AS Geography - Atmosphere and Weather

Latitudinal variations in the ITCZ

occur as a result of the movement of the overhead sun. -June: ITCZ lies further north -December: Lies in the southern hemisphere -Seasonal variations in the ITCZ is greatest over large land masses e.g. Asia -Contrast, over the Atlantic and Pacific oceans its movement is far less

Natural causes of climate change

-Variations in the Earth's orbit around the Sun -Variations in the tilt of the Earth's axis -Variations in solar output (sunspot activity) -Changes in the amount of dust in the atmosphere (partly due to volcanic activity) -Changes in the Earth's ocean currents as a result of continental drift

Absolute humidity

Amount of water in the atmosphere.

Night-time energy budget: Latent heat (Condensation)

At night water vapour in the air close to the ground can condense to form dew as the air cooled by the cold surface. Heat is released during this process

Coriolis effect

Deflection of moving objects caused by the easterly rotation of the earth. Northern hemisphere- Air moving from high to low is deflected to the right of its path and to the left in southern hemisphere

Influence of ocean currents on the global distribution of temperature, pressure and wind

Effect of ocean currents on temp. depends upon whether the current is cold or warm. Warm currents from equatorial regions raise the temp. of polar areas - effect only noticeable in winter. Cold currents, can reduce summer temp. but only if the wind blows from the sun to the land

Night-time energy budget: Sub-surface supply

Heat transferred by the sun to the surface during the day. May be released back to the surfaces at night which can off set the night time cooling at the surface

Factors affecting evaporation

Initial humidity of the air- if air is very dry then strong evaporation occurs; if it is saturated then very little occurs. Supply of heat - the hotter the air, the more evaporation that takes place. Wind Strength - under calm conditions air becomes saturated rapidly and therefore little evaporation occurs.

Freezing

Liquid turns to a solid when its temp. is lowered below freezing point. Heat is released

Land breeze

Occurs at night when the land cools faster than the sea at night. Creates a situation which is the opposite to day time - where the air above the sea is actually warmer at night than the air above the land

Monsoons

Occurs in south-east Asia due to: -The extreme heating and cooling of large landmasses in relation to smaller heat changes over adjacent sea areas. -The northward movement of the ITCZ during northern hemisphere summer. -The uplift of the Himalayas which 6 million years ago became high enough to interfere with the general circulation of the atmosphere. -Overall there is a dramatic increase in rainfall.

Orographic uplift of air

Orographic uplift is forced uplift of air on meeting a topographic obstacle such as a mountain front.

Dry adiabatic lapse rate (DALR)

Parcels of dry air cool at a higher rate than surrounding air approx. 10 degrees Celsius per 1000m

Urban climates

Refer to the changes in temperature, humidity, wind patterns, precipitation and air pressure that are noticeable over large urban areas during high-pressure conditions.

Precipitation

Refers to all forms of deposition of moisture from the atmosphere in either solid or liquid states. It includes rain, hail, snow and dew. For any type of precipitation to form, clouds must first be produced.

Relative humidity

Refers to the water vapour present, expressed as a % of the maximum amount air at that temp. can hold

Factor affecting air movement: Pressure gradient

The driving force is the pressure gradient, i.e. the difference in pressure between any two points. Air blows from high pressure to low pressure. Globally, very high-pressure conditions exist over Asia in winter due to the low temperatures. By contrast, the mean sea-level pressure is low over continents in summer. High surface temperatures produce atmospheric expansion and therefore a reduction in air pressure.

Adiabatic lapse rate

The rate at which atmospheric temperature decreases with increasing altitude in conditions of thermal equilibrium.

Daytime energy budget: Latent heat (Evaporation

The turning of liquid water into vapour it consumes a considerable amount of energy. When water is present at the surface, a proportion of the incoming solar radiation will be used to evaporate it. Consequently, that energy will not be available to raise local energy levels and temp.

Over the equatorial trough pressure is low, at around 1008-101 mb.

Trough coincides w/ the zone of maximum insolation. Northern Hemisphere in July it is well north of the equator. Southern Hemisphere (January) it is just south of the equator because land masses in the southern hemisphere are not of sufficient size to displace it southwards

Warm fronts

Usually show up on the tail end of precipitation and fog. As they overtake cold air masses, warm fronts move slowly, usually from north to south. Because warm fronts aren't as dense or powerful as cold fronts, they bring more moderate and long-lasting weather patterns. Warm fronts are often associated with high-pressure systems, where warm air is pressed close to the ground. High-pressure systems usually indicate calm, clear weather.

Daytime energy budget: Incoming solar radiation

The main energy input. Affected by latitude, season and cloud cover. The less cloud cover there is and/or the higher the cloud, the more radiation reaches the earth's surface

Reasons why urban climate is generally warmer

-Heat produced by human activity -Buildings having a high thermal capacity in comparison with rural areas - up to six times greater than agricultural land -Fewer bodies of open water (therefore less evaporation) and fewer plants (therefore less transpiration) -The composition of the atmosphere, involving the blanketing effect of smog, smoke or haze -Less thermal energy required for evaporation and evapotranspiration due to the surface character, rapid drainage and generally lower wind speeds

Influence of latitude on the global distribution of temperature, pressure and wind

Areas that are close to the equator receive more heat than areas that are close to the poles due to two reasons: 1)Incoming solar radiation is concentrated near the equator but dispersed near the poles 2)Incoming solar radiation near the poles has to pass through a greater amount of atmosphere and there is more chance of it being reflected back out of space.

Variations in temperature and wind: Height above sea level

Atmosphere is heated from ground level upwards via long-wave radiation. The higher up a mountain you go, the smaller the surface area available to heat the atmosphere above. This, in combination with a decrease in the ability of the air to retain heat results in lower temperatures.

Jet streams and Rossby waves

Between the different atmospheric cells high up in the tropopause at a height of about 10km are the jet streams, named the polar jet stream (40-60°N+S) and the subtropical jet stream (25-30°N+S). These jet streams move air at a high speed (up to 300km/h) around the Earth horizontally and give rise to Rossby waves. Rossby waves are waves or zigzags in the jet streams as the travel around the Earth. The number of waves varies throughout the year but usually in summer it's between four and six while in winter it's three. Rossby waves are formed by major releif barriers (like mountains), thermal differences and uneven land-sea interfaces.

Night-time energy budget: Sensible heat transfer

Cold air moving into an area may reduce temperatures whereas warm air moving in will raise temp.

Air flow over an urban area

Disrupted - winds are slow and deflected over buildings. Large buildings can produce eddying. Severe gusting and turbulence around tall buildings causes strong local pressure gradients from windward to leeward walls. Deep, narrow streets are much calmer unless aligned with prevailing winds to funnel flows along them - the 'canyon effect'.

Katabatic wind

During a clear evening, the valley losses heat through radiation. Surrounding air cools and becomes dense. It begins to drain and sink down the valley sides and along the valley floor as a mountain wind or a katabatic wind. This gives a temperature inversion and may create fog or a frost hollow. These winds are usually strong if they blow over glaciers or snow covered slopes.

Daytime energy budget: Long wave radiation

Emitted by the surface and passes into the atmosphere, and eventually into space. Downward-directed stream of long-wave radiation from particles in the atmosphere. Difference between two streams is known as net radiation balance. During the day, since outgoing stream is greater than incoming one, there is a net loss of energy from the surface

Daytime energy budget: Surface absorption

Energy arriving at the surface has the potential to heat that surface, as heat is absorbed by it. Nature of the surface has an effect e.g. surfaces can conduct heat rapidly into lower layers of the soil, its temperatures will be low. If heat is not carried away quickly, it will be concentrated at the surface and result in high temperatures

Variations in temperature and wind: Distance from land and sea

Land and sea have vastly different specific heat capacities (the amount of energy needed to raise 1kg of a substance by 1 degree). They have different abilities to absorb, transfer and radiate heat energy. Generally, land surfaces respond to heating on a daily basis (diurnal) meaning that differences between day and night temperatures can be into double figures, but sea surfaces respond over a period of months and retain heat for longer. The sea heats up and cools down more slowly than the land, acting to moderate temperatures for coastal locations.

High pressure areas

High pressure areas have higher than average pressures which are characterized by descending air and stable weather conditions.

Stability

Relates to the atmospheric conditions associated with dry, descending air which is characterised by calm conditions and relatively clear skies. Occurs when ELR < DALR and the SALR. If a parcel of air is displaced upward, it immediately gets cooler and denser and sinks. The only time when stable air can rise is when it is forced to e.g. over high ground

Local winds: Meso level

Land and Sea Breeze: The result of differential heating and cooling between land surfaces and adjacent sea areas. The small pressure differences result in gentle breezes in the day on the coasts blowing from sea to land. At night. The sea retains heat longer than the land, the pressure gradient reverses and so to the wind direction. The land breeze is gentler than the two and dies away by sunrise.

Influence of land/sea distribution on the global distribution of temperature, pressure and wind

Land heats and cools more quickly than water. Water heats more slowly because it is clear so the sun's rays penetrate to a greater depth (distributing energy over a larger volume) and tides and currents cause the heat to be further distributed. Water takes up heat and gives it back much more slowly than land. In winter, in mid-latitudes, sea air much warmer than land air, so onshore winds bring heat to the coastal lands. By contrast, during the summer, coastal areas are much cooler than inland sites.

Anabatic wind

Likely to blow in mountainous areas during times of calm, clear, settled weather. During the morning, valley sides are heated by the sun, especially if facing the sun and lacking vegetation. The air in contact will heat, expand and rise creating a pressure gradient. By the time of maximum heating a uphill wind will blow up the valley. Anabatic wind is conditionally unstable. Cumulus clouds and warm conditions will occur.

Night-time energy budget: Long-wave radiation

Loss at night (terrestrial radiation) as nights are often cloudless and there is nothing to return the long wave radiation back to the surface. On cloudy nights, energy loss is reduced

Variations in temperature and wind: Angle of the overhead sun, latitude and thickness of atmosphere:

Lower latitudes (equatorial regions) have higher temperatures than higher latitudes (Poles) this is as a result of the amount of heating that each area receives. Places near the equator receive direct heat on a small surface area, and experience little energy loss via absorption, scattering and reflection, as there is a relatively small amount of atmosphere to pass through. Towards the Poles, the surface area to be heated increases, as does the amount of atmosphere to pass through, increasing losses via, absorption, scattering, and reflection.

Conditional instability

Occurs when ELR lies between SALR and DALR. Moist saturated air will rise, and dry unsaturated air will sink. Air is therefore stable in respect to the dry rate and would normally sink to its original level. If air becomes saturated, owing to being forced to rise to higher elevations, it may become warmer than surrounding air and would continue to rise of its own accord.

Instability

Occurs when a parcel of air is warmer and therefore less dense than the air above causing rising and expansion. Uplift and adiabatic cooling of moist air occurs. Air is unstable if ELR is greater than DALR. Unstable air tends to occur on v. hot days when ground layers are heated considerably

Factors affecting condensation

Occurs when a) enough water vapour is evaporated into an air mass for it to become saturated or b) when temp. drops so that the dew point is reached Cooling occurs in three ways: radiation cooling of air, contact cooling of air when it rests over a cold surface and adiabatic cooling of air when it rises Requires particles or nuclei onto which the vapour can condense

Condensation

Occurs when either enough water vapour is evaporated into an air mass for it to become saturated or when the temperature drops so that dew point is reached.

Radiation fog

Occurs when the ground loses heat at night by long wave radiation. Occurs during high pressure conditions associated w/ clear skies

Temperature inversions

Occurs when warm air meets a cold surface and rapid condensation occurs .During the day the ground is heated by the sun's short wave radiation. After a short time it heats the air above it when it emits long wave radiation. At night the ground surface and the air lose the heat energy they have absorbed during the day. However, the ground loses heat energy faster than the air as it is a more efficient conductor of heat

Cold front

Often come with thunderstorms or other types of extreme weather. They usually move from west to east. Cold fronts move faster than warm fronts because cold air is denser, meaning there are more molecules of material in cold air than in warm air. Strong, powerful cold fronts often take over warm air that might be nearly motionless in the atmosphere. Cold, dense air squeezes its way through the warmer, less-dense air, and lifts the warm air. Because air is lifted instead of being pressed down, the movement of a cold front through a warm front is usually called a low-pressure system. Low-pressure systems often cause severe rainfall or thunderstorms.

Sea breeze

On a warm day along the coast, differential heating of land and sea leads to the development of local winds known as sea breezes. Land is heated quicker than the sea and so the air above the land is warmer than the air above the sea during the day. As air above land surfaces is heated by radiation from the sun, it expands and begins to rise, being lighter than the surrounding air

Saturated adiabatic lapse rate (SALR)

Parcels of saturated air cool at approx. 40 - 90 degrees per 1000m. Saturated air cools at a lower rate than the dry air because condensation is occurring so the air releases heat offsetting the cooling processes

Subtropical high-pressure belts (STHP) are a permanent feature, especially over ocean areas

Southern Hemisphere: Almost continuous at 30 degrees' latitude. Northern Hemisphere: At 30 degrees the belt is more discontinued because of the land. Over oceans, high pressure occurs at discrete cells e.g. Pacific highs. Over continental areas e.g. Sahara major fluctuations occur: Higher pressure in winter and summer lows because of overheating

ITCZ

The Inter-Tropical Convergence Zone (ITCZ) is a band a few hundred km wide in which winds from the tropics blow inwards, convergence and then rise, forming an area of low-pressure

Environmental lapse rate (ELR)

The actual temperature decline with height - on average this is 6degrees per 1000 metres.

Factor affecting air movement: Pressure and wind

The basic cause of air motion is the unequal heating of the Earth's surface. The major equalising factor is the transfer of heat by air movement. Variable heating of the earth causes variations in pressure and this in turn sets the air in motion. There is thus a basic correlation between winds and pressure.

Contrast of urban and rural climates

The contrasts between urban and rural areas are greatest under calm, high-pressure conditions. The typical heat profile of an urban heat island shows the maximum at the city centre, a plateau across the suburbs and a temperature cliff between the suburban and rural area. Small-scale variations within the urban heat island occur, with the distribution of industries, open space, rivers, canals and so on.

How is heat transferred by ocean currents

The earth receives energy from the sun as insolation; some is lost through atmosphere but overall surface has a net gain of energy, particularly in the tropics. Ocean currents transfer heat horizontally, meaning that the ocean currents re-distribute heat from the tropics to higher altitudes. An example of this is the North Atlantic Drift.

Latitudinal pattern of radiation excesses and deficits

The energy excess/deficits represents the balance between short wave radiation received and long wave radiation. Tropical and equatorial areas receive the most solar radiation due to the overhead sun and the earth's tilt. The greater amount of atmosphere to heat at the poles plus the seasonal tilt away from the sun produces less solar radiation and hence and energy deficit.

Tropical cyclones

These are systems of intense low pressure zones, also known as hurricanes, typhoons and cyclones. They occur: Over warm tropical oceans where sea temperatures exceed 26oC. In autumn where sea temperatures are at the highest. In the trade wind belt, where surface winds warm as they blow towards the equator. Between latitudes 5o and 20o N or S of the equator. Any closer to the equator and the coriolis force is insufficient to enable the 'spin'.

Advection fog

Warm air passes over a cold surface it is chilled. Condensation takes place as the temp. of air is reduced and the air reaches dew point (the temp. at which relative humidity is at 100%)

Anti cyclones

This is a large mass of subsiding air which produces an area of high pressure on the Earth's surface. Air in the upper troposphere has little moisture. As it descends, the air warms at the DALR, so dry conditions result. Pressure gradients are gentle which results in weak winds or calms. Winds blow outwards and clockwise in the northern hemisphere. Weather conditions: Summer = due to absence of cloud there is intense insolation. Hot, sunny days. Rapid radiation at night leading to inversions and the formation of dew and mist. Winter = Cloudless skies but with little incoming radiation in the day due to angle of earth. Temperatures are cool. Absence of cloud at night means fog and frost develops.

Evaporation

Water changes from liquid to a gas, heat is absorbed. Evaporation depends on initial humidity of air - if air is very dry then strong evaporation occurs; if it is saturated then very little occurs

Sublimation

Transition of a substance directly from solid to a gas without passing through the liquid state or vice versa

Temperate latitudes pressure is generally lower than in subtropical areas

Unique feature is the large number of depressions (low pressure) and anti cyclones (high pressure), which do not show up on a map of mean pressure. Northern Hemisphere: Strong winter low pressure zones over Icelandic and Oceanic areas. But over Canada and Siberia high pressure dominates due to the coldness of the land. In summer high pressure is reduced esp. over continental areas. In polar areas pressure is relatively high throughout the year esp. over Antarctica because of the coldness of the land mass

Daytime energy budget: Sensible heat transfer

Used to describe the transfer of parcels of air to or from the point at which the energy budget is being assessed. If relatively cold air moves in, energy may be taken from the surface, creating an energy loss. If warm air rises from the surface to be replaced by cooler air, a loss will occur - the process is convection transfer. During the day it is responsible for removing energy from the surface and passing it to the air

Daytime energy budget: Reflected solar radiation (Albedo)

Varies with colour - light materials are more reflective than dark materials. Amount of incoming solar radiation that is reflected by the surfaces. Lighter surfaces e.g. fresh snow has an avg. albedo of 75-90%. Darker surfaces e.g. grass has an avg. albedo of 20-30% and dry concrete albedo is 17-27%. Solar energy (shortwave radiation) is reflected from earth back into space

General circulation model

Warm air is transferred pole wards and is replaced by cold air moving towards the equator. Air that rises is associated with low pressure, whereas air that sinks is associated with high pressure. Low pressure produces rain, while high pressure produces dry conditions. There are three major cells present: Hadley, Ferrell and Polar. They shift northwards and southwards throughout the year due the shift in location of the sun's rays focusing most intensely on the Earth's surface.

Mist and fog

•Cloud at ground level •Mist occurs when visibility is between 1000m and 5000m •Fog occurs where visibility is below 1000m •Fog is thicker cloud cover than mist •They form at ground level because air can only hold a certain amount of water •Colder air can hold less moisture than warmer air •Once this maximum amount of moisture is reached, air is saturated and water vapour in air turns to liquid •This is when clouds form as condensation of water vapour to water droplets occur