The climate of West Africa is largely determined by two large air masses: a warm, dry, dusty air mass formed over the Sahara and a moist tropical air mass formed over the South Atlantic. The alternation of these air masses causes dry seasons and wet seasons in much of the region. Another aspect of these weather systems is the trade winds. As Akin Mabogunje explains, they were a major factor in delaying European exploration of the West African coastal region:

These winds in the north tend to blow from the north-east. In the south their initial direction is from the south-east, but on crossing the Equator they are forced to veer to the right and so become south-west in direction. Highly regular in their flow, they . . . have played very little part in the history of West Africa compared with their role in East Africa. There...