Unit 3: Atmosphere and Weather Flashcards
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Diurnal
The 24 hour period of day and night
Radiation
The energy coming from the Sun is short-wave radiation
This energy heats the ground and is reradiated as infrared long-wave radiation known as terrestrial radiation
Some of this energy escapes into space by much of it is reflected back to the surface by the atmosphere. This is counter-radiation. Without counter-radiation the Earth would be about 25C cooler
Radiation balance
(incoming solar radiation + atmospheric counter radiation) - (reflected solar radiation + outgoing terrestrial radiation)
Incoming shortwave radiation
Of 100%:
6% scattered at the edge of atmosphere
20% reflected by clouds
4% reflected by surface
51% heats land or sea
3% absorbed by clouds
16% absorbed by water vapour, dust and CO2
Outgoing long-wave radiation
Of 100%:
10% passes into space
90% absorbed by atmosphere and reradiated back to surface
Albedo
The proportion of the solar radiation from areas of the Earths surface
0 = all absorbed
1 = all reflected
Surface without snow or ice absorbs more heat
Surface with snow and ice reflects more heat
Latent heat
Most of the net radiation balance energy is used to evaporate water turning it into water vapour
When heat is used to evaporate water it becomes a hidden latent heat. When water vapour condenses back into water droplets, it release that heat back into the atmosphere
Sensible heat
The heat people actually feel
When air and water moves from one place to another taking it with it
Night-time energy budgets
Consists of 4 components:
-long-wave Earth radiation
-latent heat transfer (condensation)
-absorbed energy returned to Earth (sub-surface supply)
-sensible heat transfer
Moisture
Absolute humidity refers to the amount of water in the atmosphere. Relative humidity refers to the water vapour present expressed as a % of the maximum amount air of that temperature can hold
Mist and fog are cloud at ground level. Mist is where visibility is between 100 m and 5000 m. Fog is when visibility is below 1000m
Temperature inversions
A temperature inversion is where temperature increases with height. Normally the opposite is expected
When the ground is cold, especially at night it might cool the air just above it. If humidity is high with will cause condensation which will result in mist or fog. Where the ground itself is below freezing frost will occur
Diurnal ranges
Difference in temperature between daytime and nighttime
Greater in areas with no clouds like deserts
The range is also strongly influenced by the sea. The sea is cooler than the land in the summer so onshore breezes often reduce daily highs in coastal areas. The opposite occurs in winter where the sea will make the weather milder
How does insolation vary in rural and urban areas?
Less is received by urban areas due to buildings
How does heat loss by evaporation vary in rural and urban areas?
The same amount of lost at night at 1 unit
More is lost by the rural area during the day at 29 units compared to the 1 unit lost by the urban area. Rural has greater albedo
Short-wave radiation reflected in rural and urban areas
More is reflected by the rural area at 24 units compared to the 5 units reflected by the urban area. This is because urban areas have a smaller albedo
Implications on heating the ground by conduction
The rural surface is heated less at 30 units since much of the incoming radiation was reflected back to space whereas the urban surface is heated more at 53 units since more radiation was absorbed. This is due to the very small albedo of the urban area
Heat given up at night in rural and urban areas
More heat is given up by the rural area at 11 units than the urban area at 22 units
Why is there more longwave radiation at night from the urban than rural area?
Heat from industrial activity, thermal properties of buildings and the evaporation of water. The tall buildings trap solar radiation
Latitude
When looking at spatial patterns in physical geography, it is more likely to be referred to as latitudinal changes (closer or further from the equator) than longitudinal changes. This is because of energy and temperature changes related to differences i solar radiation
What is insolation?
Incoming solar radiation
Atmospheric temperature balance
The atmosphere constantly receives solar energy but until recently, the atmosphere was not getting any warmer. There has been a balance between inputs (insolation) and outputs (re-radiation)
Under natural condition, this balance is achieved by:
1. Radiation: Mostly short-wave from the sun
2. Convection: The transfer of heat nut the movement of gases or liquids
3. Conduction: The transfer of heat by contact
Latitudinal vibration in insolation
Although overall there is an energy balance within the world, there are clear latitudinal and seasonal differences. At the tropics there is an excess of radiation (positive budget) but in higher latitudes there is a deficit of radiation (negative budget)
Latitudinal contrasts in insolation
The depth of atmosphere the insolation has to pass through to reach the Earth’s surface also increases because of the angle it reaches the earth so there is more scatter and absorption
The surface area covered by the same amount of insolation increases with latitude because of the curve of the Earth’s surface
Seasonal variations in insolation
Due to the tilt of the earth, as latitude increases, insolation decreases so there will be many hours of sunlight in the summer and very few in the winter. In the Northern hemisphere, summer is in the months around June, July and August but these will be the winter months for the Southern hemisphere. At the equator, insolation is relatively constant throughout the year since there is not much variation in incoming sunlight due to the tilt of the earth towards the sun, meaning that there will be almost exactly 12 hours of day and 12 hours of night all year. The latitude of places in the equator is 0
Annual temperature patterns
There pattern reflect the decrease of insolation from equator to poles. There is little seasonal variation at the equator but in mid or high latitudes large seasonal differences occur due to the decrease in insolation from the equator to poles and changes in the length of day. There is also a time lag between overhead sun and period of maximum insolation - up to 2 months - because the air is heated form below not above. The coolest period is after the winter solstice (shortest day) since the ground loses heat even after insolation has resumed. Over oceans, the lag is greater due to differences in specific heat capacity
Horizontal energy transfers
To achieve equilibrium and ensure that nowhere gets progressively hotter or colder, a horizontal transfer of energy from the equator to the poles takes place via winds and ocean currents
Influences on atmospheric heat transfers
Pressure variations - air blowing from high to low pressure areas
Ocean currents - can either warm up or cool down the overlying air
Atmospheric pressure
Measured in millibars
Traditionally measured with a barometer
Shown by isobars on a map
Pressure adjusted to mean sea levels (MSL) to account for impact of elevation
Average global MSL is 1013 mb but can range from 1060 to 940. Some intense storms can bring even lower pressure for short periods
Trend of rising or falling pressure more significant than actual numbers
Falling pressures normally bring bad weather while rising pressure often results in better, calm weather
Surface pressure belts
Sea-level pressure conditions show marked differences between the hemispheres. In the N hemisphere there are greater seasonal contrasts. High pressure can be reduced by altitude. The differences are mostly related to unequal distribution of land and sea because oceans are more equable in temperature and pressure variations.
A more permanent feature is the subtropical high-pressure belts (STHP) especially over oceans. In the S hemisphere there are nearly continuous at 30 latitude. Generally pressure is about 1026 mb. In N hemisphere at 30 the belt is more discontinuous because of the land. High pressure only occurs over ocean as discrete cells. Over continental areas major fluctuations occur
What is pressure like over the equatorial trough?
Pressure is low. This coincides with the zone of maximum insolation. In July in N hemisphere it is well north of the equator but in January in S hemisphere it is just south of the equator because land masses in S hemisphere are not of sufficient size to displace it south. The doldrums refers to the equatorial trough over seas where slack pressure gradients have a becalming effect
What is pressure like in temperate latitudes?
Generally less than subtropical areas. The most unique feature is the large number of depressions (low pressure) and anticyclones (high pressure) which don’t show up on a map of mean pressure. In summer, high pressure is reduced. In polar areas, pressure is quite high throughout the year
What is the intertropical convergence zone?
It is a band of low pressure around the earth which generally lies near to the equator. The trade winds of the N and S hemispheres come together which leads to the development of frequent thunderstorms and heavy rain. These can reach and sometimes exceed 16km in height above the surface. Air that is forced to rise along the ITCZ moves towards the poles and slowly descends leading to large areas of high pressure in the subtropics and bring largely benign weather conditions to places like the Azores. The resulting circulation that forms with air converging near the surface around the equator and diverging above is the Hadley cell
How does the ITCZ move?
Moves throughout the year and follows the migration of the Suns overhead position typically with a delay of about 1-2 months. As the ocean heats up more slowly than land, the ITCZ tends to move further north and south over land areas than over water. In the July and August, the ITCZ lies well north of the equator
Why is the ITCZ also called doldrums
The winds along the band of low are typically calm, trapping ships and leaving them stranded
How do seasonal ITCZ shifts affect weather?
Drastically affects rainfall in equatorial nations causing the wet and dry seasons of the tropics rather than the cold and warm seasons of high latitudes. Long term changes can result in severe drought and floods
What are monsoons?
A monsoon climate is characterised by a dramatic seasonal change in the direction of prevailing wind of a region which brings a marked change in rainfall. The monsoon climate result sin ghi annual rainfall totals. They lead to distinct wet and dry seasons in many areas throughout the tropics and are most often associated with the Indian Ocean. Conditions are best developed in the subtropics. The rainy seasons associated with monsoon winds is the main feature. During the winter monsoon, large areas of high pressure remain persistently over Asia pushing cold, dry air south to the tropics providing the region with its dry season
Summer monsoons
Associated with heavy rainfall. Usually happens between April and September. As winter ends, warm, moist air from the SW Indian ocean blows towards countries. It brings a humid climate and torrential rainfall. They depend on this rain for agriculture since they don’t have large irrigation, lakes, rivers or snowmelt. Aquifers are shallow. It fills these for the rest of the year. Dairy farms rely on it to keep cows healthy and fed. Electricity is mainly produced by HEP driven by water collected during monsoons. This powers hospitals, schools and businesses and helps economies develop. When late, the economy suffers. Heavy monsoons cause great damage. Urban streets flood and entire neighbourhoods can be drowned. In rural areas, mudslides can bury villages and destroy crops
Winter monsoons
Lasts from October to April. Blows form NE and winds start over Mongolia and NW China. They are less powerful since the Himalayas prevent much of the wind and moisture reaching the coast and keeps some places warm all year. Associated with droughts
Why do monsoons happen?
The 2 strongest influences are the seasonal march of the ITCZ above and below the equator that affects all tropical wet and dry climates magnified by the seasonal heating and cooling of the Asian landmass. As the global seasons alternate summer between the N and S hemispheres the ITCZ follow with it tracking north in the northern summer and south in the southern summer. This band is almost always accompanied by heavy thunderstorms and consequential rainfall as the hot and moist air is unstably thrust into the cold upper atmosphere
What is climate?
Climate averages are very closely related to the latitude of a place as this influences expected average temperature and precipitation. Climate is based on long-term average conditions for a particular place. Usually averages are collected using daily statistics for precipitation and temperature over a 30 year period
What is weather?
Weather is driven more by air masses, fronts and the position of the jet stream. It is based on short-term variation in atmospheric conditions such as precipitation, temperature, air pressure, wind speed/direction, humidity and cloud cover
Air masses
Mid latitude places are influenced by air masses. These are:
A continental scale body of air
A body of air with uniform temperature and humidity characteristics
Air whose temperature and humidity reflect the air mass source area
They are names based on whether they originated over land or the ocean and the latitude of the source area. These given an air mass its temperature and humidity characteristics. Cold air masses generally originate from the north and warm from the south
Track modification and stability
Maritime tracks cause air to pick up additional moisture as water evaporates from the ocean; continental tracks means air remains quite dry. Cold air masses that track over warmer areas will have ait at their base warmed, leading to instability; conversely air masses tracking towards colder places experience cooling at their base and remain stable
Weather fronts
When weather is associated with only 1 air mass, conditions are relatively stable and there may not be much change in weather. However most weather occurs at the boundary between 2 air masses (front). Where they meet there can be large differences of temperature, humidity and air pressure. These cause cloud formation, precipitation and wind along the meeting zone
Jet streams
The movement of air in the atmosphere is determined by the general circulation of 3 atmospheric cells. Hadley, Ferrel and Polar (equatorial, tropical and polar air masses). The boundaries between cells are fronts and at high altitudes are jet streams of sub-tropical and polar
What are the characteristics of jet streams?
Fast moving, high altitude tunnels or ribbons of wind that blow along the boundary of atmospheric cells
Blow west to east
Can be several 100km wide but only a few 1000m deep
Wind speeds average about 100mph but can be over 250mph
What are jet streams caused by?
The temperature difference between a warmer and colder air mass meeting. The temperature difference leads to a difference in air pressure (pressure gradient force) and winds blow across this
The temperature difference between equatorial and subtropical air is small so the subtropical jet streams are quite weak
The temperature difference between tropical and polar air especially in winter can be very large so the polar jet stream can be very fast and powerful
Rossby waves
More powerful polar jet streams are Rossby waves or planetary waves. The polar jets path meanders as it travels west to east usually between 4 and 6 meanders. These are caused by differences in the way land and oceans are heat up and cooled down which disrupts the flow. The exact position of jet streams and Rossby waves is dynamic changing daily and unpredictably. It changes seasonally as a result of the annual migration of the heat equator which is more predictable
Depression tracks
They incorporate fronts where warm and cold air meet. Warm, cold and occluded fronts all bring rainfall caused when warm air rises, cools and condenses as it moves over cold air. The track of depression is influenced by the position of Rossby waves. A very strong jet stream at high altitudes pells air up and out of a depression lowering air pressure and strengthening winds. Rossby waves and the polar jet stream can enter periods of stability. A large northward meander loops allowing warm Tm air to move up and over it. The Rossby wave meander prevents Pm and depressions
In an Arctic blast, polar outbreak or arctic plume, deep southward meanders develop in the jet stream allowing areas of cold polar and arctic air to extend south. Brings very cold weather and snowfall. These are associated with the polar vortex circulation. When this weakens it can send areas of cold air further south than usual and large pockets of cold air become detached
Oscillations
In the North Atlantic there are medium-term oscillations between different atmospheric states similar to the ENSO phenomenon in the Pacific Oceans
Arctic oscillation (AO)
North atlantic oscillation (NAO)
Atlantic multidecadal oscillation (AMO)
What does AMO affect?
Summer weather patterns, especially the strength and frequency of hurricanes and summer rainfall in Europe. The AO and NAO affect winter weather and the occurrence of Arctic Blasts
During a positive NAO phase…
There is a strong Tm Azores high and Pm low pressure area over Iceland
The pressure gradients between these 2 ar masses leads to a powerful polar jet stream
Depressions and fronts race across the Atlantic to the UK bringing wet, windy but mild weather
Pc air moves into Europe from the east because the jet stream is to the north
During a negative NAO phase…
The Azores high and Icelandic low pressure areas are weaker than normal
The jet stream begins to meander north and south because of a weak Polar Vortex
Leads to cold outbreaks and arctic blasts
Polar jet stream loops south bringing depressions
Global warming and NAO phases
Negative NAO values have become rare since around 1970 whereas strong values have become more common. A strongly positive winter NAO may be the new normal (wet and windy conditions associated with frequent Atlantic depressions). This is an early sign of global warming and might suggest that winters dominated by battles between Pm and Tm air will become more common than blasts of Am and Pc air
How latitude affects pressure
At the equator, the warm surface causes low pressure and rising air. At the poles, cold air produces high pressure and sinking air. If the earth did not rotate, this would describe global circulation. Due to the Coriolis effect, air rises near the equator but rather than flowing to the poles, deflection produces sinking air around 30 north and south (Hadley)
Rising air in the Hadley cell along the equator produces clouds, thunder and rain in the ITCZ, sinking air near 30 cause high pressure areas with clear skies
In general, air glows out of the subtropical highs along the surface, rises at the polar front causing the subpolar low
How ocean currents affect pressure
Oceans store heat better than the atmosphere so in some areas, the air is being heated from above by the sun and below by the ocean. This creates a bigger high pressure than usual which makes winds stronger. Warm water is evaporated into the atmosphere
When pressure affects ocean salinity:
-high pressure leads to clear skies
-high insolation
-more water converts into water vapour
-salinity increases in left over water
How to distribution of land and sea affects pressure
At sea level there is the greatest amount of atmospheric pressure
Air warms quickly over land and rises creating low pressure areas while air over water tends to stay cooler and not rise creating an area of relatively higher pressure
These differences drive different weather patterns depending on the pressure
How latitude affects temperature
-angle of the sun
-thickness of the atmosphere
At the equator the sun is very direct because of the angle of the earth and there is less atmosphere to penetrate
The opposite is true for the poles where the angle is greater so the radiation must penetrate through more atmosphere making temperatures cooler
More atmosphere means energy is scattered so less reaches the ground
Low sun angle makes it more easily reflected by snow and ice/less absorbed
How ocean currents affect temperature
The temperature of the ocean will either warm or cool the air just above it
Ocean currents make cold areas warmer and warm areas colder
Warm equatorial currents will warm cooler places in the winter in tropical climates. Winds must be blowing the warm/cool air from above the currents onto land
Oceans absorb much of the suns radiation, warming the planet and allowing warm water to be transported
How the distribution of land and sea affects temperature
Land near the sea will have less variation in temperature compared to inland
It takes 5x as much energy to raise water by 2 than land
The ocean retains heat and radiation longer than land. This is especially noticeable in winter when night is longer so land loses more heat
The further a place is from the sea, the more variation it will experience in temperature since they are not affected by ocean currents
How latitude affects wind belts
Winds at the ITCZ are generally light but can be broken by strong westerlies in summer
Mid-latitudes experience faster flow than wind belts at the equator
Trade winds are those which flow in low latitudes
When there is summer in SH there is cooling in NH causing increased differences between polar and equatorial air that leads to stronger high westerlies in winter in NH
Trade winds are NE in NH and SE in SH
Trade winds are associated with the Hadley cell, prevailing westerlies with the Ferrel cell and polar easterlies with the polar cell
How ocean currents affect wind belts
Prevailing winds blowing steadily across the sea causes surface ocean currents
Trade winds blow W to E in NH just above the equator. They pull surface water causing currents. The Coriolis force deflects them right to move north. Westerlies at 30 push them east forming a closed clockwise loop
In the Sh the Coriolis force bends currents to the left forming a counter-clockwise loop
These are gyres due to their circular flow
Wind drives subpolar gyres away from coastal areas
Wind moves from high to low pressure. Winds in the Pacific push warm surface water into the warm region exposing colder deep water behind which keeps the patten
How the distribution of land and sea affects wind belts
General winds are influenced by continents
Close the the Earth’s surface, winds over oceans are stronger than over land
Land surfaces are rougher due to mountains and forests, slowing winds
A lack of land in the SH (40 south of the equator) is the cause of a band of strong westerly winds
Monsoons occur when there is a reversal of a wind system
SE trade winds from the SH cross the equator in July, deflected to the right to become SW winds in the NH due to the Coriolis force
