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At the Equator, there is a discontinuous belt of low pressure. Due to the vertical rays of the Sun, the Earth receives maximum heat in this region. As a result, the air in this area is always hot. The hot air readily expands, rises up and flows towards the Poles, creating a low pressure in the region. This equatorial low pressure belt is located on either side of the geographical equator in a zone that extends from 0º to 5°N and 5°S latitude. These are regions of calm with light variable winds and quiet, stable weather conditions. However, though it is a region of calm, local thunderstorms occur frequently.
This belt of Equatorial low pressure of frequent calm conditions is termed as ‘Doldrums’. As there is no horizontal movement of wind, the sailors in the old days found them stranded in this calm region. This led to the term ‘being doldrums’, which means to ‘stagnate’.
The Equatorial low pressure belt is always slightly north of the geographical Equator, due to the greater amount of land in the Northern Hemisphere and largely coincides with the belt of the higher temperature.
It is not also stationary, since there is a seasonal shift of this belt with the northward (summer solstice) and southward (winter solstice) migration of the sun. Sometimes during northern summer, this belt extends up to 20°N in Africa and to the north of tropic of Cancer in Asia. On the other hand, during southern summer this low pressure belt shifts to 10° to 20°S latitude.
