Mali's climate and vegetation vary from north to south.
Northern Mali is arid and "deserted" of vegetation. It is a true desert, a part of the Sahara. Few people live there. Southern Mali is wetter, and natural vegetation is increasingly abundant. The short grasses and shrubs that mark the Sahel give way to the tall grasses of the savannah further south. It is in the southern part of the country, and along the course of the Niger, that most of Mali's people live.
Mali's climate is marked by distinct wet and dry seasons.
Winters are dry. Summers are rainy. Rains arrive first, and in greater amounts, in the south, and later in much smaller amounts in the north. However, rainfall is not very dependable. In some years, there is enough; in others, there is not. Also, when it does rain, it often comes in torrential downpours, which erode the land and leach out nutrients from the soil. In addition, rainfall is spotty--heavy in one area, light only a few miles away. Thus, one village may have abundant crops, while a neighboring village is still waiting for its first rain.
During the dry winter season, days are generally in the mid-70s F. At night, however, it gets extremely cold. The lack of clouds allows the heat that has built up during the day to escape from the earth's surface. This is called radiation cooling. During the April to June dry season, daytime temperatures become much hotter--more than 90°F--as hot, dry winds blow from the northeast. These winds are known as the harmattan. They are followed by the wet season, when daytime temperatures become slightly cooler--in the mid-80sF.