Unit 3: Atmosphere and Weather Flashcards
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1
Q
Diurnal
A
The 24 hour period of day and night
2
Q
Radiation
A
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
3
Q
Radiation balance
A
(incoming solar radiation + atmospheric counter radiation) - (reflected solar radiation + outgoing terrestrial radiation)
4
Q
Incoming shortwave radiation
A
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
5
Q
Outgoing long-wave radiation
A
Of 100%:
10% passes into space
90% absorbed by atmosphere and reradiated back to surface
6
Q
Albedo
A
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
7
Q
Latent heat
A
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
8
Q
Sensible heat
A
The heat people actually feel
When air and water moves from one place to another taking it with it
9
Q
Night-time energy budgets
A
Consists of 4 components:
-long-wave Earth radiation
-latent heat transfer (condensation)
-absorbed energy returned to Earth (sub-surface supply)
-sensible heat transfer
10
Q
Moisture
A
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
11
Q
Temperature inversions
A
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
12
Q
Diurnal ranges
A
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
13
Q
How does insolation vary in rural and urban areas?
A
Less is received by urban areas due to buildings
14
Q
How does heat loss by evaporation vary in rural and urban areas?
A
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
15
Q
Short-wave radiation reflected in rural and urban areas
A
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
16
Q
Implications on heating the ground by conduction
A
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
17
Q
Heat given up at night in rural and urban areas
A
More heat is given up by the rural area at 11 units than the urban area at 22 units
18
Q
Why is there more longwave radiation at night from the urban than rural area?
A
Heat from industrial activity, thermal properties of buildings and the evaporation of water. The tall buildings trap solar radiation
19
Q
Latitude
A
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
20
Q
What is insolation?
A
Incoming solar radiation
21
Q
Atmospheric temperature balance
A
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
22
Q
Latitudinal vibration in insolation
A
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)
23
Q
Latitudinal contrasts in insolation
A
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
24
Q
Seasonal variations in insolation
A
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
25
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
26
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
27
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
28
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
29
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
30
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
31
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
32
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
33
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
34
Why is the ITCZ also called doldrums
The winds along the band of low are typically calm, trapping ships and leaving them stranded
35
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
36
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
37
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
38
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
39
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
40
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
41
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
42
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
43
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
44
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
45
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
46
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
47
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
48
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
49
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
50
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)
51
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
52
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
53
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
54
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
55
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
56
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
57
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
58
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
59
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
60
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
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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
62
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
63
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
64
Rossby waves
Also called planetary waves, are a type of inertial wave naturally occurring in rotating fluids. These are large-scale fast-moving belts of westerly winds which follow a ridge and trough pattern. These led to the 3 cell model. They are meandering river of air formed by westerly winds. There are 3-6 waves in each hemisphere. They are formed by major relieft barriers (mountain), thermal differences and uneven land-sea interfaces. As the pattern becomes more exaggerated, it leads to blocking anticyclones (prolonged periods of unusually warm weather)
65
The general circulation model
Warm air is transferred poleward and is replaced by cold air moving to the equator
Air rising is associated with low pressure and sinking air with high pressure
Low pressure produces rain, high pressure produces dry conditions
66
Cells
Hadley cell produced by direct heating at the equator. Air is forced to rise by convection, travels polewards and sinks at the subtropical anticyclone. They interlink with a mid-latitude cell, rotating it in the reverse direction into the polar cell. The north/south component of the Hadley cell is meridional flow. The polar cell is found in high latitudes. These winds are generally easterly. Air over the North pole continually cools are subsides creating high pressure. Air above the polar front flows back to the north pole creating a polar cell. Between the Hadley and polar cells is the Ferrel cell driven by the movement of the other 2 cells. The Hadley cell is thermally direct and the Ferrel cell is thermally indirect
67
Tropical latitudes and winds
Easterly surface winds are replaced by westerly winds above especially in winter. At the ITCZ convectional storms lift air into the atmosphere which increases air pressure near the tropopause causing winds to diverge at high altitudes. They move out of equatorial regions towards the poles, losing heat by radiation. As they contract more air moves in and the weight of the air increases the air pressure at the sub-tropical high pressure zone. The denser air sinks causing stability
68
What is ENSO?
The climate system around the equatorial pacific undergoes irregular changes every 3-7 years. The surface waters of the ocean and atmosphere interact and reinforce changes in the other. Positive feedback amplifies small changes in ocean temperatures. El nino occurs when sea surface temperatures are 0.5 degrees higher than average. This represents a huge store of energy in the deep ocean. Water heats lower than land and stores heat for longer. Surface will remain warm and sustain an ENSO event. Air pressure changes in response to sea surface temperatures and if the pressure gradient decreases, winds have less strength and can reverse. The winds pull a high cell of warm water across the Pacific from west to east during El Nino
69
Causes of ENSO
Has been happening for at least 125000 years. Has been operating since the Pacific boundary was formed. The scale of the Pacific could be responsible for ENSO. Planetary scale equatorial waves take time to cross the pacific so the ocean adjust slowly to wind variations giving temperature and pressure anomalies time to develop. This is positive feedback because deviation from the equilibrium position of the atmosphere-ocean coupled system because reinforced over time
Land masses bordering the narrower Indian and Atlantic have a greater effect on seasonal climate than the Pacific. Heating of land in the summer and cooling in winter means the land-sea temperature contrasts compete with an interrupt the larger ocean-atmosphere interactions needed for ENSO
70
The neutral ENSO phase
During normal Pacific circulation (Walker), NE trade winds blow from E to W across the Pacific driving warm, moist air and warm surface water to the west Pacific, deepening the thermocline. This creates warm water at depth. The warm water drives atmospheric convection forming warm moist air upwards to form cumulonimbus clouds that produce heavy rain (tropical rainforest in N Australia and Indonesia). Once the air releases its moisture, it travels east at the top of the troposphere before descending over the cooler east Pacific. In the ocean the col Humboldt current passes north along the South American coast. Cold water from deep is drawn up the replace the westward surface waters. This upwelling brings nutrients from depth and supports fisheries
71
El nino ENSO phase
Starts when trade winds die down or reverse, allowing warm surface water to move east and convection produces rain clouds over the Peruvian and Chilean coast. Warmer coastal water are less rich in nutrients and can badly affect fisheries
72
La nina ENSO phase
Follow El nino when the Walker circulation reverts to the neutral phase but is intensified by very strong westerly trade winds. Sea surface temperatures are warmer than average in the west Pacific and cooler in the east with upwelling of cold water from the deep ocean also contributing to lower sea temperatures
73
Indications of El nino
Water temperatures at the surface and depth
The amount of stored energy in the ocean
Ocean currents
The strength of the Walker circulation
Atmospheric air pressure
Upper atmosphere wind strength
Cloudiness in the tropics
74
The Southern Oscillation Index
A measure of intensity of the Walker circulation that controls the strength of ENSO. Measures the difference between sea-level air pressure between Tahiti and Darwin. Monthly averages are used. Below -8 = El nino. +8 = La nina. Disadvantage is that they are quite south of the equator whereas ENSO is closer
75
Equatorial SOI
Differences in sea-level air pressure between 2 large regions centered over the equator are used
76
Sea surface temperature index
The ocean shows the first indication of an ENSO so sea surface temperatures have been recorded in zones along the equator with cargo ships
77
Outgoing longwave radiation index
Satellites can monitor outgoing longwave radiation. It is reflect off cloud tops and gives a measure of convection and therefore thunderstorm activity, allowing mapping of wetter and drier regions of the Pacific. Above-average thunderstorm activity is associated with warmer sea temperatures
78
Wind index
Wind strength in the upper and lower atmosphere indicates the strength of Walker circulation
79
Evaporation energy
Occurs when vapour pressure of a surface exceeds that in the atmosphere and aims to equalise this pressure. Energy is required for the transfer of water from one state to another. In evaporation, heat is absorbed and becomes latent heat. It takes 600 calories of heat to change 1g of water from a liquid to a vapour. This would cool 1kg of air by 2.5C
80
Condensation energy
Occurs when either enough water is evaporated into an air mass for it to be saturated or when the dew point is reached. When condensation occurs, the latent heat in the water vapour is released. This causes a rise in the surrounding air temperature. Heat is also released if water vapour is converted straight to ice (deposition). This creates rime ice and is especially common at high altitudes and latitudes
81
Sublimation energy
When ice turns straight into water vapour. This takes energy from the surroundings and cools it
82
Freezing and melting energy
When liquid water turns to ice, heat is released increasing air temperature. Air temperature is probably still very low. When ice melts to water, heat is absorbed from the atmosphere and the air cools
83
Factors affecting evaporation
Initial humidity of the air: If air is very dry, strong evaporation occurs; if saturated then little occurs
Supply of heat: The hotter the air, the more evaporation that takes place
Wind strength: Under calm conditions, the air becomes saturated rapidly
84
Factors affecting condensation
Radiation cooling of the air
Contact cooling of the air when it rests over a cold surface
Adiabatic cooling of air when it rises
Required tiny particles or nuclei which the vapour can condense onto. Common in lower atmosphere. Some particles are hygroscopic (water seeking)
85
Why sublimation occurs
When a compound's vapour pressure equals its applied atmospheric pressure. Most solids do not have an appreciable vapour pressure at easily accessible temperatures so the ability to sublime is common. Those than can usually have weak intermolecular forces in the solid state
86
Why deposition occurs
When water vapour changes directly from a gas to a solid. Occurs when gas particles becomes very cold.
87
Precipitations
All forms of deposition of moisture from the atmosphere in either solid or liquid states. This includes rain, hail, snow, sleet and dew
For all types except dew, clouds must be produced first
88
Formation of water droplets
Once small water droplets condense from water vapour, they float in the air and form clouds
Droplets then coalesce to become bigger droplets
When they become too heavy they fall as rain
89
Mechanisms for rainfall
Bergeron theory: where ice crystals are formed in clouds at high altitude, they grow by condensation and fall eventually melting before reaching the ground after passing through warmer air
Condensation on extra large hygroscopic nuclei
By sweeping whereby a falling droplet sweeps up other. as it falls
The growth of droplets by electrical attraction
90
Types of rainfall
Frontal
Relief
Convectional
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Convectional rainfall
When land becomes very hot it heats the air above it. This air expands and rises. As it rises, cooling and condensation occurs. If it continues to rise, rain will fall. It is very common in tropical areas and is associated with the permanence of the ITCZ. In temperate areas it is more common in summer
92
Frontal rainfall
Occurs when warm air meets cold air. The warm air, being lighter and less dense is forced to rise over the cold, denser air. As it rises it cools, condenses and forms rain. It is more common in middle and high latitudes where warm tropical air and cold polar air converge
93
Orographic rainfall
Air may be forced to rise over a barrier. As it rises it cools, condenses and forms rain. There is often a rain shadow effect where the leeward slope gets a relatively small amount of rain. Altitude is important especially on a local scale. Increases in precipitation up to 2 km. Above this rainfall decreases since air temperature is so low
94
Thunderstorms
These are special cases of rapid cloud formation and heavy precipitation in unstable air conditions. This is intense convectional rainfall
This instability causes strong updraughts within cumulonimbus clouds
Most common in tropical and warm water area where the air can hold large amounts of water. They are rare in polar regions for the opposite reasons
95
3 stages of a thunderstorms
Developing stage
Mature stage
Dissipating stage
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Developing stage
Updraught caused by uplift. Energy (latent heat) is released as condensation occurs. Air becomes unstable. Rainfall occurs as cloud temperature is less than 0C. The strength of uplift prevents snow/ice
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Mature stage
Sudden onset of heavy rain and maybe thunder and lightning. Rainfall drags cold air down with it. Upper parts of the cloud may reach to tropopause. The clouds spreads giving an anvil shape
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Dissipating stage
Downdraughts prevent further consecutive instability. New cells may be initiated by the meeting of cold downdraughts from cells some distance apart triggering the rise of warm air in between
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Lightning and thunder
Lightning occurs to relieve the tension between charged areas. The upper parts of the cloud are positive, the lower parts are negative. The very base of the cloud is positive. The origin of the charges is thought to be due to condensation and evaporation. Lightning heats the air to very high temperatures. Rapid expansion and vibration of the column of air produces thunder
100
Clouds
Millions of tiny water droplets held in suspension. Can be classified as:
1. Form or shape: stratiform (layers) or cumuliform (heaped)
2. Height: low (<2000m), medium/alto (2000-7000m) and high (7000-13000m)
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Difference clouds
High clouds consist mostly of ice crystals. Cirrus are wispy clouds and include cirrocumulus and cirrostratus. Alto or middle-height clouds consist of water drops at 0C. Low clouds indicate poor weather. Stratus clouds are dense, grey and low lying. Nimbostratus are those that produce rain. Stratocumulus are long clouds tolls and a mixture of stratus and cumulus. Vertical development suggests upward movement. Cumulus are flat-bottomed and heaped. They indicate bright, brisk weather. Cumulonimbus produce heavy rainfall and thunder
In unstable conditions, uplift is mainly convection causing cumulus clouds
With stable conditions, stratiform clouds occur
Where fronts are involved a variety of clouds occur
Relief causes stratiform or cumuliform depending on air stability
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Banner clouds
Formed by orographic uplift in stable air. Uplifted moist air stream reach condensation at summit forming small cloud. Downward air sinks and cloud disappears. Wave clouds reflect influence of topography on air flow
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Cirrus clouds
Wispy, feathery and composed entirely of ice crystals. First sign of an approaching warm front or upper-level jet streak
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Cirrostratus clouds
Form a widespread, veil layer. Light is refracted when it passes through so a halo may form. As a warm front approaches, cirrus thicken to cirrostratus which may become altostratus
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Cirrocumulus clouds
Layered clouds permeated with small cumuliform lumpiness. Localised ascent and descent
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Altostratus clouds
Possess a flat and uniform texture in mid-levels. They indicate the approach of a warm front and may thicken and lower into stratus, then nimbostratus causing rain or snow. They do not produce significant precipitation at the surface but may sprinkle
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Altocumulus clouds
Heap clouds with convective elements. May align in row suggesting locally descending, drier air air. With some vertical extent may have more instability which could be boundary-layer based (am) and be released into deep convection (pm)
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Stratus clouds
Uniform and flat producing a grey layer of cloud cover which may have no precipitation or light precipitation or drizzle
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Stratocumulus clouds
Individual cloud elements characteristic of cumulo clouds in a continuous distribution characteristic of strato clouds. Can also be called cloud clumps which thick and thin areas. Ahead or behind a frontal system
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Nimbostratus clouds
Generally thick, dense stratus or stratocumulus clouds with steady rain or snow
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Cumulus clouds
Significant vertical development is cumulus congestus or towering cumulus. If enough instability, moisture and lift are present, strong updraughts develop leading to a depp cumulonimbus cloud. Cloud electrification occurs in these due to collisions between changed water droplet, graupel and ice crystals causing lightning and thunder
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Wall clouds
A localised layering from the rain free base of a strong thunderstorm Lowering denotes a storm updraught where fast rising air causes low pressure below the main updraught, enhancing condensation and formation under the base. Can lead to tornado formation due to cyclonic rotation
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Shelf clouds
Low, horizontal, wedge-shaped clouds and leading edge of a thunderstorm and wind. No tornadoes
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Fractus clouds
Low, ragged stratiform or cumuliform clouds unattached from thunderstorms or cold air bases
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Mammatus clouds
Drooping underside of a cumulonimbus clouds in latter development. No severe weather
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Contrail clouds
Narrow, elongated formed as jet aircraft or exhaust condenses in cold air at high altitudes
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Fog clouds
Layer of stratus clouds on or near the ground
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Hole-punch clouds
Formed when water temperature in the cloud is below freezing but water has not frozen. When freezing starts, surrounding vapour will descend leaving a rounded hole
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What is rain?
Liquid drops of water with a diameter of 0.5-5mm. It is heavy enough to fall to the ground
Drizzle is rainfall with diameter less than 0.5mm
Varies in amount, seasonally, intensity, duration and effectiveness (more the PEVT or not)
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Hail
Is alternate concentric rings of clear and opaque ice formed by raindrops being carried up and down in vertical air currents in large cumulonimbus clouds
Freezing and martial melting may occur several times before the pellet is large enough to escape the cloud
As raindrops are carried high in the cumulonimbus cloud they freeze
Hailstones may collide with supercooled water which freeze on impact and form a layer of opaque ice around the hailstone
As it falls the outer layer may melt but freeze again with uplift
This can occur many times before the hail falls to the ground when its weight can overcome the strong updraughts of air
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Snow
Is frozen precipitation. Snow crystals form when the temperature is below freezing and water vapour is converted into a solid
Very cold air contains limited moisture so heaviest snowfall occurs when warm moist air is forced over high mountains or when warm moist air comes into contact with very cold air at a front
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Dew
The direct deposition of water droplets onto a surface
Occurs in clear, calm anticyclonic conditions (high pressure) where there is rapid radiation cooling by night
Temperature reaches dew point and further cooling causes condensation and direct precipitation onto the ground and vegetation
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Radiation/ground fog
Radiation fog is formed in low lying areas during calm weather especially in spring and autumn. The surface of the ground, cooled rapidly at night by radiation cools the air immediately above it. This air flows into hollows by gravity and is cooled to dew point. Ideal conditions are a surface layer of moist air and clear skies allowing fast radiation cooling
Requires cold, clear night with light winds. Heat radiations away cooling the ground and air
Heavier cold air flows downwards
Fog forms as air cools to its dew point. Usually less than 200ft deep
Rising sun raises temperature to above dew point and fog evaporates
Fog won't form when its windy as cold air near ground and warm above mix
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Advection fog
Formed when warm moist air flows horizontally over a cooler land or sea surface. Steam fog is very localised. Cold air blows over much warmer water. Evaporation from the water quickly saturates the air and resulting condensation leads to steaming. It occurs when very cold polar air meets the surrounding relatively warm water
Warm humid air pushed inland during winter
Warm air is cooled by the cold ground below and forms fog
Can cover wide areas in winter and close airports
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Fog
Cloud at ground level. The decrease in temperature in the lower layers of air causes air to go below dew point. With light winds, fog forms close to the water surface but with stronger turbulence the condensed layer may be uplifted to form a low stratus sheet. As the sun rises, radiation fog disperses. Under cold anticyclonic conditions in late autumn and winter fog may be thicker and more persistent and around large towns smog may develop under an inversion layer. This means that cold air is found at ground level whereas warm air is above it unlike normal. In industrial areas, emissions of sulphur dioxide act as condensation nuclei and allow fog to form. Along motorways the heavy concentration of vehicle emissions does the same. In coastal areas the higher maximum temperatures means that condensation during high pressure is less likely. Commonly occurs over the sea in autumn and spring because contrast in temperature between land and sea is significant. Warm air from over the sea is cooled when it moves onto land during anticyclonic conditions. In summer the sea is cooler than land so air is not cooled when it blows into land. In winter there is more low pressure causing stronger winds and mixing air
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Anticyclones
Fog is more common in anticyclonic conditions. Anticyclones are stable high pressure systems with clear skies and low winds. Clear skies allow maximum cooling by night. Air is rapidly cooled to dew point, condensation occurs and fog is formed
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Greenhouse effect
Natural phenomena and warms up the earth by 33C on average
Some infrared radiation passes through the atmosphere but most is absorbed and re-emitted in all directions by greenhouse gas molecules and clouds. The effect of this is to warm the earths surface and lower atmosphere
Solar radiation powers the climate system. Some is reflected by the earth and atmosphere. About half is absorbed by the earths surface and warms it. Infrared radiation is emitted from the earths surface
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Enhanced greenhouse effect
Less heat escapes into space
More re-emitted hear
More greenhouse gases
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Greenhouse gases
Methane
Nitrous oxide
Carbon dioxide
Ozone
Chlorofloro carbon
Carbon monoxide
Sulphur dioxide
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Global warming potential
The GWP of human generated greenhouse gases is a measure of how much heat each gas traps in the atmosphere relative to CO2
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Main causes of climate change
Generating power
Manufacturing
Deforestation
Transportation
Food production
Power
Overconsumption
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Core evidence of climate change
Evidence for the greenhouse effect has been taken from ice cores 160000 years ago. These show the earths temperature closely paralleled to CO2 and methane levels. Changes in these gases were part of the reason for the large global temperature swings between ice ages and interglacial periods
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Measurements of CO2
Measurement began in 1957 in Hawaii. The chosen site was away from sources of industrial pollution. Levels show a clear annual pattern associated with seasonal changes in vegetation especially in NH. By the 1970's there had been a long term increase in CO2 levels
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Core in Antarctica and Greenland
Studies of cores of ice packs show that CO2 between 10000 years ago and 19th century was stable (270ppm). By 1957 it was 315ppm and has risen to. 360ppm. The extra CO2 has come from burning fossil fuels (coal) and the disruption of rainforests for every tonne of carbon burned 4 tonnes of CO2 were released. By 1980, 5 gigatonnes of fuel were burned every year. 1/8 of CO2 produced is absorbed by natural sinks like vegetation and plankton
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Other causes of climate change
Change in albedo affects the amount of solar energy absorbed at the surface. Sulphur aerosols emitted largely in fossil fuel combustion can modify clouds and lower temperatures. Changes in ozone in the stratosphere due to CFC's may influence climate
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Climate change since the industrial revolution
Since the industrial revolution combustion of fossil fuels and deforestation have increased 26% of CO2 in the atmosphere. CFC emissions as aerosol propellants, solvents, refrigerants and foam-blowing agents are well known. They were not present before 1930. Sources of methane and nitrous oxides are less well known. Methane concentrations have more than doubled because of rice production, cattle rearing, biomass burning, coal mining and natural gas ventilation. Fossil fuel combustion may have contributed through chemical reactions which reduce the removal rate of methane. Nitrous oxide has risen by 8% since pre-industrial due to humans. Effect of ozone is strongest in upper troposphere and lower stratosphere
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Arguments of climate change
There are natural causes:
Variations in earths orbit
Variations in tilt of earths axis
Variations in solar output
Changes in amount of dust in atmosphere
Periods of extreme volcanic activity
Changes in ocean currents as a result of continental drift
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Complexity of climate change
Scale: includes atmosphere, oceans and land of the whole worls and interations between these is complicated
Causes are natural and anthropogenic
Have complex feedback mechanisms
Processes are long term so might be a long time before full impacts are seen. Improvements may also take a long time
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Periods of climate change
Most agree that the rise in CO2 and other greenhouse gases is causing global warming. The temperature is not steadily rising and has been higher than today. Since the end of the Little Ice Age 200 years ago there have been 2 warming periods. 1860-1880 and 1910-1940 just as big as the last 30 years. Greenhouse gases are one of several influences on temperature. Others are the amount of heat emitted by the sun, distance of earth from the sun, tilt of earths axis, changes in amount of volcanic dust in the atmosphere and movement by continental drift. Global warming in the last 35 years has been slower than predicted
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Issues with climate change policies
No clear increase in frequency or severity of storms or droughts and no acceleration of sea-level rise
Could be a danger in bias of research for grants
unknown is the extent to which global warming and associated changes can be seen as slow and gradual over many years or if there is a tipping point where problems will escalate. Farming zones will shift polewards 250km for each 1C of warming. MIC's and LIC's will suffer more. Uncertainty of manageable warming. Level recommended by IPCC is 2C but debate about if this is too much
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LICS problems with climate policies
LIC's object to MIC's and HIC's suggesting to burn less coal, oil or gas. Developed countries are prosperous because of the economic progress they made by burning fossil fuels. Disadvantages of global warming to which humans can adapt are outweighed by benefits of fossil fuels. Unless all countries work together the efforts of those cutting back will have little impact. Fossil fuels are still the cheapest and most reliable energy in many countries. Hard to store electricity so conventional power stations burning coal, oil or gas are more reliable than wind farms. At a global level renewable energy lie wind and solar have not contributed much to cutting carbon emissions. Wind generates 1% of worlds energy, solar even less
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Effects of increased global temperature change
Rise in sea level causing flooding. 200 million could be displaces
200 million at risk of being driven from homes by flood or drought by 2050
4 million km^2 of land, 1/20 of world population threatened by floods from melting glaciers
Increase in storm activity
Changes in agricultural patterns
Reduced rainfall causing drought
4 billion could suffer from water shortages if temps rise by 2C
Drop in crop yields
200 million more exposed to hunger if temps rise by 2C. 550 million of 3C
Extinction of up to 40% of wildlife species if temps rise by 2C
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Stern review
Was a report by Nicholas Stern that analysed financial implications of climate change
-climate change in fundamentally altering the planet
-risks of inaction are high
-time is running out
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Stern review 3 main problems
Effects vary with degree of temp change. Report states that climate change poses a threat to the world economy and it will be cheaper to address the problem than to deal with the consequences. Global warming arguments seemed straight fight between science to act and economics not to
Stern review says doing nothing about climate change would lead to a reduction in global per person consumption of at least 5% now and forever. Global warming could deliver on economic blow of 5-20% of GDP to world economies due to natural disaster and the creation of hundreds of millions of climate refugees displaced by sea-level rise. Dealing with the problem will cost 1% of GDP
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Main points of Stern report
Carbon emissions have already increased global temps by over 0.5C
With no action to cut greenhouse gases it will warm by another 2-3C in 50 years
Temp rise will transform the physical geography of earth and the way we live
Floods, disease, storms and water shortages will be more frequent
Poorest countries will suffer earliest and most
Effect could cost the world 5-20% of GDP
Action to reduce greenhouse gas emissions and worst of global warming costs 1% of GDP
With no action each tonne of CO2 emitted will cause at least $85 of damage
Levels of CO2 in atmosphere should be limited to equivalent of 450-550ppm
Action should include carbon pricing, new technology and robust international agreements
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Global temperature rise predictions
Falling crop yields in many areas especially LEDC's
Possible rising yield in high latitudes
Falling yields in MEDC's
Small glaciers disappear
Fall in water availability
Sea levels threaten cities
Damage to coral reefs
Rising number of extinct species
Rising intensity of storms, forest fires, droughts, flooding and heatwaves
Increasing risk of dangerous feedbacks and abrupt large shifts in climate system
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Why do urban climates occur?
Occur as a result of extra sources of heat released from industry, commercial and residential buildings, vehicles, concrete, glass, bricks, tarmac etc.
Some of these absorb large quantities of heat and release them slowly by night
The release of pollutants help trap radiation in urban areas
Urban microclimates can be very different than rural ones
Greater amounts of dust mean an increasing concentration of hygroscopic particles
There is less water vapour but more CO2 and higher proportions of noxious fumes
Discharge of waste gases by industry is also increased
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How do urban heat budgets differ from rural ones?
By day the major heat source is solar energy and in urban areas brick, concrete and stone have high heat capacities
1km of an urban area contains a greater surface area than 1km of a countryside
This allows a greater area to be heated
There are more heat-retaining materials with lower albedo and better radiation absorbing properties in urban areas
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Moisture in urban and rural areas
In urban areas there is a relative lack of moisture due to a lack of vegetation and a high drainage density. There are decreases in relative humidity in inner cities due to the lack of moisture and higher temperatures. This is partly countered in very cold, stable conditions by early onset on condensation in low-lying and industrial areas
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Storms in urban and rural areas
There are more intense storms especially during summer nights due to greater instability and stronger convection above built-up areas. There is a higher incidence of thunder but less snowfall. Even if snow falls it melts rapidly
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EVT in urban and rural areas
Little energy is used for EVT so more is available to heat the atmosphere. This adds to head produced by industry and cars
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Cooling in urban and rural areas
At night the ground radiates heat and cools. In urban areas the release of heat by buildings offsets the cooling process and some industries, commercial activities and transport continue to release heat at night
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Radiation in urban and rural areas
There is greater scattering of shortwave radiation by dust but higher absorption of large waves due to surfaces and CO2. There is more diffuse radiation with local contrasts due to variable screening by tall buildings in shaded narrow streets. Reduced visibility from industrial haze
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Clouds in urban and rural areas
Higher incidence of thick cloud cover in summer because of increased convection and radiation fog or smog in winter because of air pollution. Concentration of hygroscopic particles accelerated onset of condensation
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The urban heat climate effect
Contrast is greatest under calm high pressure conditions
Typical heat profile of an urban heat island shows a maximum at the city centre, plateau across suburbs and temperature cliff between suburban and rural areas
Small scale variations within the urban heat island occur with industry distribution, open spaces, river, canals etc
The heat island is a feature delimited by isotherms in an urban area
This shows that the urban area is warmer than surrounding rural areas especially by dawn inn anticyclonic conditions
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What is the heat island effect caused by?
Heat produced by human activity: low level of radiant heat can be up to 50% of incoming energy in winter
Changes of energy balance: buildings have a high thermal capacity up to 6x greater than agricultural land
Effect on airflow: turbulence of air may be reduced overall but buildings cause funnels
Fewer bodies of open water so less evaporation and fewer plants so less transpiration
Atmospheric composition: blanketing effect of smog, smoke or haze
Reduction in thermal energy required for evaporation and EVT due to surface character, rapid drainage and lower wind speeds
Reduction of heat diffusion due to changes in airflow patterns as a result of urban surface roughness
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Air flow in urban areas
Urban areas may develop a pollution dome
Highest temperatures over city centre or downwind of the city if a breeze is present
Pollutant may be trapped under the dome
Cooler air above prevents pollutants dispersing
Pollutants prevent some incoming radiation passing through reducing the impact of the urban heat island
By night the pollutants may trap some longwave radiation escaping, keeping urban areas warmer
Airflow over an urban area is disrupted
Winds are slow and deflected over buildings
Larger buildings produce eddies
Severe gusting and turbulence around tall buildings causes strong local pressure gradients from windward to leeward walls
Deep narrow streets are calmer unless aligned with prevailing winds to funnel flows along them
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How are urban climates changing?
The nature of them is changing
With coal declining as a source of energy there is less SO2 pollution so fewer hygroscopic nuclei and less fog
Increase in cloud cover has occurred due to:
Greater heating of air
Increase in pollutants
Frictional and turbulent effects on airflow
Changes in moisture
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