SEASONAL CHANGES IN PRESSURE ON THE GLOBE

As the greater part of the globe experiences seasonal changes in temperature, it is to be expected that the pressure belts and their associated winds will also be affected.

1) On 21 June, the sun is overhead at the Tropic of Cancer. This is summer in the northern hemisphere. At this time, the Doldrum low-pressure belt swings north.

However, although the sun has moved north through 23½° since 21 March, the Doldrum belt only extends about 5° north.

In this season, intense overheating of the Asian landmass, particularly in the Indian subcontinent, leads to a sharp fall in pressure. This is also true, to a lesser extent, of the south-west United States. These low-pressure areas link up with the equatorial low-pressure area.

2) The high-pressure belt of the Horse Latitudes also moves north, but because of overheating in the continental interiors, it only exists in the northern hemisphere as cells of high pressure centred over the Pacific and Atlantic Oceans.

In the southern hemisphere where it is winter, the Horse Latitudes are occupied by an almost continuous high- pressure zone.

3) The low-pressure areas of the depression zone in the northern hemisphere have also swung north and are now in the vicinity of the Arctic Circle.

Atmospheric Pressure in January

1) On 21 December, the sun is overhead at the Tropic of Capricorn. This is the southern hemisphere’s summer. At this time, the Doldrum low-pressure belt swings south, lagging behind the overhead sun. Because of the smaller extent of the southern continents, the low-pressure belt is not well developed. However, increased insolation has produced a significant fall in pressure over Africa and Australia.

2) In the Horse Latitudes of the southern hemisphere, the increased insolation has weakened the sub-tropical high-pressure belt so that it is no longer continuous. It has moved further south and only individual cells of high pressure remain. These are over the South Pacific and South Atlantic Oceans.