Vertical circulation cells

Hadley

At the Equator, temperatures are relatively high. Land and water masses heat up and a sa result these warm the air over them. As the air warms, it expands and becomes less dense. It therefore rises and leaves a low pressure cell behind. As the air rises it cools at the environmental adiabatic lapse rate (approximately 10° per 1000 metres) and becomes less able to hold water vapour. Once the air becomes saturated, any further cooling leads to precipitation. Nearer the ground, the evacuated air leaves a low pressure cell which in turn, draws higher pressure air from the higher pressure cells to the north and south of the Equator (at approximately 30° N and S). As a result, cool air in the higher atmosphere at approximately 30° latitude, descends creating the high pressure cells. As the air descends it warms and becomes more able to hold water vapour. Therefore climates at these latitudes are predominantly dry.

This sets up a large convection current of air moving away from the Equator towards 30° N and S at ground level and air moving towards the Equator in the upper atmosphere. This circulation pattern is known as the Hadley Cells and occur between the Equator and latitudes 30° N and S.

Ferrel

At latitudes 30° N and S, air descends vertically. Once it reaches the ground it is now under increasing pressure Ð hence the predominance of high pressure cells. Some of this air under high pressure will move towards the Equator as part of the Hadley circulation, and some of the air will move away to the higher latitudes of 60° N and S, where the air pressure is predominantly low. This establishes another vertical circulation pattern known as the Ferrel Cell which occur between the latitudes of 30° and 60° north and south of the Equator.

Polar

At the North and South pole, air is exceedingly cold and dense. It therefore descends and we tend to find relatively weak high pressure cells in the lower atmosphere at the polar regions. Again for similar reasons as before, a circulation of global air masses move from the polar regions (high pressure) towards the low pressure regions that are found at latitudes 60° N and S. This cold air mass from the polar regions is more dense than the warmer air masses established at lower latitudes. Therefore at these latitudes (60°) we tend to see cold polar fronts pushing through and forcing up the warmer air, causing frontal lifting which in turn leads to increased precipitation.