Large-scale air circulation

Warm air rises, cold air sinks. We might expect air heated near the equator to rise and spread towards the poles at a high level. As it moves it would cool, finally descending in the polar regions, to displace cold polar air towards the equator.

In real life this 'single cell' circulation cannot be sustained over the long distances between the equator and the poles, within a relatively shallow atmosphere. The circulation breaks up into three cells in each hemisphere. Rising air at the equator descends around latitude 30°, continues towards the pole near the surface until about latitude 60°, then rises before continuing towards the pole in the upper atmosphere.

The return flow reverses this and so forms a double figure-eight circulation.

This simple principle is complicated by other factors. The earth's rotation from west to east causes it to slide along below overlying air.

The earth is a sphere rotating on its axis every 24 hours; the surface near the equator moves at about 1500km/h. Near the poles, where the radius of the earth along a parallel of latitude is much less than near the equator, the rotational speed is correspondingly less.

As surface air moves from polar regions where its easterly speed roughly matches that of the earth, it moves to lower latitudes where the rotational speed is greater. It therefore tends to get left behind: to a person on the surface, the wind appears to blow from the east.

In the middle latitudes, where surface air is going towards the poles, it is moving into regions of lower rotational speeds, and is therefore travelling faster than the new surface. An observer here senses the wind blowing from the west - the so-called westerlies or 'roaring forties.'

Finally, in tropical regions, the surface air is again slowly moving away from the pole and so once again has an easterly direction. But now the north-south variation of latitudinal radius is small, and the easterly component of the wind is small, creating the gentle 'trade winds' which merge into the doldrums at the equator.

The simple north-south air movement is also complicated by uneven distribution of land surfaces.

The other major effect comes from the changing seasons. The earth's axis is tilted at 66 1/2° to the path it traces around the sun during a year. In midsummer the pole in one hemisphere is tilted at 23 1/2° towards the sun, so the sun shines perpendicularly on latitude 23 1/2° in that hemisphere, instead of on the equator. Six months later the other pole is tilted towards the sun, and the sun shines perpendicularly on the equivalent latitude in the other hemisphere.

Therefore the tropical uprising of heated air does not remain at the equator, but swings backwards and forwards on an annual cycle between the Tropic of Cancer and the Tropic of Capricorn. Planetary air circulation likewise moves backwards and forwards across the latitudes, as do the monsoon rains.

In summer the 'roaring forties' are further toward the pole, and in winter they move into middle latitudes. Subtropical regions have wet summers and dry winters, while temperate latitudes have clear skies in summer and wet winters.

In summer, much of tropical Australia is affected by thunderstorms.(photograph by Bob Mossel)