Seasons & Nature diary Following the rhythms of the ITCZ

107mm in 24 hours. That is the enormous quantity of water that fell on Cayenne on 17 January last. That gives an idea of how appropriate the adjective ‘humid’ is when describing the tropical climate of French, where average annual rainfall is between 1700mm in the North East and 3800mm in the region of Cacao-Régina. As a point of comparison, the middle of Brittany has about 1400mm per year.But French Guiana is not a country of monsoons. Rather it is a place of alternating seasons where the key parameter is not temperature or wind but the amount of rainfall. As a first step to understanding the rhythm of the seasons in this part of the world, it is necessary first to explain a meteorological phenomenon well-known to climate specialists – the ITCZ, or Inter-Tropical Convergence Zone. In the North Atlantic the north-easterly tradewinds are caused by the Azores High. To the south the equally well-known South Atlantic High causes the south-easterly trade winds. These two winds meet in a cloudy, humid depression commonly called the ‘meteorological equator’. This gives rise to the ITZC. The ITZC more or less follows the geographical equator, though to be precise this zone of low pressure is found at about latitude 5° north. It is the way the ITZC shifts around this position which determines the climate in French Guyana. This zone tends to follow the sun, drifting north in summer and south in winter. In this way the movement of the ITZC affects the life of the ...

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