inter-tropical convergence zone

That part of the tropics where the opposing north-east and south-east trade winds converge. It is not a continuous belt, more like a necklace with groups of clouds as the ‘beads’; in places there may be two or more ‘strings’. The zone is narrower over the oceans, and broader over the continents, where other wind systems may be involved; in West Africa the ITCZ is the convergence of the harmattan and Guinea monsoon. Here, pressure is low, humidity is high, and spasmodic rain is associated with shallow depressions.

The ITCZ moves north and south with the seasons; moving more over land, and arriving in the summer in each hemisphere. Its position is affected by the apparent movement of the overhead sun, the relative strengths of the trade winds, and the changing locations of maximum sea-surface temperatures. This means that the movements of the ITCZ are highly unpredictable. If it moves well away from the equator it brings unusually heavy rainfall; this caused floods in Khartoum in August 1988. If it stays close to the equator, droughts occur beyond, as in Ethiopia in 1984. The ITCZ can draw in moist air from the sea, bringing rain, and is more active in mountainous regions. However, in the dry interiors of continents it may not even bring cloud. Over the oceans, the ITCZ is broad, and often loses its identity. Winds are then absent, and such windless regions are known as doldrums.