ZIMSEC O Level Geography Notes: Pressure
- Air has weight and therefore exerts pressure, called atmospheric pressure onto the earth’s surface.
- The pressure is not the same for all regions nor is it the same for any one region all the time that is pressure is higher for one part of the year than other times of the year.
- Atmospheric pressure is affected by: altitude, temperature and the earth’s rotation.
- Altitude refers to height above sea level.
- Air pressure is higher at sea level than it is at the top of a mountain.
- This is because at sea level air has to support greater weight of air than air at the top of the mountain.
- The molecules of air at sea level push outwards with a force equal to that exerted by the air above it whereas air at the top of a mountain pushes outwards with much less force because the weight of the air bove it is less.
- This explains why air pressure increases when air descends.
- When air descends its volume decreases but the number of molecules remain the same in number resulting in more molecules occupying a smaller volume.
- Conversely when air rises its volume increases and its outward force is spread over a larger area thus its pressure decreases.
- The temperature of air rises when its pressure rises.
- The temperature of air falls when its pressure falls.
- The pressure of air falls when its temperature rises.
- The pressure of air rises when its temperature falls.
- If only temperature affected pressure there would be a belt of low pressure around the earth at the equator, two belts of high pressure one over both the North and South Poles.
- Because all because altitude and the earth’s rotation also affect pressure the resulting pattern is not as simple as this.
The influence of the earth’s rotation on pressure.
- The rotation of the earth causes the air at the poles to be “thrown” away towards the equator.
- In theory this would result in air piling at the equator creating a region of High pressure at the equator.
- The reality is much more complicated as other factors like temperature also affect pressure.
- Low temperatures at the poles cause the air to contract and high pressure develops as a result.
- High temperatures along the equator cause the air to expand and low pressure, called the doldrum low pressure (the ITCZ),develops.
- Air blowing away from the poles crosses parallels and creates low pressure belts along 60°N and 60°S.
- As air moves in from the poles more air moves in from higher levels to take its place.
- Some of this comes from the rising low pressure air along 60°S and 60°N.
- Air rising at the equator spreads out and moves towards the poles.
- As it does so it crosses parallels that are getting shorter and has to occupy less space resulting in an increase in pressure as it contracts.
- This happens near 30°N and 30°S and the air begins to sink where it builds up sub-tropical high pressure belts called horse latitudes.
- Some of the High pressure air in latitudes 30°N and 30°S moves over the surface towards the equator and some of it towards the poles.
- The air that moves towards the equator replaces the air that rises there.
- The air moving towards the poles reaches latitudes 60°N and 60°S where it replaces the air that rises there.
- Winds blow over the surface blow from high pressure to low pressure areas.
- At high levels blow from low pressure systems to high pressure systems.
- In each hemisphere there are three pressure systems: Polar High Pressure, Temperate Low pressure and Sub-Tropical High pressure and in the midst of both poles is the Equatorial Low pressure.
- This is all assuming the earth’s surface was flat and uniform i.e. if the earth was an isotropic plane.
- See the diagram below for a simplified view of the earth’s major pressure belts.
Actual pressure systems
- The earth’s surfaces is not uniform: there are water and land masses of different shapes and sizes.
- The earth’s is also tilted at an of 66.5 degrees and the earth and land masses are heated and cool at different rates.
Pressure belts and winds
- The earth’s (planetary) wind systems are caused and controlled by the major pressure belts outlined above.
- The pressure patterns and winds are shown in the two diagrams below.
- These patterns in pressure and winds change seasonally according to changes in temperature.
- Due to the rotation of the earth winds are deflected to the right in the northern hemisphere and to the left in the southern hemisphere.
- Planetary winds are sometimes known as prevailing winds because they blow more frequently than most winds.
- Winds are named after the direction which they blow.
- There are 3 major wind systems in each hemisphere:
- In the Northern Hemisphere:
- North East Polar winds: blow from the polar high pressure towards the temperature low pressure in latitude 60°N (temperate low).
- South West Winds: blow from the sub-tropical high pressure regions in latitude 30°N towards the temperate low pressure zone.
- North East Trade Winds: blow from the sub-tropical high pressure towards the doldrums along the equator.
- In the Southern Hemisphere:
- South East Polar winds: blow from the polar high pressure towards the low temperate pressure regions in latitude 60°S.
- North East Winds: blow from the sub-tropical high pressure in latitude 30°S towards the temperate low pressure.
- South East Trade Winds: blow from the sub-tropical high pressure regions towards the doldrums.
Pressure belts in diagrams
Conditions-November to March
- High temperatures occur over central and southern Africa.
- This results in low pressure developing over this part of the continent.
- At the same time temperatures are lower over the South Atlantic and Indian Oceans and pressure over these oceans is relatively high.
- North Africa is much cooler than the rest of Africa and high pressure therefore develops.
- Winds blow out from the tropical continental high pressure arr mass over northern Africa.
- Some of these winds blow towards the low pressure area to the south.
- The winds blow from the north east over West Africa.
- Because they originate from the desert the winds are dry and dusty.
- They are called harmattan winds.
- They are prevented from going further south by winds blowing form the South Atlantic Ocean.
- The North-East Trade Winds affect the east cost of Africa , while the South-East trade winds operate along the South-East coast.
- Both winds make for the low pressure over Central Africa.
- During this period, south-west winds blow across the south-west tip of the continent.
Conditions from May to September.
- High temperatures over northern Africa give rise to the development of a low pressure system.
- High pressure lies over southern Africa and over the Indian Ocean to the east.
- Winds blow from the tropical maritime air mass over the South Atlantic Ocean, towards the continental low pressure air mass over the Sahara.
- These winds start as South-East trade winds but as they move north of the equator they are drawn across the coast of West Africa towards the Sahara low pressure.
- They now blow from the south west and become the south west monsoonal winds.
- They are warm and moist.
- During this period areas South of the Sahara are warm and dry.
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