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CfE H Geography: Physical Environments - Atmosphere
Transcript of CfE H Geography: Physical Environments - Atmosphere
The sun is the main source of energy to the earth and this energy drives the atmospheric system which makes us able to live on earth!
This is an additional tool to assist with revision after looking through your notes, books and past papers. It is not a replacement for the above!
Solar radiation passes through atmosphere and makes it's way to the earth's surface but:
- some is reflected by clouds
- some is scattered by gas particles in the air
- some is reflected by the earth's surface
These are described as the albedo effect and about 30% of solar energy is lost to this. In addition to this:
- some is absorbed by clouds (~3%)
- some is absorbed by dust, water vapour and other gases (~17%)
This leaves around 50% of the solar radiation to be absorbed by the earth's surface.
Some of the solar radiation absorbed by the earth's surface is emitted back into the atmosphere.
- Around ~6% is lost directly to space
- The remaining ~94% is absorbed by water vapour, carbon dioxide and other greenhouse gases in the atmosphere before it is re-radiated.
This is known as the 'Greenhouse Effect', and it slows down the rate at which radiation is lost from the atmosphere. Without the 'Greenhouse Effect', average global termperatures would be around 30c cooler.
Some parts of the earth receive more solar radiation than others e.g. there is an energy surplus at the equator but energy deficits at the poles
From 35N and 35S towards the poles, there is an energy deficit as the amount of incoming solar radiation is less than the outgoing radiation. There is an energy surplus at the equator as there is far more incoming solar radiation than outgoing. This extreme is caused by:
- The curvature of the earth - as you move away from the equator the earth's surface is sloped increasingly away from the sun. Therefore, the solar radiation is more concentrated at the equator. The curvature of the earth also means that the incoming solar radiation has to pass through more atmosphere at the poles, therefore more is absorbed by clouds, water vapour, gases etc.
- Between the tropics the sun is overhead during the day all year round, meaning insolation is more focused and temperatures higher than at the poles, where the sun hits the surface at the lower angle.
- For six months of the year, the poles are in darkness, receiving little or no solar radiation so have very low temperatures.
If the global differences in energy surplus and energy deficit were the only processes in operation, the equator would become increasingly warmer and the poles increasingly cooler.
However, energy is transported around the earth through atmospheric circulation (accounting for about 80%) and oceanic circulation (remaining 20%).
The Three Cell Model
Formation of the Hadley Cell (1)
Insolation in tropical areas causes warm air to rise and spread polewards, carrying heat energy.
The Three Cell Model
Formation of the Hadley Cell (2)
Air cools and begins to fall at about 30ºN and 30ºS of Equator. Cooled air returns to the Equator.
This circulation of air is caused by solar heating.
Heat energy is transferred from the Equator to sub-tropical latitudes.
It is called the
The Three Cell Model
Formation of the Polar Cell (1)
Intensely cold, dense air sinks at the poles, then blows as surface winds towards the Equator.
The Three Cell Model
Formation of the Polar Cell (2)
At about 60ºN and 60ºS, the cold polar air is warmed in contact with the earth’s surface.
This warmed air rises and returns polewards, carrying heat energy.
This circular motion is called the
The Three Cell Model
Formation of the Ferrel Cell (1)
The Hadley Cell is driven by differences in heat energy at the Equator.
As the air in the Hadley Cell falls at about 30ºN and 30ºS, it pulls the air beside it down as well, due to friction
The Three Cell Model
Formation of the Ferrel Cell (2)
The Polar Cell is driven by differences in heat energy. Cold polar air falls and spreads towards the Equator.
As the air in the Polar Cell rises at about 60ºN and 60ºS, it pulls the air beside it up as well, due to friction.
The Three Cell Model
Formation of the Ferrel Cell (3)
Unlike the Hadley and Polar Cells, the Ferrel Cell is not driven by differences in heat energy.
The Ferrel Cell is caused by friction where air is in contact with the other two cells.
The Hadley Cell drags air down at about 30ºN and S.
The Polar Cell causes an uplift at about 60ºN and S.
The transfer of heat from equatorial to polar areas
Where air carrying energy from the Equator in the Hadley Cell comes into contact with air in the Ferrel Cell, there is a transfer of heat energy into the Ferrel Cell.
There is a similar transfer of heat energy from the Ferrel Cell to the Polar Cell.
In this way, heat energy is transferred from the Equator, where there is a surplus of energy, to the poles where there is a deficit.
The corresponding movement of colder air
In the Polar cell cold air from polar regions flows to mid-latitudes as polar easterly winds.
In the Ferrel Cell there is a movement of cold air at high altitude.
In the Hadley Cell, cooler air moves from the sub-tropics to the Equator.
Associated pressure belts
Rising air at the equator causes the equatorial belt of low pressure.
Descending air at about 30ºN and 30ºS causes the sub-tropical belt of high pressure.
Rising air at about 60ºN and 60ºS causes a mid-latitude belt of low pressure.
Descending air at the poles causes the polar high pressure areas.
Associated wind surface patterns
Winds always blow from high pressure to low pressure. They are deflected because of the Coriolis Force which come about because of the rotation of the earth.
Winds in Northern Hemisphere are deflected to the right.
Winds in the southern hemisphere are deflected to the left.
These wind belts shift seasonally.
Position of the three cells in December
The sun is overhead at the Tropic of Capricorn, 23ºS of the Equator.
The cells shift southwards as the heat equator is in the southern hemisphere.
Position of the three cells in June
The sun is overhead at the Tropic of Cancer, 23ºN of the Equator.
The cells shift northwards as the heat equator is in the northern hemisphere.
The Inter-Tropical Convergence Zone (ITCZ)
IN THE NORTHERN HEMISPHERE
OVER WEST AFRICA
The sub-tropical high pressure belt develops over the Sahara so is hot and dry. This is known as continental Tropical (cT) air.
IN THE SOUTHERN HEMISPHERE
OVER WEST AFRICA
The sub-tropical high pressure belt develops over the Atlantic so is warm and moist.
This is known at maritime Tropical (mT) air.
IN THE NORTHERN HEMISPHERE:
The winds that blow to the equatorial low pressure belt are called the North East Trade Winds
IN THE SOUTHERN HEMISPHERE:
The winds that blow to the equatorial low pressure belt are called the South East Trade Winds
The line along which they converge (meet) is called the INTER-TROPICAL CONVERGENCE ZONE.
This is often abbreviated to ITCZ.
The ITCZ in December
In December, the zone of maximum insolation (solar energy) is south of the Equator. This means that the wind belts shift southwards.
This means that winds blow out of the sub-tropical high pressure area over the Sahara, and take dry air from the continental Tropical (cT) air mass across most of West Africa. This causes a dry season.
Moist air from the maritime Tropical (mT) air mass from the Atlantic cannot reach far inland, where there is a dry season.
The ITCZ in June
By contrast, in June, the zone of maximum insolation is well to the north of the Equator. This means that the wind belts shift northwards.
Moist maritime Tropical air from the Atlantic now reaches far inland, where there is a rainy season. These winds flow northwards to the ITCZ to replace air that has become unstable and risen.
The winds blow out of the sub-tropical high pressure area over the Sahara, now only affect the northern part of sub-Saharan Africa.
Position of the ITCZ in December
In December the sun is overhead in the southern hemisphere.
The ITCZ is found to the south, where there is maximum insolation.
The sea stays a fairly constant temperature, so the ITCZ runs just along the coast in WAfrica.
Only the coastal fringe receives rain from the unstable mT air at this time of year
Further north, the area is under the influence of the Harmattan, (stable, dry cT air blowing out of the Saharan high pressure area).
Migration of the ITCZ from December to June
Between December and June, progress through the Earth’s orbit causes the sun to migrate northwards.
As it does so, the ITCZ also moves further north, allowing moist mT air to reach progressively further inland, bringing the rainy season to West Africa.
By late June, the sun begins to migrate southwards, and so does the ITCZ, following the zone of maximum insolation.
As the ITCZ moves further south, the Harmattan carries dry, stable cT air further south, bringing the dry season across more and more of West Africa.
Shape of Continents
Ocean current direction is modified by the shape of the continents.
Consequently the major oceanic basins have huge, roughly circular shaped loops of water called gyres.
A good example of this is the Gyre that forms around the Ivory Coast in West Africa.
Oceanic currents play an important
role in redistributing energy. They make
sure that the low latitudes (equator) does not become too hot and that the high latitudes (poles) do not become too cold.
Energy is redistributed in the oceans by ocean currents, this is called oceanic circulation.
This huge movement of water can result in warm water being transferred towards the poles and cool water being transferred
towards the equator.
The Thermohaline Conveyor
An ocean current is more or less a permanent or continuous, directed movement of water that flows in one of the Earth’s oceans.
Ocean currents can flow for thousands of kilometres. They are very important in determining the climates of the continents, especially climate regions next to oceans.
Ocean currents are driven by thermohaline circulation.
(thermo = heat, haline = salt) The heat and salinity
determine the density of the sea water.
Uneven heating of surface water in high and low latitudes sets up convection currents which transfer energy.
The water round the Poles is more dense than at the
equator because it contains more salt. Salt does
In simple terms:
How do the ocean currents work?
Global winds cause frictional drag on large water surfaces.
Ocean currents therefore tend to follow prevailing wind directions.
The Coriolis Force
The Coriolis force means that winds are deflected to the right (clockwise) in the Northern Hemisphere and to the left (anti-clockwise) in the Southern Hemisphere.
Ocean currents are also deflected to the right (clockwise) in the Northern Hemisphere and to the left (anti-clockwise) in the Southern Hemisphere.
Exam Style Question
exchanges that result in the Earth’s surface absorbing only 51% of the solar energy that reaches the outer atmosphere
Reflection from the atmosphere accounts for a 26% reduction in the amount of solar energy. Large amounts are reflected by clouds, gas and dust in the atmosphere.
A further 6% is reflected from the earth’s surface and emitted as long wave radiation.
The amount of reflectivity is linked to the Earth’s albedo. At the poles this is much higher than the equator due to ice and snow and the high reflective quality of white surfaces.
Absorption by the atmosphere reduces the solar energy by a further 18%. The majority of absorption is by gases, cloud and dust in the atmosphere.
This means that only around 50% of the solar energy which reaches the edge of the atmosphere is actually absorbed by the earth’s surface.
Warm air rises at the Equator and travels in the upper atmosphere to around 30◦ N and S, cools and sinks. Air moves from the tropical high to the low pressure area at the equator creating the Hadley cell/Trade Winds.
Cold air sinking at the poles moves to 60 N/S to form the Polar Cell/Polar Easterlies. The cold air from the poles meets warmer air from the tropics, causing air to rise creating the Ferrel Cell low pressure. Air is moved from the tropical HP, towards LP at the polar front, forming the westerlies.
This convergence causes the air to rise, with some of this flowing in the upper atmosphere to the Poles where it sinks, forming the Polar cell. Easterly winds blow away from the high pressure at the Pole.
Warm air from the Equator is distributed to higher and cooler latitudes and cold air from the Poles distributed to lower and warmer latitudes.
Due to the Coriolis effect winds are deflected to the right in the northern hemisphere.
Variations in Insolation
As the earth is a sphere, there are variations in the amounts of
received in different places.
The bands near the north pole have to heat a larger area due the curve of the earth.
The band at the equator can concentrate its heat in a smaller area.
We can also compare energy received by the earth (insolation) with energy lost by radiation (terrestrial radiation).
solar energy received by the earth.
solar energy lost by radiation.
At the equator: insolation is greater than terrestrial radiation (due to darker forest material and higher land concentration), leading to a surplus in energy.
At the polar zones, insolation is weak and terrestrial radiation is high (due to lighter light-reflecting snow + lower land concentration), leading to an energy deficit.
Exam Style Question
Explain why there is a surplus of solar energy in the tropical latitudes and a deficit of solar energy towards the poles.
You may wish to use annotated diagram or diagrams.
Sun‘s angle in the sky decreases towards the poles due to the curvature of the Earth, which spreads heat energy over a larger surface area. Sun‘s rays are concentrated on tropical latitudes as the intensity of insolation is greatest where rays strike vertically.
Sun‘s rays have less atmosphere to pass through at the tropics, so less energy is lost through absorption and reflection by clouds, gas and dust.
Albedo rates differ from the darker forest surfaces at the tropics absorbing radiation, in contrast to the ice-/snow-covered polar areas reflecting radiation.
Tilt of the axis results in the Sun being higher in the sky between the tropics throughout the year, focusing energy.
No solar insolation at the winter solstices at the poles producing 24-hour darkness, whereas the tropics receive insolation throughout the year.
The highest annual rainfall is in Lagos with more than 1500 mm/approximately 1600 mm; this decreases further North, Ouagadougou has less than 1000mm / approx. 900 mm and Tombouctou less than 250 mm / Approx. 240 mm.
This is because the maritime tropical air mass brings large amounts of rain to areas near to the coast. The peak in rainfall is earliest in Lagos with a peak in June, while the peak rainfall does not occur until August for the other two selected cities. This is due to the ITCZ moving northwards in line with in the thermal equator.
The thermal equator moves south in the second part of the year, this accounts for the second peak rainfall in Lagos in October. Rainfall is heaviest when the two air masses (maritime tropical and continental tropical) meet forming the ITCZ, this accounts for the high rainfall in Lagos in June of 480 mm (approx.).
As this band of rain moves northwards it brings rain with it causing a peak in Ouagadougou in August of 200 mm (approx.). As the band of rain does not fully reach Tombouctou it does not get the full effect of the rainfall and receives a small amount of only 90 mm (approx).
As the sun is overhead in the southern hemisphere, it is the south that is hottest, (shown by the red areas). The Sahara stands out as a cooler, (lighter coloured), area.
AFRICA – TEMPERATURES IN JANUARY
In July, with the sun overhead north of the Equator, the Sahara is clearly much hotter than the rest of the continent.
AFRICA – TEMPERATURES IN JULY
Currents flow in loops or gyres; clockwise in the Northern Atlantic, formed with warm water from the Gulf of Mexico (Gulf Stream/North Atlantic Drift) travelling northwards and colder water moving southwards.
Currents from the Poles to the Equator are cold currents whilst those from the Equator to the Poles are warm currents. Cold water moves southwards from Polar Latitudes e.g. the Labrador current.
Ocean currents are greatly influenced by the prevailing winds, with energy being transferred by friction to the ocean currents and then affected by the Coriolis effect and the land masses which deflect the ocean currents.
Due to differential heating, density differences occur in water, resulting in chilled polar water sinking, spreading towards the Equator and displacing the less dense, warmer water upwards.