base and heaped upper layers.
184
cuvette
A small tube of square cross-section, sealed at one end, made of plastic, glass or quartz and designed to hold samples for spectroscopic experiments.
185
cyclone
An extensive region of low pressure on a mean sea-level pressure weather map into which the air spirals anticlockwise in the Northern Hemisphere (and clockwise in the Southern Hemisphere). Also referred to as a 'low'.
186
cyprinid
Cyprinidae is a family of freshwater fish and its members are commonly called cyprinids.
187
Darcys law
A relationship that, in various forms, can be used to predict the rate of flow or the mass flux of a liquid through a porous medium. One version of Darcy's law states that the speed of water movement, v, down a slope through saturated soil between two points in a catchment that are a distance l metres apart, is partly determined by the difference in height, h, between them and the hydraulic conductivity of the soil, K, such that:
188
dark reactions
The second part of photosynthesis. These biochemical processes consume the hydrogen atoms and the high-energy electrons formed in the light reactions of photosynthesis, by combining them with carbon dioxide to make sugars.
189
decomposers
Organisms, primarily bacteria and fungi, that feeds on dead organic matter by secreting enzymes which digest it externally, releasing simple inorganic compounds such as CO2, water, NO3-, NH4+ and PO43-.
190
deflation
Erosion of unconsolidated material by wind.
191
deflected
(climax). A stable result of successional change in vegetation when external forces (usually human-derived) have prevented the development of the climatic climax vegetation type - typically by repeatedly grazing, mowing or burning an area. In the Teign catchment, grassland is a deflected climax because the climatic climax of deciduous forest is prevented from developing by grazing.
192
delayed throughflow
(Abbreviation: Qdt) The fraction of throughflow water (Qt) that travels slowly through the less permeable underground layers, see also quick throughflow (Qqt).
193
delta
A body of sediment deposited by a stream where it flows into standing water.
194
denitrification
Process in the nitrogen cycle carried out by microbes in anaerobic conditions, where nitrate acts as an oxidizing agent, resulting in the loss of nitrogen as it is converted to nitrous oxide (N2O) and nitrogen gas.
195
dependent variable
The variable plotted on a graph that depends on the variable fixed by the investigator, called the independent variable. The dependant variable is normally plotted on the y-axis (vertical axis) of a graph.
196
desert pavement
A natural residual concentration of closely packed pebbles, boulders, and other rock fragments on a desert surface where wind and water action has removed all smaller particles (by deflation).
197
detritivores
Soil organisms, primarily invertebrates including earthworms, centipedes, nematodes and mites, that consume dead or waste organic matter (detritus).
198
detritus
Fragments of dead or waste organic matter (such as fallen leaves, dead bodies, faeces).
199
dewpoint temperature
The temperature to which the drybulb temperature of an air sample must fall to produce saturation, at constant barometric pressure and absolute humidity.
200
diamicton
An unsorted, unstratified sediment regardless of origin.
201
diatoms
One of the most common type of phytoplankton and a major group of algae.
202
diffuse layer
Heterogeneous, loosely-held ions forming a layer of electric charge between an exchange surface and the soil solution.
203
diffuse source
Diffuse pollution can be caused by a variety of activity that has no specific point of discharge, for example, agriculture.
204
discharge
Water that runs from the land along channels such as streams and rivers. It is also referred to as streamflow or runoff.
205
dolomite
Carbonate rock-forming mineral of composition CaMg(CO3)2, or carbonate rock composed predominantly of this mineral. It occurs in limestone that has been dolomitised by the action of magnesium-bearing fluids.
206
Doppler effect
Where waves (of any kind, e.g. electromagnetic radiation, sound) have been reflected off an object and are then shifted in wavelength by an amount proportional to the object's speed. An object moving towards an observer will produce a reduction in wavelength, whereas an object moving away will increase the wavelength.
207
Doppler Radars
Radars designed to sense the component of the wind that moves towards or away from them, by tracing particulate material suspended in the flow - such as raindrops.
208
Draas
Large-scale dunes with wavelengths measured in kilometres and heights measured in tens or hundreds of metres.
209
Drainable water
The amount of water in a soil between saturation and field capacity - i.e. the amount that drains out of a saturated soil under gravity.
210
drawdown
A lowering of the water table of an unconfined aquifer or a lowering of the potentiometric surface of a confined aquifer, caused by the pumping of ground-water from wells.
211
drift
All glacial, glaciofluvial and glaciomarine deposits
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drumlin
A long streamlined hill composed of rock debris moulded into a mound by a glacial ice sheet.
213
dry adiabatic lapse rate
(Abbreviation: DALR) The rate at which an unsaturated parcel of air drops in temperature as it ascends through the atmosphere, exchanging no heat (i.e. adiabatic) with its surroundings. The DALR is 9.8 °C km−1.
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dry air
All constituents of air, except water.
215
drybulb temperature
The temperature of air, usually measured inside a weather screen.
216
dry bulk density
The oven-dry mass of soil per unit bulk volume. The dry bulk density (sometimes referred to simply as bulk density) of a soil depends on the density of the soil particles and on their packing arrangement.
217
dry deposition
The transfer of airborne particles or gas directly from the atmosphere to the Earth's surface (be it plant, soil, ocean, etc.).
218
Dystrophic
dystrophic
In this content, dystrophic is used to describe a brown water lake that is low in oxygen and supports little life due to the high levels of organic matter.
219
ecological dynamics
Processes by which species distributions change in time and space.
220
ecosystem
A unit of any size (e.g. a cowpat or the Earth itself) in which living organisms interact with one another and their abiotic environment. The defined unit is used to study food webs, energy flows and nutrient cycles.
221
Ecosystem services
Services provided by ecosystems that are of benefit to humans (e.g. flood prevention).
222
ecotones
Habitats that are transitional between two distinct vegetation types and form a vegetation gradient between them.
223
ectothermic
Relating to an organism that typically operates at the ambient temperature of its environment.
224
eddy viscocity
Internal friction between the molecules of a liquid that transfers momentum.
225
E horizon
E horizon
Horizon characterised by maximum eluviation (washing out) of silicate clays and iron and aluminium oxides; commonly occurs above the B horizon and below the A horizon.
226
Ekman drift
Ekman drift
| The mean current across the Ekman layer.
227
Ekman layer
The depth of influence of the Ekman spiral.
228
Ekman spiral
The vertical spiral pattern of water velocities that develops in the upper ocean as a result of the Coriolis force acting on moving water. The pattern develops to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere.
229
Ekman transport
The volume of water transported by the Ekman drift.
230
electromagnetic radiation
A form of radiation that involves variations in electric and magnetic effects, and is emitted by all objects with a temperature above zero kelvin. Electromagnetic radiation is divided into subranges according to the wavelength of the waves that comprise the radiation, and includes radio waves, microwaves, infrared radiation, visible light, ultraviolet (uv) radiation, X-rays and gamma rays.
231
electromagnetic spectrum
The range of electromagnetic radiation, ranging from gamma rays at short wavelengths (high frequencies) through X-rays, ultraviolet radiation, visible light, infrared radiation and microwaves to radio waves at long wavelengths (low frequencies).
232
electron
electrons
| A negatively charged particles, found outside the nucleus of an atom.
233
Eluvial
eluvial
Meaning 'washed out', describes where material has been removed in solution and suspension from a layer of the soil. (Elluvial contrasts with 'illuvial' which means 'washed into'.
234
Endemic
endemic
| Term describing a species whose range is confined to a given region.
235
endocrine disruptors
endocrine disruptors
Chemical that at certain doses, can interfere with the hormone system in animals (including humans) causing cancerous tumours, birth defects and other developmental disorders.
236
endotherm
An organism that raises its internal temperature by increased rates of respiration.
237
endothermic
Relating to an organism that maintains a stable body temperature by generating heat internally as required.
238
englacial
Refers to any location within the body of a glacier.
239
enzymes
Specialised proteins that catalyse a biochemical reaction.
240
epiphytes
epiphytes
A plant that does not normally root in the soil but grows upon another plant, or other object, remaining independent of it except for support.
241
equatorial
equatorial
| The region at and near to the Equator, between the tropics at 23.4° N and 23.4° S.
242
equatorial trough
An west-east elongated pressure minimum, which stretches around the Earth's surface within the lower latitudes of the tropics. It is associated with a belt of cloud and showery rain.
Permalink: Equatorial Trough
243
equilibrium line
equilibrium line
Conceptual line on a glacier surface separating accumulation and ablation areas, where net accumulation equals net ablation and net mass balance is therefore zero.
244
Erg
erg
| Wind-blown deposits of sand greater than 30 000 km2 in area.
245
erosion
The process by which weathered rock is transported by wind, water or ice and deposited elsewhere.
Permalink: erosion
246
erratics
erratics
| Rocks that have been transported by glaciers away from their origin and deposited in a region of dissimilar rock.
247
esker
esker
| A long, narrow, often sinuous ridge of stratified glacial drift.
248
Eukaryotes
eukaryotes
Organisms that have their DNA packaged within a nucleus rather than free in their cytoplasm. Examples are protists, plants, animals and fungi. All multicellular organisms are eukaryotes.
249
eutrophic
eutrophic
| Term describing water that is rich in nutrients.
250
eutrophication
eutrophication
| The process of nutrient enrichment and its effect on living organisms.
251
evaporation
evaporation
| The process of a liquid changing into a gas.
252
evaporite
evaporite
| Evaporite deposits are rocks created by evaporation of seawater (e.g. gypsum, CaSO4).
253
evapotranspiration
evapotranspiration
The process by which water moves from the land surface into the atmosphere as vapour. It combines both direct evaporation from wet surfaces and transpiration from vegetation.
254
excess precipitation
excess precipitation
(Abbreviation: Pe) Term used in the context of the Horton and Hewlett hypotheses to represent the difference between the rate at which water falls on the ground and the rate at which infiltration occurs. Excess precipitation is then identified as overland flow.
255
Exchangeable cation buffer range
exchangeable cation buffer range
The soil solution pH at which changes in pH are buffered by the release of basic cations from pH-dependent exchange sites (primarily on organic matter), in exchange for hydrogen ions in solution. Occurs across the range of commonly occurring pH levels in soil.
256
exploitable storage
exploitable storage
The exploitable storage of water in an aquifer is the volume of water it will yield. It is described by
V multiplied by Y divided by 100
where V is the volume of the aquifer that is being exploited, and Y is the specific yield.
257
extrusive
extrusive
| Description of igneous rocks that have been erupted as magma at the Earth's surface.
258
eye
eye
The central core of intense tropical cyclones that have reached hurricane strength. The eye is typically -50km in diameter and is characterised by deep sinking motion, generally clear skies and light winds.
259
eyewall cloud
eyewall cloud
The cylinder-like ring of deep, extremely vigorous thunderclouds that encircles the eye of a hurricane, typhoon or cyclone. It is associated with the ring of most devastating surface winds and most intense precipitation.
Permalink: eyewall cloud
260
facilitation
facilitation
One model of ecological succession in which early successional species alter their environment in such a way that it becomes more favourable for other (late successional) species to become established.
261
family
family
| A level of biological classification intermediate between genus and order.
262
felsenmeer
felsenmeer
| A surface of angular, shattered rock produced by frost weathering.
263
felsic
felsic
A pale-coloured igneous rock, with a high proportion of feldspars and quartz (silica). Examples are rhyolite and granite.
264
ferromagnesian
ferromagnesian
| Minerals which are rich in magnesium and iron. Also described as mafic. They are characteristically dark.
265
fertility
fertility
The quality of a soil that enables it to provide essential chemical elements in quantities and proportions for the growth of specified plants.
266
fetch
fetch
| The distance of open water in one direction over which wind can blow.
267
field capacity
field capacity
The amount of water retained in soil after it has been saturated and allowed to drain freely for two or three days. At field capacity, the forces holding water to soil particles are approximately equal to the downward forces of gravitational pull.
268
fixed nitrogen
fixed nitrogen
| Collective term for nitrogen compounds that are biologically active.
269
fjord
fjord
Where U-shaped glacial trough valleys with steep sides and a flat floor meet the coast, the resulting bay is called fjord.
270
flocculation
flocculation
| Separation of colloidal particles from a liquid to form loose aggregations (Flocs).
271
floodplain
floodplain
That portion of a river valley, adjacent to the channel, that is built of unconsolidated sediments derived from the river and that may be submerged periodically by flooding.
272
flowers
flowers
Structures that aid the sexual reproduction of plants and encourage animals, particularly insects, to transport pollen between them.
273
food chain
food chains
A sequence of organisms, starting with a primary producer or detritus, with the following organisms in the chain all heterotrophs: some chains feature a sequence of herbivores and carnivores, and some a sequence of detritivores and decomposers.
274
food web
food web
A network of organisms within a particular community, which demonstrates the reliance of each on other organisms (or a source of detritus) in the web as a food source.
275
fractional crystallisation
fractional crystallisation
The physical separation of crystals from the magma in which they occur. Because the first crystals to form are less rich in silica than the remaining magma, fractional crystallisation leads magma to become enriched in silica.
276
fragmentary rocks
fragmentary rocks
| See sedimentary rocks.
277
frazil ice
frazil ice
| The initial form of sea ice: a slurry-like suspension of ice crystals.
278
freeze thaw weathering
freeze–thaw weathering
A form of physical weathering through the pressure generated by the expansion of freezing water within cracks or pores in rocks as it freezes.
279
frequency
frequency
(Of a wave, f) The number of complete cycles of a wave that pass a fixed point in unit time. Conventionally measured in the unit s−1.
(Of a species) The proportion of samples in which a given species occurs.
280
frost cracking
frost cracking
| Fracturing of the ground by thermal contraction at sub-freezing temperatures.
281
frost heave
frost heave
An upwards swelling of soil during freezing conditions caused by an increasing presence of ice as it grows towards the surface.
282
frost shattering
frost shattering
| See freeze thaw weathering.
283
fruit
fruit
| The material surrounding the seeds of angiosperm plants. It is derived from maternal tissue.
284
fully mixed (or vertically mixed) estuaries
```
fully mixed (or vertically mixed) estuaries
An estuary type dominated by large tidal currents and having low river flow, such that the waters are well mixed and the salinity does not vary with depth in the estuary channel, but increases towards the sea.
```
285
functional group
functional group
A group of covalently bonded atoms that exist together in a molecule, which usually behaves chemically in a particular way, i.e. shows a specific chemical functionality.
286
functional redundancy
functional redundancy
A term describing ecosystems in which there are several species performing the same function. Therefore, if one species declines or disappears, another species replaces it (e.g. insects as food for the greater horseshoe bat). It is an important factor in determining ecosystem stability.
287
functional types
functional types
| Grouping of species that perform similar functions or employ similar survival strategies.
288
fundamental niche
fundamental niche
| The range of environments in which a given species could survive in the absence of competition from other species.
289
gelifluction
gelifluction
| The seasonal freeze–thaw action upon waterlogged soils which induces downslope movement.
290
Genera
genera
| The plural form of genus. A classification by type, in this case of clouds.
291
Genus
genus
A level of classification intermediate between species and family. It is a group of species that have many features in common and can on occasion interbreed.
292
Geoconservation
geoconservation
| Geological and geomorphological conservation or Earth heritage conservation.
293
Geodiversity
geodiversity
The variety of rocks, minerals, fossils, landforms, sediments and soils in an area, together with natural processes, such as erosion and landslips.
294
Geopark
geopark
| An area with an outstanding geological heritage that is internationally significant.
295
geophytes
geophytes
| Plant that has its buds below ground during its dormant phase; one of Raunkaier’s plant life-forms.
296
geostationary orbit
geostationary orbit
An orbit at 6.7 Earth radii from the centre of the Earth above the Equator, such that a satellite completes one orbit in 24 hours and hence remains stationary above a fixed point on the Earth's surface.
297
gibbsite
gibbsite
An aluminium oxide clay mineral, in which aluminium ions form octahedra by sharing OH− ions. These octahedra combine to form sheets that are stacked on top of each other and held in place by hydrogen bonds. Gibbsite is common in highly weathered soils, such as oxisols.
298
Glacial marine drift
glacial marine drift
| Drift deposited in the sea from floating ice bodies.
299
Glacial trough valley
glacial trough valleys
| A glacially eroded U-shaped valley.
300
Glaciated
glaciated
| Terrain formerly covered by glaciers or ice sheets, bearing their imprint in terms of landforms and sediments.
301
Glaciarised
glacierised
| Terrain currently covered by glaciers or ice sheets.
302
Gley soils
gley soils
Soils that developed under poor drainage; the waterlogging and anaerobic conditions resulting in the reduction of iron to give the soils a characteristic blue-grey colour.
303
gneiss
gneiss
A medium- or coarse-grained metamorphic rock, composed of alternating bands of felsic and mafic minerals, formed at very high pressures and temperatures during regional metamorphism.
304
goethite
goethite
A common iron oxide clay mineral, in which Fe3+ ions share O2− and OH− ions and are arranged in octahedra. Goethite is a weathering product of moderate climates, and gives many mid-latitude soils their characteristic brown colour.
305
graded stream
```
graded stream
A stream (or river) that has a concave longitudinal profile (i.e. a gradient that decreases from source to mouth) and in which inputs and outputs of sediment load are balanced.
```
306
granite
granite
A coarse-grained, felsic, intrusive, igneous rock with a chemical composition similar to that of rhyolite; formed as the result of slow cooling at depth beneath the Earth's surface.
307
ground layer
ground layer
The lowest layer of a plant community, usually comprising mosses and rosette herbs (e.g. less than 0.1 m above the ground surface).
308
ground moraine
ground moraine
| Unconsolidated material deposited directly beneath the base of a glacier.
309
groundwater
groundwater
The water that penetrates the ground surface and is contained in pore spaces in rocks located below the water table, or located in a confined aquifer.
310
groundwater flow
groundwater flow
| (Abbreviation: Qg) The flow of water through a saturated underground region.
311
guild
guild
A group of species that exploit the same kind of resources in comparable ways e.g. snakes that hunt anuran prey using venom.
312
gullyy erosion
gully erosion
Process whereby water accumulates in narrow channels and, over short periods, may remove soil from these channels to considerable depths.
313
gymnosperms
gymnosperms
A plant with vascular tissue and which produces seeds, but the seeds are naked (i.e. not covered by fruit). All conifers are gymnosperms.
314
gyre
gyre
| A large-scale circulatory feature of the ocean circulation, usually extending across many thousands of kilometres.
315
habitat
habitat
The environment in which an organism lives. The term is most strictly applied to the range of environments a particular species inhabits, but in practical use it is often defined by reference to a community rather than an individual species.
316
habitat fragmentation
habitat fragmentation
The process by which once extensive areas of relatively uniform habitat are broken up (usually by human activity) into small blocks separated by tracts of a very different habitat type.
317
hadley cells
Hadley cells
| The cellular motion of air in the north-south vertical plane that occurs within the tropics.
318
half life
half-life
| The time taken for half of the number of atoms of the parent radioactive isotope to decay into its daughter isotope.
319
halophytes
halophytes
| Any plant that grows in saline environments and readily takes up salts.
320
hanging valley
hanging valley
A valley eroded by a small tributary glacier that joins the main valley of the larger glacier at a higher elevation than the larger valley floor.
321
hazard
hazard
| A substance or environmental agent or condition with a potential to cause harm
322
head
head
| (In the context of water) The difference in height between two points on a sloping water table.
323
helical flow
helical flow
| Movement of water within a stream that occurs as spiral flows.
324
hemicryptophytes
hemicryptophytes
Plant that has its buds at the level of the ground surface (often hidden by snow or litter) during its dormant phase; one of Raunkaier’s plant life-forms.
325
hemisphere
hemisphere
One of two halves of the Earth, divided by the Equator; known as the Northern and Southern Hemispheres. The Western and Eastern Hemispheres are separated by the prime meridian (0° of longitude).
326
herbivore
herbivores
| Organism that primarily eats plant material (e.g. the water vole, Arvicola terrestris).
327
herb layer
herb layer
The layer of a plant community composed primarily of herbaceous species including grasses (e.g. 0.1–1.0 m above the ground surface).
328
heterogeneous
heterogeneous
| Term describing a substance that has different characteristics in different locations (i.e. non-uniform).
329
heterotrophs
heterotrophs
| Organisms that obtain energy and materials from eating the tissues or products of other organisms.
330
homogenous
homogeneous
| Term describing a substance that has identical characteristics everywhere (i.e. uniform).
331
horizons
horizons
A soil layer approximately parallel to the land surface with characteristic features that often gives information about the processes that formed the soil.
332
horn
horn
| A pyramid-shaped mountain peak created by several glaciers eroding away different sides of the same mountain.
333
horton hypothesis
Horton hypothesis
An explanation of the way that rainfall reaching the surface of the ground divides into a component that flows over the surface and a component that penetrates the ground. The idea is based on the way that the ability of the ground to absorb water changes with the progress of a rainstorm.
334
humidity
humidity
| A measure of how close the air is to saturation with water vapour.
335
humus
humus
The stable, dark-coloured organic material that accumulates as a by-product of decomposition of plant or animal residues added to soil. The term is often used synonymously with soil organic matter.
336
hurricane
hurricane
A tropical cyclone in the North Atlantic or the northeast Pacific Ocean. Hurricanes in the Atlantic Ocean can have landfalls in southeastern USA, the Caribbean and Central America.
337
hybrids
hybrids
The offspring of parents from different genetic lines. These may be from within a species, from different species (interspecific hybrid) or even different genera (intergeneric hybrid).
338
hydrated oxide-clay minerals
hydrated oxide-clay minerals
Alteration minerals formed by severe weathering, including hydrated iron (III) oxides (goethite and hematite) and hydrated aluminium oxide (gibbsite). Dominant in highly weathered soils, especially in the tropics.
339
hydration
hydration
Part-physical, part-chemical union between an ion or compound and one or more water molecules, the association being stimulated by the attraction of the ion or compound for either the hydrogen or the unshared electrons of the oxygen in the water.
340
hydraulic conductivity
hydraulic conductivity
The volume of water that will flow through a unit cross-sectional area of rock per unit time, under a unit hydraulic gradient and at a specified temperature.
341
hydraulic gradient
hydraulic gradient
The slope of the water table, defined as h/l, where h is the height difference and l is the length difference between two points.
342
hydraulic radius
```
hydraulic radius (R)
A characteristic of a cross-section of a river, defined as the cross-sectional area of the river divided by the wetted perimeter.
```
343
hydrogen-abstraction reactions
hydrogen-abstraction reactions
In an atmospheric context, a reaction in which the hydroxyl radical reverts back to a stable water molecule by abstracting a hydrogen atom from some other molecule (e.g. methane or other hydrocarbons).
344
hydrogen bond
hydrogen bond
Weak, directional bond formed between slightly positively charged hydrogen atoms (attached to strongly electron-attracting atoms like oxygen and nitrogen), and similar strongly electron-attracting (and therefore slightly negatively charged) atoms, usually in other molecule.
345
hydrograph
hydrograph
| A plot of river discharge at a particular cross-section of river against time.
346
hydrographic section
hydrographic section
The standard way for presenting a series of CTD (seawater conductivity and temperature, and depth) measurements taken across an ocean or a feature in the ocean.
347
hydrological cycle
hydrological cycle
Also called the water cycle. Circulation of water from the ocean through the atmosphere to the land and ultimately back to the ocean.
348
hydrologically effective precipitation
hydrologically effective precipitation
| Effective precipitation. The difference between precipitation and actual evapotranspiration.
349
hydrology
hydrology
| The study of the occurrence, distribution and chemistry of all waters of the Earth.
350
hydrolysis
hydrolysis
The reaction between water and a compound (commonly a salt or mineral) in which the hydroxyl from the water combines with the cation from the compound undergoing hydrolysis to form a base; the hydrogen ion from the water combines with the anion from the compound to form an acid. In this way, metal cations within mineral structures are replaced by hydrogen ions in soil water.
351
hydrosphere
hydrosphere
| Those parts of the Earth that are mainly water, such as the oceans, ice caps, lakes and rivers.
352
hydrostatic balance
hydrostatic balance
| A force balance between gravity and the vertical pressure gradient.
353
hydrostatic pressure
```
hydrostatic pressure (P)
A component of water potential, which relates to the pressure being applied to a unit of water (such as from the weight of the water column above it, a force applied via a piston, or from the elasticity of plant cell walls).
```
354
hydrothermal vents
hydrothermal vents
| Fissures on the seafloor out of which flows water that has been heated by underlying magma.
355
hydrous minerals
hydrous minerals
| Minerals characterised by the inclusion of –OH groups or water molecules in their crystal structures.
356
hydroxyl radicals
hydroxyl radicals
Molecular fragment, HO•, derived from a water molecule by stripping off a hydrogen atom (complete with its single electron). Known as 'the atmosphere's detergent' because attack by HO• initiates the removal from the atmosphere of many substances released by human activities, as well as most natural trace gases. The primary source of HO• in the troposphere is photolysis of ozone, followed by reaction of the excited oxygen atoms so formed, with water vapour.
357
hygrometer
hygrometer
| An instrument used for measuring the moisture content of air.
358
hypertrophic
hypertrophic
| A hypertrophic body of water has excessively been enriched with nutrients.
359
ice shelf
ice shelf
| A sometimes large, flat, floating expanse of glacial ice supplied from inland accumulation.
360
ice streams
ice streams
| A region within a grounded ice sheet where the ice moves much faster than in the regions on either side.
361
icings
icings
Sheet-like accumulation of ice formed at the ground surface in winter, where water issues from the ground (e.g. from a spring).
362
ideal gas
ideal gas
A gas in which molecular interactions are negligible. The gas therefore behaves according to the ideal gas equation of state.
363
ideal gas of state equation
ideal gas equation of state
The equation
PV = nRT
which applies to a sample of ideal gas. Here, P is the pressure, V the volume, n the quantity of gas (expressed in moles), T the temperature and R the universal gas constant (8.314 J K−1 mol−1).
364
igneous rock
igneous rocks
Rocks formed by the cooling and crystallisation of magma (ignis is Latin for 'fire'). They are usually characterised by an interlocking crystalline texture, with crystal size dependent mainly on rate of cooling.
365
illite
illite
| A 2 : 1 clay mineral, with negatively charged layers held together by potassium ions (K+).
366
illuvial
illuvial
A soil horizon (generally designated the B horizon) that is enriched by the deposition of clays, humus, and/or iron and aluminium oxides washed down from upper (eluvial) horizons.
367
independent variable
independent variable
The variable plotted on a graph that is fixed by the investigator. The independent variable is normally plotted on the x-axis (horizontal axis) of a graph.
368
indicator species
indicator species
Organisms living in the water that can be used to monitor particular environmental conditions, for example levels of organic pollution.
369
indicator values
```
indicator values
Qualitative value (ranking) applied to a plant species, which is based on a subjective assessment of an aspect of its habitat requirement. For example, Ellenberg’s indicator values relate to temperature regime, water status, light availability, soil pH, etc.
```
370
inductively coupled plasma-atomic emission spectroscopy (ICP-AES)
inductively coupled plasma-atomic emission spectroscopy (ICP-AES)
An atomic emission technique which uses a hot plasma to excite atoms and ions and looks at the emission spectra to identify metals present in the sample.
371
infiltration
infiltration
The process whereby water from rainfall, irrigation, snowmelt, etc. on the soil surface vertically enters the soil pores and becomes soil water.
372
infiltration capacity
infiltration capacity
The capacity of a particular piece of ground to absorb rainfall; this is not a constant, but changes with time, e.g. during the course of a rainstorm
373
insolation
insolation
| The total amount of solar radiation energy received on a given surface area per unit time.
374
interception
interception
| The process by which water fails to reach the ground surface, mainly due to obstruction by vegetation.
375
interception loss
interception loss
The amount of water that fails to reach the ground surface as a result of being caught by vegetation or other obstruction.
376
intermediate
intermediate
Rocks with a composition between felsic and mafic. The dominant minerals of this group of rocks are feldspar and amphibole. Rock examples are andesite and diorite.
Permalink: intermediate
377
interspecific competition
interspecific competition
The process of trying to obtain a limiting resource when organisms from another species are trying to obtain the same thing.
378
intrusive
intrusive
Term describing igneous rocks formed by the cooling and crystallization of magma beneath the Earth's surface. Coarse-grained intrusive rocks, such as granite and gabbro, must have cooled slowly.
379
ion
ion
| An atomic- or molecular-sized particle carrying a charge.
380
ionic bonding
onic bonding
| The attraction of positive for negative charges that holds crystal structures together.
381
ionic materials
ionic materials
| Chemical compounds that gives rise to ions in aqueous solutions.
382
ion selective probe
ion-selective probe
An analytical device that converts the concentration of a specific ion dissolved in solution into a measurable electrical potential.
383
iron buffer range
iron buffer range
The soil solution pH (below about pH 3.2) at which changes in pH are buffered by the dissolution of iron oxides through the overall reaction:
FeOOH(s) + 3H+(aq) ⇌ Fe3+(aq) + 2H2O(l)
384
irradiance
irradiance
The power of electromagnetic radiation per unit area (radiative flux) incident on a surface. The SI unit of irradiance is the W m−2.
385
isobars
isobars
| Lines of constant pressure, typically on a mean sea-level surface pressure map.
386
isohalines
isohalines
| Contour line joining points of equal salinity either on maps or in vertical sections.
387
isohyet
isohyet
Lines that connect points receiving equal amounts of rainfall, in the same way that isobars on a weather map connect points of equal pressure
388
isotherms
isotherms
| Contours of constant temperature.
389
isotopes
isotopes
Atoms of the same chemical element, i.e. with the same number of protons (see mass number), which have different numbers of neutrons.
390
isotropy
isotropy
| The condition in which the hydraulic properties of an aquifer are equal in all directions.
391
isovel lines
isovel lines
| Lines connecting points over the cross-section of a river that have the same flow rate.
392
jointing
jointing
A fracture pattern of roughly parallel and fairly evenly spaced cracks, usually perpendicular to bedding, which occurs due to a release of pressure when rocks are exposed to the surface by uplift or erosion. Jointing often initiates the process of physical weathering.
393
jokulhlaup
jokulhlaup
| Also jökulhlaup) An Icelandic word for a flood generated from a glacier (e.g. from a subglacial volcanic eruption
394
kame
kame
| Mound or hill of sorted glacial drift formed by glaciofluvial sediments.
395
kaolinite
kaolinite
A 1 : 1 clay mineral; the most weathered of the aluminosilicate clays, in which the layers are held together by hydrogen bonding only.
396
katabatic
katabatic
Moving downhill, as in a katabatic wind, which is a cold wind that blows down a slope.
Permalink: katabatic
397
kettle hole
kettle hole
| A basin created by the melting of subsurface ice.
398
kettle lake
kettle lake
| A water-filled kettle hole.
399
kingdom
kingdom
The highest level of biological classification. The six kingdoms are progressively subdivided into phyla, classes, orders, families, genera and species.
400
labile pool
labile pool
In relation to soil, nutrients that can move readily between the soil solids (e.g. exchange surfaces, very soluble minerals and rapidly decomposable organic matter) and the soil solution; readily available to plants.
401
lag time
lag time
| The period between the time of the maximum rainfall rate and the time of peak discharge of a river.
402
laminar
laminar
The type of flow in which the fluid particles follow paths that are smooth, straight, and parallel to channel walls (cf. turbulent flow).
403
laminations
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laminations
Very thin (less than a millimetre in depth) individual beds in fine-grained sedimentary rocks like mudstones.
```
404
lapse rate
lapse rate
| The rate of temperature change with increasing height (°C km−1).
405
latent heat of vaporisation of water
latent heat of vaporisation of water
The energy required to completely convert one kilogram of water to steam without an increase in temperature.
The energy required to completely convert one kilogram of water to steam without an increase in temperature.
406
lateral moraine
lateral moraine
| Unconsolidated material deposited along the sides of glaciers.
407
laterite
laterite
An iron-rich subsoil layer which, when dried, hardens irreversibly. Laterites occur under intensive tropical weathering regimes.
408
latitude
```
latitude
Angle defining (together with longitude) the position of a point on the Earth measured from the Equator to the pole.
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409
law of density draining
law of drainage density
| Drainage density is the total stream length divided by the basin area.
410
law of stream lengths
law of stream lengths
The law of stream lengths suggests that higher order streams are longer than lower order streams and the increase in length occurs in a regular way.
411
layered silicate
layered silicate
| A silicate mineral with crystal structure like the micas or the aluminosilicate clays.
412
leachates
leachates
| Solutions that form as water percolates through landfill waste.
413
lee
lee
| The downwind side of an object.
414
legumes
legumes
A member of the plant family Fabaceae, which is capable of using atmospheric nitrogen through a mutualistic relationship with a bacterium.
415
lenticils
lenticels
| An opening in the bark of woody plants that allows gas exchange.
416
levees
levées
| A deposit of sand or mud built up along either side of a river or stream.
417
lichens
lichens
An organism that although treated as a distinct organism and classified by species, actually represents a mutualistic association between two species from entirely different kingdoms (a fungus and an alga).
418
life forms
life forms
Description of a plant’s pattern of growth that reflect similarities of function rather than genetic relatedness. The term is often used with reference to the position of the perennating buds with respect to the ground surface (e.g. Raunkaier’s classification).
419
life form spectrum
life-form spectrum
A breakdown of the number of species classed within each of the different life-form categories (e.g. Raunkaier’s classification) for a particular vegetation type.
420
life time
lifetime
The timescale for a particular atmospheric species to be destroyed in the atmosphere via chemical or photochemical processes.
421
life zones
life zones
Term to describe geographical regions characterised by a distinct set of animals and plants e.g. alpine life zone is characterised by high elevation, cold and windy climate with no trees
422
lifted condensation level
lifted condensation level (LCL)
The height at which an air parcel, lifted adiabatically from the surface, would become saturated (i.e. relative humidity = 100%).
423
light reactions
light reactions
The first stage of photosynthesis, in which water is broken into its component parts, hydrogen and oxygen, by the energy from light. The products are used in the second stage, known as the dark reaction, to make sugars.
424
limestone
limestone
One of the major classes of sedimentary rocks, consisting mainly of calcium carbonate, formed from the skeletal remains of fossil plants or animals, or by chemical precipitation.
425
limiting factor
limiting factor
A factor that is available in minimum quantity with respect to the requirements of an organism. This may be nutrients such as carbon dioxide or calcium, or physical factors such as light, temperature, water, etc.
426
limiting nutrient
limiting nutrient
The nutrient that is available in minimum quantity with respect to the requirements of an organism, often nitrogen or phosphorus
427
linear dunes
linear dunes
| Linear dunes are long, straight or slightly sinuous sand ridges. They are found in areas with two wind directions.
428
lithification
lithification
| The processes which transform unconsolidated sediments into sedimentary rocks.
429
lithomorphic soils
lithomorphic soils
Immature soils, underlain by bedrock or broken rock at shallow depth. One of the six categories of the current classification of British Soils (Avery, 1990).
430
loam
loam
| The textural classification for soil with moderate amounts of sand, silt and clay
431
loess
loess
| Fine, wind-blown sedimentary material covering large areas of central Asia and North America.
432
longshore current
longshore current
| A current, within the surf zone, that flows parallel to the coast.
433
longshore drift
longshore drift
| The movement of material along a beach.
434
lower atmosphere
lower atmosphere
| The troposphere.
435
luxury consumption
luxury consumption
Uptake of a mineral element by a plant in excess of its current requirement. The excess is stored within the plant tissue as a buffer against a subsequent period of shortage.
436
lysimeter
lysimeter
An instrument that enables precipitation rates to be measured. A basic lysimeter comprises a pan set in the ground to collect precipitation, which can be weighed at regular intervals.
437
Ma
Ma
| A measure of geological time; 1 Ma = one million years.
438
macroaggregates
macroaggregates
| Soil particles joined together to form larger discrete particles of various shapes and sizes, visible to the naked eye.
439
macropores
macropores
Large pores within a medium. In soil, macropores are typically the result of cracks or fissures resulting from soil drying and from biological activity, such as earthworm burrowing and the spaces left by decayed plant roots
440
mafic
mafic
A dark-coloured igneous rock, with a high proportion of magnesium and iron, from the initial letters of magnesium and ferrum (iron). Examples are basalt and gabbro.
441
magma
magma
Molten rock, generally containing suspended crystals and dissolved gases. If magma is extruded onto the surface, it is usually known as lava.
442
magmatic fluid
magmatic fluid
The hot mineral-rich watery fluid left over when a considerable amount of magma has crystallised. A source of ore minerals.
443
man made soils
man-made soils
Soils that have been extensively modified by human land-use practices. This may include material additions, such as fertilisers, and mechanical disturbances, such as ploughing. It also includes soils under residential or other developments.
444
manning roughness coefficient
Manning roughness coefficient
(Abbreviation: n) The proportionality constant of the Manning equation that relates the speed of water flow in a channel to the hydraulic radius and the gradient of the water surface.
445
mann whitney u test
Mann–Whitney U-test
| Test for significant differences between medians of two samples with no assumptions of distribution in the samples.
446
mantle
mantle
| The concentric layer of the Earth lying beneath the crust.
447
maritime
maritime
| (Of an air mass) An air mass that forms over a surface that is predominantly ocean.
448
mark release capture
mark–release–recapture
| A method for determining the size of populations and the longevity and movement of individuals.
449
mass balance
mass balance
| The sum of accumulation plus ablation on a glacier surface.
450
massive
massive
| Applied to rock formations of great thickness with no discernible divisions.
451
mass mixing ratio
mass mixing ratio
| The mass of a particular constituent divided by the total mass of all other constituents in a mixture.
452
mass number
mass number
| The number of neutrons and protons in a nucleus.
453
matric pressure
```
matric pressure (m)
A component of water potential which relates to the forces by which water adheres to solid surfaces, e.g. soil particles or xylem tubes.
```
454
meandering channels
meandering channels
| Sinuous-shaped stream channel.
455
mean sea level pressure
mean sea-level pressure (MSLP)
The surface pressure corrected to the value that it would have at mean sea level. A surface of constant geopotential, thus largely removing topographic effects on the surface pressure map.
456
mean trophic rank
mean trophic rank (MTR)
The MTR is an assessment of the trophic status and impact of eutrophication in river or lakes. It is based on the species and abundance of aquatic plants.
457
medial moraine
medial moraine
| Moraine formed on top of existing glaciers when two alpine glaciers flow together.
458
mercury barometer
mercury barometer
| Standard instrument for measuring barometric pressure.
459
meristems
meristems
The growing point of plants where new structures such as leaves and flowers are initiated. A dormant meristem is termed a bud.
460
mesopause
mesopause
| The boundary between the mesosphere and the thermosphere.
461
mesosphere
mesosphere
| The layer of the atmosphere above the stratosphere - meaning 'middle' sphere.
462
mesotrophic
mesotrophic
A mesotrophic lake has an intermediate level of productivity and is commonly clear with beds of submerged aquatic plants and intermediate levels of nutrients.
463
metamorphic aureole
metamorphic aureole
| The zone of contact-metamorphosed rocks surrounding an igneous intrusion.
464
metamorphic rock
metamorphic rock
Rock whose texture and/or mineralogy has been changed by the action of heat and/or pressure (usually both are involved). Metamorphic rocks can be derived from sedimentary rocks or igneous rocks (or pre-existing metamorphic rocks).
465
meteorology
meteorology
| The study of the atmosphere and of weather.
466
methyl radical
methyl radical
| Molecular fragment, •CH3, derived from a methane molecule (CH4) by breaking one of the C–H bonds.
467
microaggregates
microaggregates
| Intimate association of soil particles, often clay–humus, not visible to the naked eye.
468
microhabits
microhabitats
A distinct environment within a general habitat type, which is exploited by particular species (e.g. hollow trees within a woodland, pockets of fine sediment on a river bed, cracks in a limestone pavement).
469
micropores
micropores
| Small pores within a soil or other medium.
470
mid-atlantic ridge
mid-Atlantic Ridge
A north–south suboceanic ridge in the Atlantic Ocean from Iceland to Antarctica on whose crest are several groups of islands.
471
middle atmosphere
middle atmosphere
| The stratosphere and mesosphere.
472
mineral
mineral
| A naturally occurring material with a defined crystal structure and chemical composition.
473
mineral elements
mineral elements
Another term used for nutrient elements, referring to the elements a plant requires for growth that originate from rock and soil, i.e. all the elements that constitute plant tissue, except carbon, oxygen and hydrogen.
474
mineralisation
mineralisation
| The conversion of an organic form of an element to an inorganic (mineral) form.
475
mineral waters
mineral waters
| Saline groundwater discharged at the ground surface.
476
minimum viable population size
minimum viable population size (MVP)
The minimum size of a population that is required for that population to persist (not go extinct) over a given period of time.
477
mixing ration
mixing ratio
The ratio of the quantity of one substance to the quantity of all substances present in a sample. The quantity in question is usually either mass or the number of atoms or molecules (which, for gases, is proportional to their volume). In particular, the mixing ratio by volume of an atmospheric constituent is defined as the number of molecules of that species as a fraction of the total number of molecules in a given volume of air. Mixing ratios of trace gases are commonly recorded in the units ppmv (parts per million 106), ppbv (parts per billion, 109) or pptv (parts per trillion, 1012).
478
moisture release curve
moisture release curve
A plot of soil moisture content against soil water potential. It is used to calculate the available water capacity of a soil.
479
molar mass
molar mass
| The mass per mole of a pure substance. The SI unit of molar mass is the kg mol−1.
480
mole
mole
The amount of a chemical substance containing 6.02 × 1023 particles of that substance (atoms, molecules, ions), equivalent to the formula mass of the particle in grams.
481
montmorillonite
montmorillonite
| A hydrous clay mineral, with negatively charged layers held together by metallic ions (Na+, Ca2+, K+) and water.
482
moraine
moraine
| An accumulation of unconsolidated material deposited by glaciers.
483
morphometry
morphometry
The measurement of the shape of any natural object is termed morphometry. When this approach is applied to streams and rivers, it is called fluvial morphometry.
484
moulins
moulins
| Points of water ingress on a glacier surface, usually vertical shafts where water flows into a crevasse.
485
mudstone
mudstone
A very fine-grained sedimentary rock containing grains < 0.2 mm in size; called shale if it is fissile i.e. splits easily into thin layers. The term mudstone encompasses siltstones (grains 0.2–0.002 mm in size) and claystones (grains < 0.002 mm in size) and those rocks containing a mixture of both clay- and silt-sized particles.
486
mutualistic
mutualistic
| An association between two species in which both partners confer some benefit.
487
mycorrhiza
mycorrhiza
| A mutualistic association between a plant’s roots and a fungus.
488
natural acid rain
natural acid rain
Rainwater that contains dissolved carbon dioxide, which forms a weak solution of H+ and HCO3− (hydrogen carbonate, or bicarbonate) ions; it has a typical pH of 5.6.
489
natural eutrophication
```
natural eutrophication
Gradual process (potentially spanning decades, centuries or even geological timescales) whereby sediment slowly accumulates in water bodies, causing the concentration of nutrient levels in the water to rise.
```
490
negative control
```
negative control
(chemistry) A way of limiting the effects that other variables might have on the variable whose effect your experiment is designed to test is by using controls. Negative controls contain all other components of the sample except the target analyte.
```
491
nephelometer
nephelometer
An instrument for measuring the size and concentration of particles suspended in a liquid or gas, especially by means of the light they scatter.
492
net primary production
net primary production (NPP)
| See primary production
493
neutrons
neutrons
| Particles that carries no charge and are found in the nuclei of all atoms except hydrogen.
494
niche separation
niche separation
Competition may lead to organisms to specialise in obtaining resources from a particular subset of their environment (e.g moths target nectar produced at night rather than during the day; fig trees target water held deep in the soil rather than shallow reserves.) This division of the available resource amongst potentially competing organisms is termed niche separation.
495
nimbostratus
nimbostratus
| A middle-level thick layer cloud from which precipitation falls.
496
nitrate leaching
nitrate leaching
| The loss of nitrogen as nitrate ions dissolved in water.
497
nitrogen fixation
nitrogen fixation
The conversion of atmospheric nitrogen into biologically active nitrogen compounds by bacteria that contain a specialized enzyme capable of cleaving the triple bond in the molecules of nitrogen gas.
498
nivation
nivation
Denudation of the land surface by a combination of processes (e.g. frost weathering, gelifluction, frost creep and meltwater flow) related to the presence of snowpatches.
499
north Atlantic deep water
North Atlantic Deep Water (NADW)
| Deep water mass formed mainly in the Norwegian and Greenland Seas.
500
null hypothesis
null hypothesis
A general or default position predicting that there is no relationship between the independent variable and the dependent variable.
501
oasis
oasis
| Water emerging from a spring or well in a desert region, some distance from the area of recharge.
502
occluded front
occluded front
A linear feature along which the warm air in a frontal depression is lifted away from the surface. It has cold air either side of it at the surface.
503
ocean conveyor belt
ocean conveyor belt
Global oceanic circulation through a series of strong currents, driven by deep water formation in the polar seas and heating of water in the tropical seas; an effect of temperature and salinity differences. Also called thermohaline circulation.
504
oceanic crust
oceanic crust
The crust making the Earth's ocean floors. It is of mafic chemical composition, and is thus poorer in silica than the continental crust.
505
octahedral sheet
octahedral sheet
| A sheet formed by joining aluminium octahedra through their oxygen atoms.
506
octahedron
octahedron
(pl. octahedra) A regular solid with eight (octa) faces. Applied to the shape of the arrangement of six oxygen atoms about an atom of aluminium in aluminosilicate minerals.
507
odd oxygen
odd oxygen
| Defined as Ox (O + O3) where 'odd' means an odd number of oxygen atoms.
508
O horizon
O horizon
The organic horizon overlaying the mineral soil horizons. It is composed primarily of decomposed organic material and frequently contains numerous soil organisms.
509
oligotrophic
oligotrophic
| Term describing water with low levels of nutrients.
510
onion skin weathering
onion skin weathering
| The concentric flaking of layers of rock from large boulders.
511
ooids
ooids
Spherical grains resembling fish roe and found in limestones; they are formed mostly by chemical precipitation. Sediments containing ooids are said to be oolitic; on compaction and burial they may form an oolitic limestone.
512
oolitic limestone
oolitic limestone
A limestone formed by the direct precipitation from solution. Ooids are formed by the gradual precipitation of calcium carbonate from warm seawater onto a nucleus. The nucleus slowly builds up a coating of calcite, developing a spherical shape as it is rolled backwards and forwards by wave action. Lithification produces an oolitic limestone.
Permalink: oolitic limestone
513
open systems
open systems
| An ecosystem that is freely exchanging energy and materials with the wider environment.
514
optimum temperature
optimum temperature
| Temperature at which a particular enzyme’s function is maximal.
515
order
order
| Level of biological classification between family and class.
516
ores
ores
A term with two slightly different meanings.
An ore deposit is a mass of ore, or a body of rock, containing ore minerals that can be mined economically.
Ore minerals are minerals from which metals may be extracted by economic methods such as mining.
517
osmosis
osmosis
Process by which water moves from a solution of low solute concentration to a solution of higher solute concentration via a semipermeable membrane.
518
osmotic pressure
osmotic pressure
A component of water potential relating to the concentration of solutes in a solution. Concentrated solutions have high osmotic pressure and tend to draw in water across a semipermeable membrane. Often denoted by the symbol π.
519
outwash
outwash
| Deposits of sand and gravel carried by meltwater originating from a glacier.
520
outwash plain
outwash plain
The stratified drift deposited by glacial streams is called outwash. If the streams have room to swing back and forth in front of the glacier, they deposit a broad spread of sediments called an outwash plain.
521
overland flow
```
overland flow
(Abbreviation: Qo) Water that reaches drainage channels by flowing directly over the land surface.
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522
oxbow lake
ox-bow lake
| A portion of abandoned stream channel filled with stagnant water.
523
oxidation
oxidation
| The addition of oxygen to, or the removal of electrons from, an atom, ion or molecule during a chemical reaction.
524
ozone hole
ozone hole
| Damage to the ozone layer in the stratosphere first observed over Antarctica.
525
ozone layer
ozone layer
| A layer within the stratosphere, at an altitude of about 30 km, where ozone concentrations peak.
526
pack ice
pack ice
| A mass of ice floating in the sea, formed by smaller pieces freezing together.
527
palaeosols
palaeosols
| Remains of soils that formed earlier in the Earth’s history; usually buried under modern soils.
528
pampas
pampas
| Local name for the natural temperate grassland found in Argentina.
529
pancake ice
pancake ice
Sea ice rind broken up into pieces a few centimetres in diameter, with upturned edges resulting from multiple collisions.
530
parabolic dune
parabolic dune
U-shaped dune common in partly vegetated areas.
Permalink: parabolic dune
531
paraglacial
paraglacial
A term describing the lingering effect of glaciers and ice sheets in the landscape: landforms, sediments and processes conditioned by former glacial occupancy.
532
parent material
Parent material
| The loose and more or less chemically weathered mineral or organic matter from which the soil develops.
533
partially mixed estuaries
partially mixed estuaries
An estuary type dominated by tidal flow, such that there is considerable, but incomplete, mixing of waters, giving a gradient from fresher water at the surface through to saltier water at the base of the channel.
Permalink: partially mixed estuaries
534
partial melting
partial melting
The phenomenon in which (at any particular pressure) a rock melts over a range of temperatures, because different minerals have different melting temperatures. The more silica-rich minerals begin to melt at lower temperatures, so the first melt to form is richer in silica than the average composition of the starting material.
535
partial pressure
partial pressure
In a mixture of gases, the partial pressure of each constituent is the pressure multiplied by the number of molecules of that constituent divided by the total number of molecules. Thus the sum of the partial pressures of all constituents is equal to the pressure.
536
particle density
particle density
| The mass per unit volume of the soil particles, usually expressed in grams per cubic centimetre (g cm-3).
537
particulate matter
```
particulate matter (PM)
Formed from the incomplete combustion of fuel in an internal combustion engine. It is made up of mainly carbonaceous material and volatile organic compounds as droplets or sorbed on solid matter.
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538
pathogenic bacteria
pathogenic bacteria
| Microbes that can cause disease.
539
peak discharge
peak discharge
| The maximum flow rate or discharge of a river.
540
peat soils
peat soils
Soils with a thick organic layer (O horizon at least 30–40 cm deep).One of the six categories of the current classification of British Soils (Avery, 1990).
541
peds
peds
A unit of soil structure formed by natural processes; often a conglomeration or aggregate of single soil particles of different sizes and shapes.
542
peneplain
peneplain
A simple, undulating lowland with an elevation near sea level, produced by continual stream erosion over a long period of time devoid of crustal movement.
543
peptide bonds
peptide bonds
| Bonds between amino acids in a protein.
544
perched aquifer
perched aquifer
A region in the unsaturated zone where the soil or rock may become locally saturated because it overlies a low-permeability unit.
545
percolation
percolation
| The downward vertical movement of water in soil.
546
perennating tissue
perennating tissue
Plant parts that survive through unfavourable seasons in a partially dormant state and are capable of growth in terms of cell division and subsequent elongation (e.g. buds, bulbs, tubers).
547
perennial
perennial
A plant that lives for several or many years. In environments with unfavourable seasons, the mature plant survives in a partially dormant state.
Permalink: perennial
548
peridotite
peridotite
A dense, coarse-grained, ultramafic rock with a crystalline texture, characteristic of the lower oceanic crust and the principal rock type forming the Earth's mantle. Peridotite is composed largely of olivine and pyroxene.
549
periglacial
periglacial
| Conditions of perennial subsurface freezing and intense frost weathering found at high latitudes and high altitudes.
550
permafrost
permafrost
| Generally regarded as permanently frozen ground, but more correctly perennially cryotic ground.
551
permanent thermocline
permanent thermocline
The region beneath the mixed layer where temperature decreases with depth.
Permalink: permanent thermocline
552
permanant wilting
permanent wilting
The water content of soils at which plant roots are unable to extract sufficient water to prevent wilting, and the plant remains wilted even if the soil water content is raised. Usually estimated as -1500 kPa.
553
permeability
permeability
| Measure of the ease with which a fluid can flow through rocks or soils.
554
peroxy radicals
peroxy radicals
Molecular fragment that contains the group –O–O•, where the terminal oxygen is involved in only one bond. Peroxy radicals, such as HO2• and CH3O2•, are formed during the atmospheric oxidation of carbon monoxide (CO) and methane (CH4) and other hydrocarbons, and play a central role in the formation of tropospheric ozone.
555
pesticide
pesticide
A wide range of organic compounds used to control weeds (herbicide), fungi and moulds (fungicide), insects (insecticide) and pests (pesticide).
556
PH
pH
A measure of the acidity of an aqueous solution, by which a concentration of H+ ions of 10−7 mol l−1 is given as pH 7 (neutral). More acidic solutions have lower pH values; more basic solutions have pH values higher than 7.
557
phanerophytes
phanerophytes
| Plant that holds its buds well above the soil surface, mostly >0.25 m; one of Raunkaier’s plant life-forms.
558
phenolic
phenolic
Chemical compounds consisting of a hydroxyl group (–OH) bonded to an aromatic hydrocarbon group (in which carbon atoms are linked together forming a ring).
559
photic zone
photic zone
| The uppermost sunlit 100 m or so of the ocean.
560
photoautotrophs
photoautotrophs
| An organism able to build its tissues from simple molecules (autotroph), using light energy.
561
photochemical
photochemical
(In reactions) In an atmospheric context, a chemical reaction triggered by the absorption of solar radiation. See also photolysis.
562
photochemicaal smog
photochemical smog
| Hazy air pollution formed from a mixture of primary pollutants occurring on hot days with bright sunshine.
563
photodissociation
photodissociation
Fragmentation of a molecule initiated by the absorption of ultraviolet (or sometimes visible) radiation. Leads to chemical change in the atmosphere by generating highly reactive free atoms and/or radicals. Also called photolysis.
564
photo-inhibition
photo-inhibition
The reduction in photosynthetic rate resulting from excess light energy (shade plants are particularly sensitive to this).
565
photolysis
photolysis
Fragmentation of a molecule initiated by the absorption of ultraviolet (or sometimes visible) radiation. Leads to chemical change in the atmosphere by generating highly reactive free atoms and/or radicals. Also called photodissociation. Also called photodissociation.
566
photo oxidation
photo-oxidation
The destruction of cell structures (primarily chlorophyll and chloroplast membranes) by spontaneous reaction with free radicals produced as a result of excess light energy being converted into chemical energy.
567
photorespiration
photorespiration
The process by which sugar is converted back to carbon dioxide, not for energy liberation purposes as in regular respiration, but simply to keep the photosynthetic machinery supplied with its raw material and hence avoid damage during periods of excess light.
568
photosynthesis
photosynthesis
| The conversion of water and carbon dioxide to sugars, using energy derived from light.
569
phyla
```
phyla
Level of biological classification between class and kingdom.
```
570
physical weathering
physical weathering
Breakdown of rock and minerals into small-sized particles, by mechanical processes such as ice formation, extreme temperature fluctuation, pressure from roots or uplift, etc. Although the material disintegrates, its chemical components remain unchanged.
571
phytoplankton
phytoplankton
| Phytoplankton are photosynthetic organisms that live in the upper layer of the water column.
572
plagioclimax
plagioclimax
Stage in a community or habitat's succession as a result of being prevented from reaching its natural climax community e.g. heathland being grazed and hence doesn't progress to woodland.
573
planetary waves
planetary waves
Large-scale, wave-like motions in the atmosphere and oceans, the dynamics of which depend on the variation of the local vertical component of the Earth's rotation with latitude.
574
plant available pool
plant-available pool
In relation to soil. Nutrients in the soil are present in ‘pools’ or ‘reservoirs’ that may or may not be accessible to plants. Nutrients in the ‘plant available pool’ are present in the soil solution in forms that can be taken up by plants from the soil.
575
plate tectonics
plate tectonics
The movement of lithospheric plates around the Earth and the processes which result from that movement. Plates interact at their edges.
576
podzols
podzols
A soil type, characterized by distinct layering of horizons as a result of minerals being leached out of the surface layers.
577
point bar
point bar
| A low ridge of sand or gravel along the inside of the bend of a meander loop.
578
point source
point source
| A point source is an identifiable localised source, for example, a factory.
579
polar
Polar
(Of an air mass) An air mass that forms at latitudes around and below the Arctic and Antarctic Circles (66.6 deg latitude).
Permalink: Polar
580
polar cells
polar cells
Over-turning circulations within the troposphere, with rising motion at about 60° latitude and descending motion over the poles.
581
polarisation
polarisation
| Partial separation of charge resulting in polarised bonds in which the bonding electrons are not evenly distributed.
582
polar orbit
polar orbit
| A satellite passes above or nearly above both poles of the body being orbited on each revolution.
583
polar stratospheric clouds
polar stratospheric clouds
Persistent ice-like clouds formed above polar regions in the lower stratosphere. These are formed by the rapid cooling of the air and the Earth's rotation which sets up strong circumpolar winds or polar vortex, circulating in the lower stratosphere.
584
pollution
pollution
Introduction of material or compounds into the environment or water supply at levels that harm the living organisms that live in or consume it.
585
pool
pool
| Scoured depression found on the bed of streams. Associated with riffles.
586
poorly sorted
poorly sorted
| Term used to describe a sediment or sedimentary rock in which the grains are of various sizes.
587
population
population
Sum total of individuals of a particular species existing in a particular locality at a particular time and able to interact.
588
population viability analysis
population viability analysis (PVA)
The study of the factors affecting population persistence.
Permalink: population viability analysis (PVA)
589
pores
pores
| Small spaces in a rock or soil.
590
porosity
porosity
The ratio of the volume of void spaces in a rock or sediment to its total volume. See also primary porosity; secondary porosity.
591
positive control
```
positive control
(chemistry) A way of limiting the effects that other variables might have on the variable whose effect your experiment is designed to test is by using controls. Positive controls contain a known amount of the target analyte.
```
592
potential evapotranspiration
potential evapotranspiration
| The maximum value of evaporation for a saturated surface, such as open water, in a given location.
593
potentiometric surface
potentiometric surface
A surface that represents the level to which water will rise if a well is drilled into a confined aquifer. In an unconfined aquifer, the water table represents the potentiometric surface.
594
pour plate method
pour-plate method
Microbiological technique designed to enumerate bacteria that are capable of growth under the conditions employed (medium, time and temperature of incubation) by mixing a sample with agar medium in a Petri dish.
595
prairie
prairie
| One of the local names for a region of natural temperate grassland in North America.
596
precipitation
precipitation
A term with two distinct meanings.
Water in liquid or solid particulate form that is deposited onto the Earth's surface from the atmosphere; includes rain, snow, hail, frost, dew, sleet, mist and fog. Precipitation over an area is measured as the equivalent depth of water.
The conversion of a dissolved substance into a solid one by chemical or physical means.
597
preferentials
preferentials
Species that occurs at a significantly higher frequency in one particular sub community than in any of the others within a particular community type.
598
present weather
present weather
| A two-digit code that summarises the weather at the time of observation.
599
pressure bomb
pressure bomb
| A scientific instrument used to measure water potential in plants by forcing water out of tissue with pressurized gas.
600
pressure solution
pressure solution
The dissolution of sedimentary mineral grains, particularly quartz and calcite, due to the pressure of overlying sediments during burial.
601
prevailing
prevailing
| (Meteorology) Wind that comes from a particular direction for 50% or more of the time.
602
primary mineral
primary mineral
Any mineral that is formed at high temperature, and has not been altered chemically since its crystallisation from magma.
603
primary pollutants
primary pollutants
Airborne gases and particles produced by the incomplete combustion of fuel in motor vehicle engines. These include CO (carbon monoxide), unburnt HCs (hydrocarbons), NO (nitric oxide and PM (particulate matter).
604
primary production
primary production
The biomass, or the energy it represents, produced by vegetation within a defined area over a defined timescale. Gross primary production (GPP) is the total energy fixed by photosynthesis. Net primary production (NPP) is GPP minus the portion used by plants in respiration (R).
605
primary succession
primary succession
| The process of succession occurring on a bare and previously uncolonised area.
606
probability of extinction
probability of extinction
| Applied to species or populations - the probability that they will go extinct within a particular time frame.
607
profile
profile
Vertical section of the soil through all its horizons and extending into the parent material. A profile can also mean the vertical gradation or sequence of a soil property, e.g. a temperature profile or pH profile.
608
proglacial
proglacial
| The area in front of a glacier, influenced by glacial processes.
609
prokaryotes
prokaryotes
Organisms that have their DNA free in their cytoplasm rather than packaged into chromosomes within a nucleus. Examples are bacteria and archaea.
610
propagules
propagules
| Plant structure that allows dispersal (e.g. seed, spore, bulbil, offshoot).
611
protons
protons
| Particles that carries a positive charge and are found within the nucleus of an atom.
612
psychrometer
psychrometer
| An instrument consisting of one standard (drybulb) thermometer and one wetbulb thermometer.
613
psychometric chart
psychrometric chart
A chart which shows drybulb temperature, wetbulb temperature, partial pressure of water vapour and relative humidity. Any two of these quantities fix a point on the chart and so allow the other two to be determined.
614
pteridophytes
pteridophytes
| A plant that has vascular tissue, but propagates by spores rather than by seeds.
615
puddling
puddling
Compaction of material, such as clay when wet, which produces a less water permeable material. Puddling of clay by trampling or other mechanical means so it can be used deliberately to create a water-tight layer for lining a pond.
616
pyramid of biomass
pyramid of biomass
As for pyramid of numbers, except that the shape of the diagram is based on the total biomass represented by the organisms at each trophic level (typical units are g m−2).
617
pyramid of energy
pyramid of energy
As for pyramid of numbers, except the shape of the diagram is based on the amount of energy utilized by each of the trophic levels during a defined period (typical units are kJ m−2 y−1).
618
pyramid of numbers
pyramid of numbers
A pictorial description of the abundance of organisms within a defined ecosystem. The autotrophs are placed at the base and sequential trophic levels placed above. The shape of the diagram is based on the number of individual organisms at each level.
619
quickflow
quickflow
| A combination of overland flow and quick throughflow measuring of the ease with which rainwater reaches rivers.
620
quick throughflow
quick throughflow
(Abbreviation: Qqt) Throughflow resulting from the relatively rapid movement of water through gaps and fissures in rock or soil.
621
radar
radar
Literally 'RAdio Detection And Ranging'. In meteorology, radars are used to sense the location and intensity of precipitation, by emitting and receiving pulses of microwave radiation.
622
radical
radical
| Molecular fragment containing an atom that is not fully bonded, and hence a very reactive chemical species.
623
radiosondes
radiosondes
| Balloon-borne instrument package that senses barometric pressure, drybulb temperature and relative humidity.
624
rainfall
rainfall
The total accumulated precipitation at a location over a defined period of time measured as an equivalent depth of water.
625
rankers
rankers
| Non-calcareous lithomorphic soils; these are soils that developed over non-calcareous rock.
626
rayleigh scattering
Rayleigh scattering
Scattering of electromagnetic waves by particles (including molecules and atoms) that are much smaller than the wavelength of that radiation. Rayleigh scattering is strongly wavelength dependent, inversely proportional to the fourth power of the wavelength, and affects short wavelengths much more than long wavelengths.
627
realised niche
realised niche
| Proportion of its fundamental niche occupied by a species when in competition with other species for limited resources.
628
recession limb
recession limb
A term applied to hydrographs that represents the period during which the river discharge is decreasing from peak discharge to baseflow.
629
recharge area
recharge area
| The area of outcrop of an aquifer at the surface, where water enters the aquifer.
630
red list of threatened species
Red List of Threatened Species
| A list of species categorised under different levels of threat.
631
redox
redox
A contraction of ‘reduction–oxidation’. Describes chemical reactions in which one reactant is reduced as the other is oxidized.
632
reduction
reduction
| The addition of hydrogen to, or the gain of electrons to an atom, ion or molecule during a chemical reaction.
633
regional metamorphism
regional metamorphism
Metamorphism that results from an increase in pressure, and usually accompanied by an increase in temperature. High pressures and temperatures are the result of deep burial and mountain-building and, as the name suggest, may affect vast volumes of rock.
634
relative humidity
```
relative humidity (r.h.)
The ratio (expressed as a percentage) of the observed absolute humidity of a sample of air to its saturation value of absolute humidity.
```
635
rendzinas
rendzinas
Calcareous lithomorphic soils; these are soils that developed over chalk limestone, or extremely calcareous unconsolidated material.
636
replicate measurement
replicate measurement
| The process of taking multiple measurements of an analysis, in order to obtain some certainty of their validity.
637
reservoir molecules
reservoir molecules
| Molecules that hold destructive Cl atoms 'in storage' as unreactive species.
638
residence time
residence time
Average length of time that an element or compound stays in a particular reservoir, i.e. a part of the local or global environment, defined according to the context. In the steady state, residence time equals the amount in the reservoir divided by the inflow or outflow rate
639
residual clay
residual clay
| Clay that remains in the locality near to where it underwent alteration from a primary mineral.
640
resilience
resilience
| (of an ecosystem) A measure of the speed with which an ecosystem recovers to a steady state following a disturbance.
641
resistance
resistance
| (of an ecosystem) A measure of the ability of an ecosystem to maintain its integrity when impacted by external factors.
642
respiration
respiration
The process by which organisms release energy stored in organic molecules, e.g. glucose, by oxidising them to carbon dioxide and water.
The energy released, which is then used to fuel the organism, is also termed respiration (R), where NPP = GPP − R.
643
response time
response time
The time required for a glacier to reach equilibrium with changed climate. It is thought that equilibrium is rarely achieved, because climate is almost constantly changing.
644
reversing dune
reversing dune
| Asymmetric ridge of sand caused by opposing wind directions.
645
rhizoids
rhizoids
A structure of lower plants such as bryophytes, which is not anatomically a root, and acts primarily to anchor the plant to the ground rather than to absorb water and minerals.
646
rhizomes
rhizomes
Specialized below-ground stem, which grows horizontally and permits a plant to spread laterally. It may also act as a perennating structures (e.g. Iris spp.).
647
rhizospheres
rhizospheres
| The narrow region of soil that is directly associated with the root.
648
riffle
riffle
| Bar deposit found on the bed of streams. Associated with pools.
649
riparian
riparian
An adjective used to describe features that are linked to a river, so riparian vegetation is composed of plants that rely on the river for their water supply.
650
rising limb
rising limb
A term applied to hydrographs that represents the period during which the river discharge is increasing from baseflow to peak discharge.
651
risk
risk
| The product of the likelihood, or probability, that a hazard might cause harm and the seriousness of the potential harm.
652
river dominated delta
river-dominated delta
| A delta where river action has the strongest control on its outline.
653
roches moutonnées
roches moutonnées
| Outcrops of tough bedrock having one smooth side and one jagged plucked side, created by the flow of an ice sheet.
654
rock
rock
A granular or interlocking aggregate of one or more types of crystals or grains. May be classified as sedimentary, igneous or metamorphic.
655
rock cycle
rock cycle
A complex cycle in which rocks are continually being formed and destroyed. Rocks exposed at the Earth's surface are subjected to processes of weathering and erosion; the particles formed are transported and deposited as sediments, which may then be buried by later sediments, compacted and subjected to high temperatures and pressures, so that metamorphic rocks form. Ultimately, partial melting may occur, yielding magma that is either erupted or intruded. Intrusive rocks may eventually be exposed at the surface, where the cycle can begin again.
656
ruderals
ruderals
One of the life strategies defined by Grime. A plant that has a short life cycle and survives unfavourable seasons as a seed (cf. therophyte under the Raunkaier classification).
657
run out distance
run-out distance
| The distance travelled downslope by avalanched snow and associated debris.
658
safe yield
safe yield
| The maximum quantity of water that can be safely removed from an aquifer annually without producing an undesired result.
659
slaine
saline
(In the context of soil) A non-sodic soil containing an enrichment of soluble salts, often resulting in impaired productivity.
660
saline intrusion
saline intrusion
The movement of seawater into freshwater aquifers, which can lead to contamination of drinking water sources and other consequences.
661
salinisation
salinisation
The process of increasing salt content; salinisation of soils can occur naturally or as a result of anthropogenic practices such as irrigation.
662
salinity
salinity
| A measure of the salt content of water, often expressed as a mixing ratio in parts per thousand.
663
salmonid
salmonid
| Salmonidae is a family of fish and its members are called salmonid.
664
saltation
saltation
| Transport of sediment initiated by moving air or water where the sediment particles bounce along the bed.
665
salt rejection
salt rejection
A process that occurs during sea ice formation where salt is pushed from forming ice into the surrounding seawater, increasing the salt concentration there.
666
salk weathering
salt weathering
| Mechanical disintegration of rock by the growth of salt crystals in pore spaces.
667
salt wedge estuaries
salt wedge estuaries
An estuary type dominated by freshwater outflow, such that seawater does not move far up the estuary channel but forms a wedge of saltwater near the estuary mouth.
668
sandstones
```
sandstone
A major class of sedimentary rocks composed mainly of quartz grains, but may also contain feldspar and clays.
```
669
saprobes
saprobes
| Fungi that obtain their nutrients from dead organic material.
670
saturated
saturated
With respect to the atmosphere, air that contains the maximum possible amount of water vapour at a given temperature and pressure.
With respect to soil water, the condition of a soil when all the pores are filled with water.
With respect to a particular ion or group of ions, the condition in which ions of the specified kind occupy all the exchange capacity.
671
saturated flow
saturated flow
| The flow of water in fully saturated pores.
672
saturated zone
saturated zone
| The zone below the water table in which all the pore spaces in rock or soil are filled with water.
673
saturation overland flow
saturation overland flow
When rainfall cannot penetrate the ground surface because that ground is already saturated with water, it flows over the ground as saturation overland flow.
674
saturation state
saturation state
The ratio of measured quantity of a gas dissolved in a fixed volume of a liquid to the quantity of that gas that would be dissolved in the liquid at equilibrium at the same temperature. Saturation state is denoted by Ω (the Greek letter omega).
675
savannah
Savannah
Tropical grassland which can range from near desert to relatively lush communities with some trees and shrubs. Repeated fires, grazing by large herbivores and local soil conditions all contribute to their maintenance.
676
schist
schist
A medium- or coarse-grained metamorphic rock with a large proportion of platy minerals such as mica, which are aligned in one direction. Fairly high temperatures (>400 °C) and pressures are required for its formation.
677
scieentific hypothesis
scientific hypothesis
A clearly stated provisional explanation for a set of observations or data, devised for the purpose of testing its validity by the collection of additional data or by conducting an experiment.
678
scouring
scouring
| The erosion of bedrock through the abrasive effects of rocks and sediments incorporated in the ice of a glacier.
679
secchi disk
Secchi disc
| A flat circular plate with quadrants painted in black and white, which is designed to measure the clarity of water.
680
secondary pollutants
secondary pollutants
| Reactions of primary pollutants in the atmosphere give rise to secondary pollutants such as ozone.
681
secondary porosity
secondary porosity
| The porosity that has been caused by fractures or weathering in a rock or sediment after it has been formed.
682
secondary succesion
secondary succession
The process of succession taking place in an area that had been vegetated previously and where some organic matter and possibly propagules remain in the soil.
683
sedimentary
sedimentary
The erosion of rocks provides material for the formation of sediments. For example, each grain of sand or mud in a river estuary was derived from the weathering and erosion of rocks further upstream. Over many centuries, these sediments may become deeply buried under more sedimentary material, causing the compaction and cementation that produces a sedimentary rock.
684
sedimentary clays
sedimentary clays
| Estuarine or ocean-floor deposits of clay minerals, which on consolidation and cementation become mudstones
685
sedimentary rocks
sedimentary rocks
They are formed from the weathered fragments of more ancient rocks, or rocks resulting from chemical and biological processes, that have been compressed and cemented together by lithification.
686
sedimentation
sedimentation
| The accumulation of sediment.
687
sediment yield
sediment yield
The mass of sediment per unit area transported by a glacier by meltwater. It should reflect the rate at which sediment is produced by erosion.
688
seeds
seeds
| Propagule produced by angiosperms and gymnosperms, which contains a new embryo plant together with a food supply.
689
seif dune
seif dune
| A long, narrow sand dune orientated parallel to the wind direction.
690
semi natural
semi-natural
A term used to describe vegetation that is composed of species native to an area (i.e. not introduced by human intervention), but whose composition has been substantially modified by human activities.
691
semipermeable membrane
semipermeable membrane
A sheet-like structure, usually of biological origin, that allows water to pass freely through it but prevents the transfer of solutes.
692
serial dilution
serial dilution
| The process whereby a sample is repeatedly diluted, sequentially. Each dilution is by the same factor, usually tenfold.
693
shade tolerance
shade tolerance
The ability of a plant to flourish under conditions of low light availability. Such plants are adapted to harvest light efficiently whilst having low respiration.
694
shale
shale
| A mudstone that is fissile, i.e. splits easily into thin layers.
695
sheeting
sheeting
A type of weathering, most commonly seen in large rocks or boulders, where concentric shells or layers are shed as the outermost portion of the rock expands when a pressure load is relieved, and breaks away from the underlying body of rock. The layers can be from a few millimetres to about a metre thick.
696
shrub layer
shrub layer
An intermediate layer within woodland vegetation, composed of woody species (often multi-stemmed) that are not tall enough to reach the high canopy (typically 1–3 m above ground surface).
697
silicate buffer range
silicate buffer range
In catchments dominated by aluminosilicate rocks, the soil solution pH (around pH 6–5) at which H+ is neutralised by weathering the silicate minerals in these rocks to form clays, such as in the example below.
2KAlSi3O8(s) + 2H+(aq) + 9H2O(l) → 2K+(aq) + 4H4SiO4(aq) + Al2Si2O5(OH)4(s)
orthoclase silicic acid kaolinite
698
silicate minerals
silicate minerals
| Minerals whose structure predominantly consists of SiO4 groups.
699
sink
sink
| Process that removes a substance from a cycle (e.g. wet and dry deposition of trace atmospheric gases).
700
sinuosity
sinuosity
| The length of a stream divided by the length of its valley. A measure of how straight or curved a stream is.
701
slaty cleavage
slaty cleavage
Closely spaced planes in a slate, caused by alignment of platy minerals under directed pressure during regional metamorphism.
702
sludge
sludge
| Nasty smelling thick liquid product of sewage treatment.
703
smog
smog
| Any type of hazy air pollution. The term was coined originally from the words 'smoke' and 'fog'.
704
soil erosion
soil erosion
The process of wearing away the soil, usually by the mechanical action of water or ice, or by particles transported by wind, water or ice.
705
soil moisture deficit
soil moisture deficit
The excess of evapotranspiration over precipitation for a given time period (usually on a monthly basis). The soil moisture deficit can be thought of as the amount of water that would be needed to fill the soil up to field capacity.
706
soil solution
soil solution
| The aqueous solution of ions and dissolved gases that is in contact with the soil.
707
solar zenith angle
solar zenith angle
| The angle between the centre of the Sun's disc and directly overhead.
708
solubility
solubility
The maximum amount of a substance that will dissolve in a given amount of solvent. Solubility varies with the temperature of the solution.
709
solutes
solutes
| An ion or molecule, such as salt or sugar, that are dissolved in a solvent such as water.
710
sources
sources
| Process that adds a substance to a cycle (e.g. sources of trace atmospheric gases, in Block 2 Part 7 Table 2.2).
711
space filling model
space-filling model
A molecular model used to display molecules in 3D by representing the atoms by spheres whose radii are proportional to the radii of the atoms and whose centre-to-centre distances are proportional to the distances between the atomic nuclei, all in the same scale.
712
species
species
The fundamental unit of classification. It describes a group of organisms that are so closely related they can freely interbreed and produce fertile offspring. Species are grouped into genera, families, orders, classes, phyla and ultimately kingdoms.
713
species trophic rank
species trophic rank, STR
The STR is a score allocated to a species based on its tolerance to eutrophication. For example, a tolerant species has a low STR and indicates a nutrient rich river.
714
specific gas constant
specific gas constant
The universal gas constant divided by the molar mass of the particular gas, or mixture of gases, under consideration. For dry air, the specific gas constant (R) is 287.1 J K−1 kg−1.
715
specific humidity
```
specific humidity (s.h.)
Mass mixing ratio of water vapour in air, usually quoted in units of g kg−1.
```
716
specific retention
specific retention
The volume of water retained around grains of rock by surface tension, expressed as a percentage of the total volume of the rock.
717
specific yield
specific yield
The volume of water that a rock or solid will yield by drainage under gravity, expressed as a percentage of the total volume of the rock.
718
spectrophototometry
spectrophotometry
An analytical technique based on the measurement of the colour intensity by passing light of a specific wavelength through the sample and measuring how much light is absorbed by the sample to give an absorbance value. Absorbance values can be related to the concentration of absorbing substances in the sample.
719
spits
spits
| Elongated ridges of sand or gravel that projects from land and ends in open water.
720
stable equilibrium
stable equilibrium
A term applied to an ecosystem when inputs of energy and materials match outputs, resulting in no net change and with the system showing a high resilience to disturbance.
721
stable isotopes
stable isotopes
| Isotopes of an element that do not undergo radioactive decay.
722
stable nutrient pool
stable nutrient pool
Soil nutrients that are unavailable to plants because they are locked up in the soil minerals (e.g. feldspar, mica), in non-exchangeable form in clays, or in resistant organic matter. These nutrients can be released only over long periods of time, through mineral weathering or the breakdown of stable organic matter. The stable nutrient pool can be considered a long-term reservoir of essential elements.
723
stand
stand
A block of vegetation that is homogenous in its structure and species composition in comparison to surrounding blocks. A stand may vary greatly in size, from a few square metres to many hectares.
724
standing crop
standing crop
| The amount of biomass (and litter) present in an ecosystem at a point in time.
725
star dune
star dune
| Isolated hill of sand with a star-shaped plan.
726
Stefan-Boltzmann law
Stefan-Boltzmann law
A law that relates the surface area of an object, A, and its temperature, T, to the total emitted radiative power, R.
R = AσT4
where σ = 5.67 × 10−8 W m−2 K−4.
727
steppe
steppe
| Local name for a type of natural temperate grassland occurring from the Ukraine to China.
728
stock solution
stock solution
| A concentrated solution that will be diluted to lower concentrations for its actual use.
729
stomata
stomata
A specialized pore within the surface layer of leaf cells. It permits gaseous interchange between the air spaces within the leaf and the atmosphere. A plant may regulate its gas exchange by altering the diameter of its stomatal pores.
730
stomatal resistance
stomatal resistance
The resistance to water loss through stomata in the leaves of a plant. Stomatal resistance depends on the number, type and size of the stomata and tends to be about ten times larger than aerodynamic resistance.
731
storm track
storm track
The line of motion of the centre (minimum barometric pressure) of a frontal depression or tropical cyclone over its lifetime.
732
stoss
stoss
| The windward side of an object.
733
straight channel
straight channel
| A stream channel with a sinuosity of < 1.5.
734
stratified
stratified
(In the context of water) Where water masses with different properties of salinity, oxygenation, density and temperature form layers that act as barriers to water mixing.
735
strafied drift
stratified drift
| Sedimentary deposits sorted and laid down by glacial meltwater.
736
stratopause
stratopause
| The boundary at the top of the stratosphere which separates it from the mesosphere.
737
stratosphere
stratosphere
The zone of the atmosphere above the troposphere, from about 11 km to 51 km, which is characterised at first by isothermal (constant temperature) conditions and then by a gradual temperature increase with increasing altitude. The Earth's ozone is concentrated here.
738
stratus
stratus
| A featureless, grey, low-level cloud with a flat base.
739
streamflow
streamflow
| Water that runs from the land along channels such as streams and rivers. It is also referred to as runoff or discharge.
740
streamlined landscape
streamlined landscape
| A landscape sculpted into smooth forms indicating erosion and a dominant direction of flow from an ice sheet.
741
stress tolerators
stress tolerators
One of the life strategy categories defined by Grime. A stress tolerator is typified by its conservative use of resources, which allows it to survive in stressful environments.
742
striations
striations
| Lines etched in bedrock underlying glaciers, by the load. These lines indicate the direction of glacial flow.
743
subcatchment
subcatchment
A sub-division of a catchment; a subcatchment consists of a hydrologic unit of land whose drainage system directs surface runoff to a single discharge point.
744
subglacial
subglacial
| Refers to any location beneath a glacier.
745
subsidence
subsidence
| Sinking of air, usually associated with adiabatic warming.
746
succession
succession
| The process of gradual change in the structure of a community, which may result in a stable climax vegetation.
747
sunflecks
sunflecks
A temporary patch of strong light, which typically occurs on a woodland floor as a result of sunlight shining through a gap in the otherwise continuous canopy of leaves.
748
surface creep
surface creep
| The slow movement of particles downwind, initiated by collisions with other grains. A form of traction.
749
survival stratedgy
survival strategy
A term used by Philip Grime to describe each of his three categories (competitor, stress-tolerator, ruderal), which are widely used as functional groups for plant species.
750
sustainable drainage systems
sustainable drainage systems (SuDS)
A system of practices and technologies used to manage surface water in a sustainable way and using techniques that mimic natural drainage systems.
751
talik
talik
| Unfrozen pocket of water within otherwise frozen ground.
752
tarn
tarn
| A glacial lake produced by scouring. Tarns are often found in cirques.
753
taxonomy
taxonomy
| The study of the principles and practices of naming and classifying organisms.
754
temperate grasslands
Temperate grasslands
Grasslands with a climate that is not extreme in terms of temperature and precipitation. Temperate climates are typically seasonal with a recognizable annual winter and summer.
755
tendency
tendency
| (Meteorology) The magnitude and the sign of a change in atmospheric pressure over a three-hour period.
756
tension
tension
| Negative water potential due to non-osmotic pressures. Sometimes referred to as ‘suction’.
757
terminal moraine
terminal moraine
| Moraine formed at the down-valley margin of a glacier.
758
terrace
terrace
| Abandoned floodplains, formed when a streams flowed at a level above the level of its present channel and floodplain.
759
tetrahedral sheet
tetrahedral sheet
| A sheet formed by joining silicon tetrahedra through their oxygen atoms.
760
texture
texture
(1) (rock) Description of the particles from which a rock is made, and the physical relationship between them. (2) (soil) The relative proportion of the various-sized particles – sand, silt and clay – that make up the mineral soil, as described in a soil texture diagram.
761
thalweg
thalweg
| A line connecting the deepest parts of a stream channel.
762
thermal inertia
thermal inertia
The resistance of a body to temperature change, which depends on its heat capacity, thermal conductivity and density. Materials with high thermal inertia show less temperature variation in a day-night cycle.
763
thermogram
thermogram
| Strip chart used in a thermograph.
764
thermograph
thermograph
| An instrument that provides a time-trace of drybulb temperature, usually for a period of one week.
765
thermohaline circulation
thermohaline circulation
| See ocean conveyor belt.
766
therophytes
therophytes
| A plant that is short-lived and survives unfavourable seasons as a dormant seed. One of Raunkaier’s life-forms.
767
three way catalytic convertor
```
three-way catalytic converter
A device (fitted into the exhaust system of a petrol-driven vehicle) for reducing emissions of the primary pollutants (carbon monoxide, hydrocarbons and NOx) that lead to ozone pollution and photochemical smog. On passing through the converter, the gases from the engine encounter catalysts that promote conversion of the pollutants to carbon dioxide, nitrogen and steam.
```
768
throughflow
throughflow
(Abbreviation: Qt) Water from precipitation that flows beneath the ground surface, but generally close to it, through unsaturated regions.
769
tidal bulge
tidal bulge
| The build-up of water due to the gravitational attraction of the Moon.
770
tidal range
tidal range
| The area of a shoreline affected by tides.
771
tide dominated delta
tide-dominated delta
A delta where tide action has the strongest control on its outline.
Permalink: tide-dominated delta
772
tides
tides
| Periodic variations in sea-level that correspond to changes in the relative position of the Moon.
773
till
till
Unstratified glacial drift deposited directly by the ice and consisting of clay, sand, gravel, and boulders intermingled in any proportion.
774
titration
titration
| The reaction of a solution of known concentration with a solution of unknown concentration until reaction is complete.
775
tolerance
tolerance
Strategies for dealing with unfavourable circumstances (e.g. high temperatures, toxicity, predation).
Permalink: tolerance
776
tombolo
tombolo
| Ridges of sand or gravel that connects an island to the mainland or to another island.
777
top down control
top-down control
| The limitation of a species distribution or abundance by a predator.
778
topography
topography
| The form of the Earth's surface, including elevation, slope and position in the landscape.
779
tortuosity
tortuosity
The actual length of a groundwater flow path, which is sinuous in form, divided by the straight-line distance between the two ends of the flow path.
780
total dissolved solids
total dissolved solids (TDS)
| The total concentration of dissolved solids in a sample of water; expressed in mg l−1.
781
trace gas
trace gas
One of the minor constituents that together make up 0.1% of the composition of dry air. Trace gases containing carbon, nitrogen and sulfur contribute to the biogeochemical cycling of these elements.
782
tracers
tracers
| Substances used to track the movement of fluid through porous rocks.
783
trade off
trade-off
Situation in which an organism increases its investment of resources to one adaptation by reducing its allocation to another.
784
traits
traits
| Properties or features of an organism that are observable.
785
transmissivity
transmissivity
The rate at which a fluid of a particular density and viscosity is transmitted through a unit width of an aquifer under a unit hydraulic gradient. It is a function of both the properties of the fluid and the nature and thickness of the porous rock.
786
transpiration
transpiration
| The flux of water from its liquid state in plants to its gaseous state in the atmosphere.
787
transpiration stream
transpiration stream
The flow of water from the soil, via the roots and shoots into the leaves, where it evaporates and is lost from the plant to the atmosphere.
788
transverse dune
transverse dune
| A large-scale sand dune that stands perpendicular to the prevailing wind direction.
789
trophic bands
trophic bands
A trophic band is the name given to define the level of nutrients measured by chemical analysis. For example, a hypertrophic lake is rich in nutrients.
790
trophic level
trophic level
Grouping of organisms within ecosystems according to their source of energy. Those of the first trophic level obtain energy from the Sun, those of the second level from consumption of organisms in the first, and those in the third by consuming those in the second, and so on. Many organisms occupy more than one trophic level (e.g. omnivores such as humans).
791
tropical
tropical
| A low-latitude region located between the Tropics of Cancer and Capricorn.
792
tropical cyclone
tropical cyclone
A general term relating to all hurricane-like storms that originate across the tropical oceans and have averaged surface winds of at least 65 knots.
793
tropical depression
tropical depression
A travelling low-pressure weather disturbance that has averaged surface winds of between 20 and 34 knots and has a few closed, circular isobars at the surface. It is often the early stage of a tropical cyclone.
794
tropical storm
tropical storm
The next, more intense, phase of a travelling low-pressure feature in the tropics after the tropical depression stage. It has averaged surface winds of between 35 and 64 knots.
795
tropopause
tropopause
| The boundary at the top of the troposphere, which separates it from the stratosphere.
796
troposhere
troposphere
The lowermost layer of the atmosphere (below about 11 km), which is marked by considerable turbulence and, in general, by a decrease in temperature with increasing height. It extends from the Earth's surface to the temperature minimum at the tropopause, and contains the bulk of the atmospheric mass.
797
trucated spur
truncated spur
| Drainage divide that has been cut off by glacial erosion.
798
tubers
tubers
An underground stem with buds that has become specialized as a storage organ and often serves as perennating tissue (e.g. potato, Solanum tuberosum, or Dahlia spp.).
799
turbidity
turbidity
| A measure of the ability of water to absorb light.
800
turbulent
turbulent
| The type of flow in which the fluid particles move along very irregular paths (cf. laminar flow).
801
typhoon
typhoon
A tropical cyclone that occurs in the northwest Pacific Ocean, often having landfall in South-East Asia and the Philippines, for example.
802
ultramafic
ultramafic
An igneous rock, such as peridotite, which has an SiO2 content of < 45% (by weight), consisting mainly of olivine and pyroxene, with minor Ca-rich feldspar but never any quartz.
803
uanvailable water
unavailable water
| Water that is held within a soil below the permanent wilting point; plant roots are unable to extract this water.
804
unconfined aqufers
unconfined aquifers
Aquifers where there are no impermeable units between the saturated zone and the surface. The water table defines the top of the aquifer.
805
universal gas constant
universal gas constant
The constant in the ideal gas equation of state that applies for any ideal gas:
R = 8.314 J K−1 mol−1.
806
unsaturated flow
unsaturated flow
| The movement of water in a soil that is not filled to capacity with water.
807
unsaturated zone
unsaturated zone
The zone between the land surface and the water table in which the pore spaces in rock or soil are filled with both water and air.
808
upper atmosphere
```
upper atmosphere
Used in this module to refer to anything above the mesosphere (the troposphere is the lower atmosphere and the stratosphere and mesosphere are the middle atmosphere) but usage in other sources varies.
```
809
UTC
UTC
Coordinated Universal Time. A standard used for reporting time of day; it is the same as Greenwich Mean Time (GMT) or Universal Time (UT).
810
variable sources area concept
variable sources area concept
| The concept that runoff-generating areas in the landscape will vary in location and size over time.
811
veering
veering
| A clockwise change of wind direction in one place over time or above one place at the same time.
812
veld
veld
| Local name for a type of natural temperate grassland in southern Africa.
813
ventifact
ventifact
| A rock or stone that has been shaped by wind-blown sediment.
814
vertical stratification
vertical stratification
Variation in ecosystem components as a response to differences in elevation. For example different fauna reside along the height of a tropical rainforest tree.
815
visibility
visibility
| The poorest horizontal visibility reported at a particular time at one point.
816
volumetric flasks
volumetric flasks
Glassware used in the laboratory to prepare solutions with accurately known concentrations. They come in a range of volumes from 10 ml to 5 l and are calibrated with a graduation mark on their neck to indicate a definite volume of liquid.
817
warm fronts
warm fronts
| The leading edge of warm air in a frontal depression.
818
warm sector
warm sector
| The region of warm air behind a warm front and ahead of a cold front in a developing mid-latitude depression.
819
water content
water content
| The proportion by mass of a soil that is consists of water in pores and capillaries.
820
water cycle
water cycle
Also called the hydrological cycle. The circulation of water from the oceans through the atmosphere to the land and ultimately back to the ocean.
Permalink: water cycle
821
water equivilent
water-equivalent
Snow and ice have very variable densities: snow tends to have densities in the range 150–450 kg m−3, and ice around 900 kg m−3. It is therefore important to standardise all mass balance quantities to their equivalent in water; depths of snow or ice are usually expressed in units of metres water-equivalent per year (m w.e. y−1).
822
water framework directive
Water Framework Directive
A key piece of European Union (EU) water legislation aiming at protecting rivers and lakes, groundwater and coastal beaches.
823
water hardness
water hardness
| A measure of the concentration of cations (commonly Ca2+ and Mg2+) in the water.
824
water mass
water mass
| A very large volume of water with uniform temperature, salinity and, therefore, density.
825
water potential
water potential
A measure of the tendency of water to move through a hydrological system. Water will move from a region with high water potential to one with a lower potential, if a pathway is available. Water potential is the sum of matrix, osmotic and gravity potential. The water potential allows determination of the water available for plants. Matrix potential describes the force by which water is held in unsaturated pores, osmotic potential describes the force by which water moves from solutions of high to low solute concentration, and gravity potential describes the force by which water moves from higher to lower levels.
826
water quality
water quality
There is no absolute definition of this term. The quality of water is a function of the extent to which it is fit for the purpose for which it is to be used (for drinking, for swimming pools, as a coolant, etc.)
827
water table
water table
The level below which all pore spaces in rock or soil are full of water. The pore water pressure at this level is equal to atmospheric pressure.
828
wave base
wave base
| The effective lower limit of wave motion, which is half of the wavelength of the waves above.
829
wave dominated delta
wave-dominated delta
| A delta where wave action has the strongest control on its outline.
830
wave height
wave height
| Vertical distance between a wave's trough and crest.
831
wavelength
wavelength
The characteristic length (λ) over which a wave repeats itself, i.e. the distance between two successive wave crests or troughs. Different types of electromagnetic radiation are characterised by a particular range of wavelengths. The wavelength and frequency (f) of electromagnetic radiation are related by the equation
c = fλ
where c is the speed of light (= 3.00 × 108 m s−1).
832
wave period
wave period
| The time taken for a wave to travel the distance of one wavelength.
833
wave refraction
wave refraction
The process by which the direction of a series of waves, moving in shallow water at an angle to the shoreline, is changed.
834
weathering
weathering
| The breaking down of rock through contact with the atmosphere, water, ice and organis
835
weeds
weeds
Plants that are in the wrong place from a human perspective. The term is only relevant to human systems where vegetation is being managed in some way.
836
wetbulb temperature
wetbulb temperature
| The temperature measured by a wetbulb thermometer.
837
wet deposition
wet deposition
The transfer of an atmospheric constituent to the Earth's surface following its incorporation into cloud droplets or falling rain.
838
Wiens law
Wien's law
A law that relates the peak wavelength, λ, of the radiation emitted by an object to its temperature T.
λT = 2.9 × 10−3 m K
839
wind mixed layer
wind-mixed layer
Surface water that has been mixed by the wind to create a layer with uniform physical properties.
Permalink: wind-mixed layer
840
wind stress
wind stress
| The frictional force that transfers energy from the wind to the surface of the water.
841
world weather watch
World Weather Watch (WWW)
| A long-term international programme aimed at expanding and improving the entire global network of weather observations.
842
yardang
yardang
| These are sculpted landforms that are streamlined by desert winds.
843
account for
Explain, clarify, give reasons for
844
analyse
resolve in to component parts, examine critically or minutely
845
Assess
determine the value of, weigh up (see also evaluate)
846
compare
look for similarities and differences betwen, perhaps reach conclusion about which is preferable and justify this clearly
847
contrast
set in opposition in order to bring out the differences sharply
848
compare and contrast
find some points of common ground between x and y and show how they differ
849
criticise
make a judgement (backed by a discussion of evidence or reason involved) about their merit of theories or opinions or truth about facts
850
define
state the exact meaning of a word or phrase
851
describe
give a detailed account
852
discuss
explain and then give two sides of the issue and implications
853
distinguish of differenciate
look at differences between
854
evaluate
make an appraisal of the worth/validity/effectiveness of something in the light of its truth or usefulness
855
examine the argument that
look in detail at the line of argument
856
explain
give details about how and why it is
857
How far
to what extent... usually involves looking at evidence/arguments for and against and weighing them up
858
illustrate
make clear and explicit, usually requires the use of carefully chosen examples
859
justify
show adequate grounds for decisions and conclusions answer the main objections likely to be made about them
860
outline
give the main features or general principles of a subject omitting minor details and emphasising structure and detail
861
state
present in a brief clear form
862
summarise
give a concise clear explaination of or account of, presenting the chief factors and omitting minor details and examples (see also outline)
863
In what sense are trees a condition for the bats?
Trees provide shade for flight paths and cover near the roost.
864
What is the most likely reason for bats needing these conditions?
To give them protection from potential predators, such as barn owls (Tyto alba).
865
For a certain organism, such as a bat, how can other organisms interact?
a resource (e.g. potential food items)
a competitor for a resource (e.g. a bird, such as a swift, which may compete for flying insects)
a beneficial helper (e.g. a tree that provides shelter)
a predator (e.g. a barn owl).
866
How does carrying capacity, bottom up control and top down control relate to each other?
As a result of these interactions, the distribution of one species can be affected by the distribution of other species, creating complex patterns across the landscape.
This brings us to a third way in which an organism’s environment can be described. Most of the resources and conditions you have met so far relate to absolute needs. These effectively determine the maximum number of individuals of a particular species that may live in a particular place, i.e. its carrying capacity. These resource-based limits on distribution are known as bottom-up controls. In contrast, if the presence of a species is limited or excluded by a predator, this is top-down control.
867
How can the long term existence of a population be threatened?
If recruitment to the population through birth or immigration does not keep pace with losses through predation, old age and disease, the long-term existence of a population is threatened.
868
In terms of top-down and bottom-up controls on other organisms, how has the shift from a hunter-gatherer to an agricultural and then an industrial economy changed the way humans influence the distribution of other species?
Hunter-gatherer communities took a proportion of the naturally occurring plants and animals around them, through harvesting or hunting. Therefore, they exerted predominantly top-down controls on other organisms.
As agriculture and industrial economies have developed, humans have made a much greater impact on the quality of the environment around them. So there has been a shift in emphasis towards bottom-up controls.
869
What bottom up and top down controls affect distribution and abundance of species?
The distribution and abundance of a species is affected by the availability of key resources and conditions (bottom-up controls) and the intensity of factors such as predation (top-down controls).
870
What is interspecific competition? What happens as a result of this?
Competition between species or interspecific competition is defined as the interaction that occurs when the abundance of two or more neighbouring species is limited by the same resource. The effect of competition is to reduce the abundance of both species because they have to share a finite resource. Either species would grow or multiply faster if the other were not there.
871
What are the limiting resources for plants?
For plants, the limiting resource is often light. It may also be water, nitrogen, phosphorus or other mineral nutrients. To avoid competing with neighbours, each species has tended to develop a slightly different requirement for each resource and a different capacity for capturing each of them.
872
What is a fundamental niche?
As a result, each species has a particular range of environments that can support it. This might differ from the range of another species, although there is often much overlap. This range is known as a species’ fundamental niche (sometimes called its physiological niche). A fundamental niche is often very broad; that is, it can encompass a wide spectrum of environments.
873
Why do some species fail to occupy its full niche?
The reason why a species fails to occupy its full potential niche is that other species compete for the same space or resource
874
What is a realised niche?
The reason why a species fails to occupy its full potential niche is that other species compete for the same space or resource. The range of environments a species actually occupies in nature is called its realised niche
875
What is competitive exclusion?
If two species, A and B, are competing for exactly the same set of resources and species A is more efficient at gathering them than species B, you would expect species B to die out. It would have been excluded from that particular habitat by species A. This theoretical concept is referred to as competitive exclusion.
876
What is niche seperation?
In this way, plants avoid competing for exactly the same resource at exactly the same time. This is referred to as niche separation. For example when two species have growth spurts in different seasons to avoid competing for resources at the same time.
877
How do species coexist without competitive exclusion?
Two species can exploit subtly different niches for examples two species can extract water and nutrients from different layers of the soil, some have growth spurts in different seasons to avoid competing for resources
