Atmospheric Circulation

Topic 2: Atmospheric Circulation You have seen that there is surplus energy at the equator and a deficiency at the poles. As the poles do not get progressively warmer or colder, a redistribution of world heat energy must occur. To create a balance there is constant energy transfer between the equator and the poles. On Earth, the atmosphere and the oceans act to transfer energy from one area to another.

Atmospheric Circulation This however is not a straightforward journey from the equator to the poles. This is because air moves from high to low pressure. The equator is an area of low pressure and the polar areas are high pressure. This is why the air moves in cells.

Atmospheric Circulation The diagram below shows a simple three-cell model of atmospheric circulation. This is also called Ferrel’s Model.

Atmospheric Circulation

1. Energy from the sun heats the air at the equator 2. The air rises 3. Because of the movement of the Earth it is pushed towards the poles 4. Because of the higher latitudes, the air cools and sinks to earth

Atmospheric Circulation At this point the air can be drawn in one of two directions towards a low pressure zone either southwards to the equatorial low (5) creating the TRADE WINDS, or northwards to the polar front creating the WESTERLIES (6).

Atmospheric Circulation 5. Air can be drawn back to the equatorial low pressure. This completes the Hadley Cell. 6. Some air moves polewards 7. Here it meets polar air and forms the polar front at D. This completes the Ferrell Cell

Atmospheric Circulation The air is lighter and forced to rise.

Atmospheric Circulation 8. Some is diverted to the high pressure zone at (4) 9. And some to the polar high pressure creating the POLAR EASTERLIES.

Ferrel Cell Polar Cell Hadley Cell Polar Cell Ferrel Cell Hadley Cell The Three Cells Together

The Hadley Cell (A) exists because of the solar energy heating the air at the equator causing the equatorial low pressure which draws in air from the sub-tropical high pressure. The Ferrel Cell (B) feeds warm air to the low latitudes and draws cold air back. The Polar Cell (C) is caused by cold dense air sinking at the poles forming an area of high pressure. Where there is an area of convergence there are high winds.

Why do the cells occur where they do? Wind always moves from HIGH to LOW pressure. There are two main areas of high pressure in the world – the tropics and the polar highs. There are two main areas of low pressure in the world – the equatorial low and the polar front. Air moves between these four areas.

Why do the cells occur where they do? Low pressure occurs where the air is forced to rise upwards. At the equator the air rises as it is heated by the sun. At the poles this is because the warmer air from the lower latitudes is pushed upwards. High pressure occurs where air falls because it has cooled because it has moved away from the equator.

World Pressure and Wind Patterns – The Coriolis Effect Mid latitude low pressure Equatorial low pressure Sub-tropical high pressure Polar high pressure

World Pressure and Wind Patterns – The Coriolis Effect The movement of air produces wind. These wind patterns show how the air moves. It always moves from high to low pressure. As the Earth rotates, other factors kick into force. One of these is the Coriolis force. As the Earth spins, winds are deflected – in the northern hemisphere to the right and in the southern hemisphere to the left.

Wind Patterns

World Pressure and Wind Patterns – The Coriolis Effect Another movement of the Earth also affects how the air circulates around the globe. The Earth does not sit in line with the sun all year – as we have already seen, it tilts. In June/July, the northern hemisphere tilts towards the sun so receives most of the sun’s energy – in December the southern hemisphere receives most of the energy. This means that the equatorial low pressure moves up and down keeping in line with the maximum insolation – the thermal equator.

Tilt Of The Earth