The nation consists of a large plateau rising gradually from sea level to almost 500 meters altitude in the north. Vegetation changes from lagoon and semitropical growth in the south to savanna grassland and scrub in the north . Mountain ranges extend along the western border and a few peaks dot the northeast corner. Four major river systems flow southward forming parallel drainage basins. The climate is generally warm and humid and is, overall, transitional from equatorial to tropical.
Seasons are more clearly distinguishable by rainfall and wind direction than by temperature.
Continental and maritime air masses, following the apparent movement of the sun from north to south, determine the cycle of the seasons that is associated with heat and cold farther from the equator. Ahead of it, a low pressure belt, or intertropical front, brings warm air, rain, and prevailing winds from the southwest. As the solar cycle reverses at midyear, the continental air mass moves southward over the nation, permitting the dry northeast harmattan to dominate. Surface winds are gentle, seldom exceeding fifteen to twenty kilometers per hour.