The climatic feature which is common to all our West African colonies is the division of the year into two well-defined seasons, the wet and the dry. Since these colonies are all situated entirely within the tropics, they are of course subject to tropical temperatures ; but temperature is influenced by local conditions such as altitude and nearness to the coast, whereas the division into two seasons is constant throughout the whole of West Africa. As a broad generalization one may say that the length of the wet season and the total rainfall decrease as one goes inland from the coast, and that at the same time the average temperature increases; the highest temperatures and the lowest rainfall are found in the extreme north of Nigeria.

The rainy season occurs during the summer months, from April to September, and during the rains the prevailing wind comes from the south-west and is therefore sometimes spoken of as the “South-west monsoon of the Coast”. West Africa during the summer is situated within the South-east Trade Wind belt, but the Trade Winds are deflected from their course by the heating up of the Sahara and blow from the south-west instead of from the south-east, bringing with them the moistureladen clouds from the ocean which cause the rainfall. After the end of September the rains cease somewhat abruptly and the wind veers round to the north or north-west. This northerly wind, known as the “Harmattan”, is in marked contrast to the south-west wind. It blows straight across the Sahara, and instead of being moist it is intensely dry and is laden with dust particles, which at times are so thick that visibility is reduced to a few hundred yards. As soon as the Harmattan sets in everything becomes desiccated and all plant growth practically ceases. Its effect at first resembles that of an early frost in England. Owing to its extreme dryness, and the amount of evaporation it causes, it is at first a cool wind and the temperature falls rapidly, especially at night; but later on, in February, March, and early April, it becomes hot and scorching; and it is during this period that the highest temperatures are recorded.