The distribution of atmospheric pressure across the latitudes is termed global horizontal distribution of pressure. Its main feature is its zonal character known as pressure belts. On the earth’s surface, there are seven pressure belts. They are the Equatorial Low, the two Sub-Tropical highs, the two Sub-polar lows, and the two Polar highs. Except the Equatorial low. the others form matching pairs in the Northern and Southern Hemispheres. There is a pattern of alternate high and low pressure belts over the earth. This is due to the spherical shape of the earth—different parts of the earth are heated unequally. The Equatorial region receives great amount of heat throughout the year. Warm air being light, the air at the Equator rises, creating a low pressure. At the poles the cold heavy air causes high pressure to be created/formed. It is also due to the rotation of the earth. In the Sub-Polar region around latitudes 60° to 65° North and South of the Equator, the rotation of the earth pushes up the bulk of the air towards the Equator, creating a low pressure belt in this region.
(i) Equatorial Low Pressure Belts
This low pressure belt extends from 0 to 5° North and South of Equator. Due to the vertical rays of the sun here, there is intense heating. The air therefore, expands and rises as convection current causing a low pressure to develop here. This low pressure belt is also called as doldrums, because it is a zone of total calm without any breeze.
(ii) Sub-Tropical High Pressure Belts
At about 30°North and South of Equator lies the area where the ascending equatorial air currents descend. This area is thus an area of high pressure. It is also called as the Horse latitude. Winds always blow from high pressure to low pressure. So the winds from sub tropical region blow towards Equator as Trade winds and another wind blows towards Sub-Polar Low-Pressure as Westerlies.
(iii) Circum-Polar Low Pressure Belts
These belts located between 60° and 70° in each hemisphere are known as Circum-Polar Low Pressure Belts. In the Sub-Tropical region the descending air gets divided into two parts. One part blows towards the Equatorial Low Pressure Belt. The other part blows towards the Circum- Polar Low Pressure Belt. This zone is marked by ascent of warm Sub-Tropical air over cold polar air blowing from poles. Due to earth’s rotation, the winds surrounding the Polar region blow towards the Equator. Centrifugal forces operating in this region create the low pressure belt appropriately called Circum-Polar Low Pressure Belt. This region is marked by violent storms in winter.
(iv) Polar High Pressure Areas
At the North and South Poles, between 70° to 90° North and South, the temperatures are always extremely low. The cold descending air gives rise to high pressures over the Poles. These areas of Polar high pressure are known as the Polar Highs. These regions are characterised by permanent Ice Caps.
SHIFTING OF PRESSURE BELTS
If the earth had not been inclined towards the sun, the pressure belts, as described above, would have been as they are. But it is not so, because the earth is inclined 23 1/2° towards the sun. On account of this inclination, differences in heating of the continents, oceans and pressure conditions in January and July vary greatly. January represents winter season and July, summer season in the Northern Hemisphere. Opposite conditions prevail in the Southern Hemisphere. When the sun is overhead on the Tropic of Cancer (21 June) the pressure belts shift 5° northward and when it shines vertically overhead on Tropic of Capricorn (22 December), they shift 5° southward from their original position. The shifting of the pressure belts cause seasonal changes in the climate, especially between latitudes 30° and 40° in both hemispheres. In this region the Mediterranean type of climate is experienced because of shifting of permanent belts southwards and northwards with the overhead position of the sun. During winters Westerlies prevail and cause rain. During summers dry Trade Winds blow offshore and are unable to give rainfall in these regions. When the sun shines vertically over the Equator on 21st March and 23rd September (the Equinoxes), the pressure belts remain balanced in both the hemispheres.