Chapter 4 Flashcards
Water is, near the surface, the atmosphere’s most abundant __________
trace gas
Evaporation
The process by which water is converted from liquid form into its gaseous state, water vapor
When the number of molecules leaving the liquid is in equilibrium with the number condensing, the air above the surface is ____________
saturated - the rate of return of water molecules is exactly equal to the rate of escape of molecules from the water
Why do we need to measure the amount of water vapor in the atmosphere?
The change of phase of water is an important energy source for storms, atmospheric circulation patterns, and cloud and precipitation formation
Water vapor is the source of all clouds and precipitation. The potential for cloud formation and dissipation depends on the amount of water vapor in the atmosphere
The amount of water in the atmosphere determines the rate of evaporation. Rates of evaporation are important to weather and many forms of plant and animal life, including humans
Water vapor is a principal absorber of longwave radiant energy. It is the most important greenhouse gas
Mixing ratio
Ratio of the wight of water vapor to the weight of the other molecules in a given volume of air
The unit of mixing ratio is grams of water vapor per kilogram of dry air
Mixing ratio is an absolute measure of water vapor
Typical values of the mixing ratio near the surface range between less than 1 gram per kilogram in polar regions to more than 15 grams per kilogram in the tropical regions
Evaporating water into the volume ____________ the mixing ratio
increases
The pressure the water molecules exert is another useful method of representing the amount of water vapor in the atmosphere. The pressure caused by these water vapor molecules is called the ____________
vapor pressure
*expressed in millibars (mb)
The vapor pressure attributable to water vapor alone is never more than about _______________
4% of 1000mb
or 40mb
Increasing the air temperature will _________ vapor pressure
increase
When air is saturated, the pressure exerted by the water vapor molecules is called the __________________
saturation vapor pressure
Most important fact about saturation vapor pressure:
It increases rapidly as the temperature increases - as the temp of water increases, the number of molecules with enough kinetic energy to evaporate from the water surface increases.
Increasing the temperature also increases the numbeer and speed of the water molecules in the vapor phase
It is more accurate to say:
a saturated parcel of warm air will contain many more water vapor molecules than a saturated parcel of cooler air
Saturation ratio
The ratio of the actual vapor pressure exerted by molecules of water vapor versus the saturation vapor pressure at the same temperature indicates just how close the air is to saturation
saturation ratio x 100% =
relative humidity
Relative humidity
Describes how far the air is from saturation
saturated air ==> relative humidity of 100% because vapor pressure equals the saturation vapor pressure
A relative humididity of 50% tells us that that the vapor pressure is _______________________
half that required for saturation
Water evaporates more slowly in air that has a ________________
high relative humditiy
Water evaporates more quickly in air that has a _________________
low relative humidity
Relative humidity is more generally an important indicator of ________________________
the rate of moisture and heat loss by plants and animals
Decrease in temperature results in an ___________ in the relative humidity
increase
Increasing the temperature ____________ the relative humidity
decreases
Condensed water is called _________
dew
Dew point
The temperature to which air must be cooled to become saturated without changing the pressure
*determined by keeping the pressure fixed because changing the pressure affects the vapor pressure and therefore the temperature at which saturation occurs
The closer the dew point is to the air temperature =>
the closer the air is to saturation
The temperature difference between the air and the dew point is called the ________________
dew point depression
The ice crystals that form are called _____________
frost
The temperature to which air must be cooled at a constant pressure to cause frost to form is called the ___________
frost point
Dew may form and then freeze if the temperature falls below freezing, forming ______________
frozen dew
Rime
opaque deposit of ice formed by the rapid freezing of water drops as they collide with an object at or below freezing
Clouds are composed of:
tiny 20-micron-sized particles of liquid water called cloud droplets and particles of ice called ice crystals
The greater the concentration of salt =>
the more the rate of evaporation is reduced
Solute effect
The ability of dissolved salt to hold onto water molecules
Curvature effect
The smaller the drop, the more curved the surface, reducing the number of neighbors for each water molecule at the surface.
This curature effect makes it easier for small drops to evaporate
Supersaturation`
The relative humidity must be higher than 100%
Nucleation
The initial formation of a cloud droplet around any time of particle
Two types of nucleation:
- homogeneous nucleation
- heterogeneous nucleation
Homogeneous nucleation
The droplet is formed only by water molecules
Requires that enough water molecules bond together to form a cluster, or particle, that then acts as a nucleus for fututher condensation
Occurs at temperatures colder than -40 degrees celsisus
Heterogeneous nucleation
Occurs when small, nonwater particles serve as sites for cloud droplet formation
These particles are usually areosols
The areosols that assist in forming liquid droplets are called _______________
condensation nuclei
There are two types of condensation nuclei:
- hygroscopic
- hydrophobic
Hygroscopic nuclei
dissolve in water
Droplet formation can occur on hygroscopic nuclei even when the relative humidity is below 100% because the solute effect reduces the rate of evaporation
Hydrophobic nuclei
Do not dissolve in water
Hydrophobic nuclei resist condensation but can form droplets when relative humidities are near 100%
____________, the particles around which the ice crystals form, are important in the beinning stages of ice crystal formation
Ice nuclei
Ice particles can form in 4 ways:
- Deposition nucleation
- Freezing nucleation
- immersion nucleation
- contact nucleation
Deposition nucleation
Ice forms from vapor by deposition onto the ice nucleus when the air is supersaturated with respect to ice
Liquid water at a temperature below 0 degrees Celsisus is referred to as ____________________
supercooled water
Freezing nucleation
The process by which a supercooled drop freezes without the aid of a nonwater particle
0 degrees Celsisus is more accurately called the ________________, not the freezing point, of water
melting point
Immersion nucleation
The nucleus is submerged in a liquid drop
Lice nuclei may also collide with supercooled drops. The drop freezes immediately on contact with the ice. This is referred to as ___________________
contact nucleation
a cloud at the ground
fog
4 different types of fog
- Radiation fog
- advection fog
- evaporation fog
- upslope fog
Radiation fogs
Tend to develop on clear nights
Light winds required because they can gently mix moist air near the ground
If the dew point temperature is approximately 8 degrees Celsisus below the air temp at sunset and if the winds are predicted to be less than 9 kilometers per hour, there is a good chance that radiation fog will form during the night
Advection fog
When warm air is advected over a cold surface, the air near the ground cools because of energy exchanges with the surface. The relative humidity increases, and an advection fog may form
Evaporation fogs
Occur in the vicinity of warm fronts and are sometimes called frontal fogs
These fogs form when water evaporate from rain that falls from warmer air above the ground into cold air near the surface
Occur only after it has been raining
Steam fog
Evaporation fog over a lake gives the appearence of steam rising out of the water and sometimes referred to as steam fog
Upslope fog
As the air rises over a mountain barrier, it expands and cools, and the relative humidity rises. If the air becomes saturated, an upslope fog forms
Orographic lifting
The air cannot go through the mountain, and so it flows over the mountain
Frontal lifting
Occurs when less dense warm air is forced to rise over the cooler, denser air
Frontal lifting is common in winter
Convection
An important lifting mechanism in summer
Solar energy passes through the atmosphere and heats the surface. The air near the surface warms, becomes less dense than the air around it, and rises
Convergence
Occurs when air near the surface flows together from different directions
Divergence
The opposite of convergence - which is the horizontal spreading out of air
In each case o flifting, the rising air creates an ____________,
updraft
the updraft keeps the cloud particles suspended in mid air despite the force of gravity that acts to bring them to the ground
Saturated Parcel of air
One in which the air contains the maximum amount of water vapor possible; its relative humidity is therefore 100%
Saturated adiabatic lapse rate
The rate that the rising saturated air parcel cools
Lifting condensation level (LCL)
The height at which water vapor in a rising parcel of air starts condensing. The bottoms of puffy flouds on sunny days are at the altitude of the LCL.
Ascending parcels that are saturated cool _________ than do unsaturated parcels
less quickly
The case where saturated air parcels are unstable, but unsaturated air parcels are stable, is called a ______________________
conditionally unstable environment
A conditionally unstable enviornment exists when its lapse rate is in between:
the saturated adiabactic lapse rate of about 6 degrees Celsius per kilometer and the dry adiabatic lapse rate of 10 degrees Celsius per kilometer
Less than saturated adiabatic lapse rate
Environment is absolutely stable
No parcels keep rising
Greater than dry adiabatic lapse rate
Environment is absolutely unstable
All parcels keep rising
Less than dry adiabatic lapse rate and greater than saturated
Environment is conditionally unstable
Only saturated parcels keep rising
Layered clouds
Much wider than they are tall
Flat bases and tops and can extend from horizon to horizon
Convective clouds
As tall, or taller, than they are wide
These clouds look lumpy and piled up
Stratus clouds
Are fog that hovers just above (rathern than on) the ground
Appear light to dark grey in color and cover the sky
Stratocumulus clouds
Low-lying clouds that cover the sky and appear white to gray in color
They are a combinaition of layered and convective cloud types
Stratocumuls clouds often appear in rows or patches.
You can distinguish stratocumuls from stratus by looking for more variations in color and a lumpier appearance
Stratocumulus clouds
Low-lying clouds with both layered and convective aspects.
Stratocumulus are distinguished from stratus clouds by variations in color across the sky
Cumulus (cu) clouds
Have well-defined, flat bases and are intricately contoured domed tops resembling cauliflower
The edges of the cloud are distinct
bases - dark gray and sunlit sides are bright white
Two basic forms of cumlus clouds
Fair-weather cumulus
Cumulus congestus
Fair-weather cumulus clouds
Symbolize pleasant weather conditions all over the world
Height similar to width
These clouds are common in summer when solar heating of the surface triggers convetion
Not deep enough to cause rain, grow into large storms
Cumulus congestus - towering cumulus
Tall relative to their width
For these clouds to form, the atmosphere must have a deep unstable layer, deeper than is required for the formation of the fair-weather cumulus
These towering clouds are common in summer and may have light rain falling from them
When cumlus gongestus form in the morning, it is a good indicator that storms may form later in the day
Nimbostratus
Deep clouds that bring precipitation and appear dark gray to pale blue in color
The cloud base is difficult to see because precipitation is falling from the cloud
Look similar to stratus, stratocumlus, or altostratus clouds
Nimbostratus clouds often precede warm fronts
Precipitation is often continuous and light to moderate in intensity
Cumulonimbus
Thunderstorm clouds
They extend upward to high altitudes, often to the tropapause and sometimes in the lower stratosphere
Produce large amounts of precipitation, severe weather, and even tornadoes
A distinguishing feature of cumulonimbus is the flattened )______________ shape of the top of the cloud
anvil
*anvil develops when the updraft slows and spreads outward hoizontally as it encounters the very stable air in the stratosphere
Underneath the anvil, sinking air may create pouches called ______________
mammatus - these clouds are not severe weather, they can form under the anvils of strong thunderstorms
Where do cumulonimbus clouds develop?
Develop in unstable, moist atmospheres and are fairly common in the United States in spring and summer
They often occur ahead of cold fronts.
In summer they can form over mountains because orographic lifting in combination with solar heating
can be isolated or in groups
Altostratus (As)
Layered clouds made up mostly of liquid water droplets
Gray to pale blue in appearence
Form when middle layers of the atmosphere are moist and slowly lifted
Observe ahead of a warm front, before the nimbostratus
Altostratus and Altocumulus are ___________ clouds
middle
Altocumuls (Ac) clouds
They can be thin or thick, white or gray, and organized in lines or randomly distributed
occur in middle levels of the atmosphere
Similar in appearance to stratocumlus, with a higher cloud base
Cirrocumulus`
Clouds are thin, white clouds that appear in ripples arranged in a regular formation.
Composed of ice crystals and occur high in the atmosphere in regions that are moist and unstable
“mackeral sky”
Cirrostratus
Can cover part or all of the sky
Uniform in appearance and can be thin or thick and white or light gray in color
Common during winter
Cirrus (Ci)
wispy, fibrous clouds that are made of ice crystals
They often occur as wisps here and there across the sky and are aligned in the same direction as the upper-level winds
Water vapor is an important greenhouse gas because _________________
it absorbs longwave energy effectively
Increases in greenhouse gases over time can result in a climate change because:
the atmosphere becomes more effective at absorbing longwave energy emitted by the surface
Evaporation depends on _____________
relative humidity
With a lower relative humidity, more __________ occurs
evaporation
Clouds also have a warming effect on atmosphere below them because they are very good emitters and absorbers of ________________
terrestrial radiation
Clouds reflect _____________
solar radiation
The size and shape of a crystal is called its ____________
crystal habit
_______________ determines the particular crystal habit of ice
Temperature
Hexagonal plate
0 to -5 degrees C
- 10 to -12 degrees C
- 16 to -25 degrees C
Needle
-5 to -10 degrees C
Dendrite
-12 to -16 degrees C
Column
- 5 degrees to -10 degrees C
- 25 to -50 degrees C
The dendrites are hexagonal with _____________
elongated branches, or fingers, of ice
They most closely resemble what we think of as snowflakes
Precipitation
Any liquid or solid water particles that falls from the atmosphere and reaches the ground
Can be long lasting and steady, or it may fall as a brief and intense shower
Precipitation removes_____________ from the atmosphere
water vapor
______________ are not precipitaiton because they do not fall from a cloud under the force of gravity
Dew and frost
_______________ are those that have temperature greater than freezing throughout the cloud
Warm clouds
____________ have temperatures that are below freezing
Cold clouds
Collision-coalescence
A process that could produce a larger drop quickly would be to combine many smaller particles
To do this the cloud particles have to bump into each other and merge together, or coalesce
Most clouds outside the tropical regions have temperatures that are ___________
below freezing
Accretion
The process of ice crystal growth by sweeping up supercooled water drops
Graupel
An ice particle produced by the accretion process that has a size between 1 and 5 millimeters and no discernible crystal habit
Aggregation
The process by which ice crystals collide and form a single larger ice particle
Probability that two crystals will stick together depends on the shape of the crystals.
Dendrites will stick together, plates probably wont
Snowflake
An individual ice crystal or an aggregate of ice crystals
Snowfalls do not consist of single crystals
Bergeron-Wegener process
Ice crystal growth process
