By applying the cluster analysis (CA) method on the observation dataset produced by the climate Research Unit (CRU) for the base period 1901-1940, four regions spread in five areas are detected in West Africa. This CA method that has been applied to geophysical research during these two last decades is among the methods more frequently used for the rainfall classification. Clusters have been defined as relative constellations of contiguous points in space. For the four regions identified in West Africa, the first one, R1, an arid land, covers essentially the north of 16.75°N from west to east of the study zone. The second region, R2, a semi-arid land with a sahelian climate, less warm than the dry climate of R1, is centered on the Chad, with almost regular extension to the west towards Mauritania, and to the east, including the north of the Central African Republic. The region three, R3, a wet land, is centered on the Ivory Coast and covers totally the Liberia, the south part of Ghana, Togo, Benin and the south-west of Nigeria. The fourth region, R4 corresponding to the wet equatorial forest, covers a part of Senegal and a part of the Central Africa towards the equator. When this zonation is superposed with the regionalization for the African atlas (2000) that divides the African continent into six climate types distributed symmetrically around the equator as equatorial, humid tropical, dry tropical, sahelian, desert and Mediterranean climate types, it appears that except the Mediterranean, our study area contains five of these six climate types. One of our zones contains the two tropical climate types. In these regions, trends have a magnitude of up to 2 K per century, with a decreasing of the precipitation since the year 1970. To observe and evaluate the spatiotemporal variations of the climatic regions, a segment of 15 years is used, going from the base period P0, to obtain P1, P2, P3 and P4 periods corresponding respectively to 1916–1955; 1931–1970; 1946–1985 and 1961–2000. The 15-year segment mean filters out inter-annual variability, which is pronounced in the original time series. The application of a segment of 15 years with overlap going from 1901 to 1940 (P0), and 1961 to 1998 (P4) throughout the periods P1, P2 and P3, shows important spatiotemporal modifications of the climatic regions. From P0 to P4, the surface of arid and semiarid lands doubles while wetlands are reduced to half. The progression of arid and semiarid land implied an impoverishment soil and the decrease of surface of lands under cultivation