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Global Atmospheric Circulation And Hazards Notes

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PHYSICAL GEOGRAPHY REVISION Theme 1: Global Atmospheric Circulation Global Heating

There is an uneven distribution of heat across the Earth

WHY?
Incoming radiation received by the Earth SUN Prime source of energy on earth and life (through photosynthesis). Affects climate- atmospheric motion (winds), ocean currents, type and amount of precipitation, temperatures.

Incoming, short wave, solar radiation. The amount of insolation received by the earth depends upon:

LENGTH OF DAY AND NIGHT Due to the earth being tilted at

23.5˚ poleward of 66.5˚ N and S, there are several months with no insolation.

THE SOLAR CONSTANT

DISTANCE FROM THE SUN

Varies according to sunspot activity but is relatively constant, affects long term climate rather than short term weather.

The eccentric orbit of the earth around the sun can cause 6%
difference in solar constant (Milankovitch)

ALTITUDE OF THE SUN IN THE SKY Each bundle has twice the area to heat up at 60˚ N than on the equator, therefore temperatures are lower. Also less atmosphere to pass through at equator therefore less heat absorbed/reflected.

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The solar energy cascade

Incoming radiation 25% absorbed by atmosphere/stratosphere 23% reflected by clouds 3% absorbed by clouds 4% reflected back by albedo surfaces 21% scatters- reaches earth as diffuse radiation 24% directly reaches earth's surface

100%

Global factors affecting insolation Factors which influence the amount of insulation received by any point, and therefore its radiation balance and heat budget, vary considerably over time and space. Long Term Factors
⇒ Height above sea level (1˚ drop in temp for every 100m above sea level)

⇒ Altitude of the sun (Lower altitude  less atmosphere to pass through and areas to heat 
warmer)
⇒ Land and Sea
⇒ Prevailing winds
⇒ Ocean currents

Heat Budget There is a positive heat balance within the tropics and a negative heat balance both at high latitudes and altitudes, so two major transfers of heat take place to prevent the tropical areas from overheating.

 Horizontal Heat Transfers
- Heat that is transferred away from the tropics
- Winds are responsible for 80% of this heat transfer and ocean currents for the rest
 Vertical Heat Transfers
- So that the earth's surface does not hotter and the atmosphere colder
- Heat must be transferred vertically
- Achieved through radiation, conduction, convection and transfer of latent heat

Tricellular

Model

Explanation of the Tricellular Model The Hadley Cell
 The meeting of the trade winds in the equatorial region forms the ITCZ
 The trade winds pick up latent heat across the warm tropical ocean and are forced to rise by violent convection currents
 The unstable warm moist air is rapidly cooled adiabatically to produce: cumulonimbus clouds, thunderstorms and low pressure characteristics
 The upward currents form the 'powerhouse of the general global circulation'
 They turn latent heat into potential energy
 At ground level, ITCZ = gentle variable winds (doldrums)
 As rising sir cools to the temperature of the surrounding environment, uplift ceases
 Begins to move away from equator
 Further cooling increases density and diversion by coriolis forces air to slow and descend The Ferrel Cell
 Remaining air is diverted polewards
 Forms westerly winds which collect moisture when crossing sea areas
 These warm winds meet cold artic air at polar front (60˚ N) and are uplifted to form are of low pressure
 Unstable conditions= mid-latitude depressions and rainfall Depressions are another method by which surplus heat is transferred. While some of this rising air eventually returns to the tropics, some travels polewards…
Polar Cell… where, having lost its heat, it descends to form a stable area of high pressure. Air returning to the polar front does so as the cold easterlies.

Climate- KANO When the ITCZ is to the south of the equator, the north-east winds prevail over Nigeria, producing the dry-season conditions. When the ITCZ moves into the Northern Hemisphere, the south westerly wind prevails as far inland to bring rain fall during the wet season.

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Temperatures are high throughout the year

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Short, slightly cooler season when the sun is overhead at the tropic

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The annual range of temperature is slightly greater (than for equatorial climates) due to the sun's slightly reduced angle in the sky for most parts of the year, the greater distance from the sea, and the less complete cloud and vegetation cover.

 In Kano the annual range is 8*C.
 Temperatures may drop slightly at the onset of the rainy season which in Kano is usually in May.  For most of the year, cloud cover is limited, allowing diurnal temperatures to exceed 25*C. The main characteristic of this climate is the alternating wet and dry seasons.

• The wet season occurs when the sun moves overhead bringing with it the thermal equator, the ITCZ, and the equatorial low pressure belt.

• Heavy convectional rainstorms can give 80% of the annual rainfall in four or five months.

• The dry season corresponds with the moving away of the ITCZ, leaving the area with the strong steady trade winds.

• The trades are dry because they are warming as they blow towards the Equator and they will have shed any moisture on distant east coasts.

• The Northern Nigeria the trade winds come from the source region of the arid Sahara desert so are very dry. (Called the Harmattan/Doctor)

• Places nearer to the desert margins tend to experience dry, stable conditions (the sub-tropical high pressure) caused by the migration of the descending limb of the Hadley Cell.

• Humidity is also low in this season.

Sometimes in West Africa the path of the easterly flowing equatorial jet stream, which is associated with the monsoon of Asia, moves further south causing a 'blocking' high pressure which prevents the 'summer' rains arriving in northern Nigeria leading to drought conditions.

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