The Lake Chad Basin embraces a great range of tropical climates from north to south, although most of these climates tend to be dry. Apart from the far north, most regions are characterized by a cycle of alternating rainy and dry seasons. In any given year, the duration of each season is determined largely by the positions of two great air masses--a maritime mass over the Atlantic Ocean to the southwest and a much drier continental mass. During the rainy season, winds from the southwest push the moister maritime system north over the African continent where it meets and slips under the continental mass along a front called the "intertropical convergence zone". At the height of the rainy season, the front may reach as far as Kanem Prefecture. By the middle of the dry season, the intertropical convergence zone moves south of Chad, taking the rain with it. This weather system contributes to the formation of three major regions of climate and vegetation.
The humid soudanian zone includes the southern prefectures of Mayo-Kebbi, Tandjilé, Logone Occidental, Logone Oriental, Moyen-Chari, and southern Salamat. Between April and October, the rainy season brings between 750 and 1,250 millimeters of precipitation. Temperatures are high throughout the year. Daytime readings in Moundou, the major city in the southwest, range from 27°C in the middle of the cool season in January to about 40°C in the hot months of March, April, and May.
The semiarid sahelian zone, or Sahel, forms a belt about 500 kilometers wide that runs from Lac and Chari-Baguirmi prefectures eastward through Guéra, Ouaddaï, and northern Salamat prefectures to the Sudanese frontier. The climate in this transition zone between the desert and the southern soudanian zone is divided into a rainy season (from June to early September) and a dry period (from October to May). In the northern Sahel, thorny shrubs and acacia trees grow wild, while date palms, cereals, and garden crops are raised in scattered oases. Outside these settlements, nomads tend their flocks during the rainy season, moving southward as forage and surface water disappear with the onset of the dry part of the year. The central Sahel is characterized by drought-resistant grasses and small woods. Rainfall is more abundant there than in the Saharan region. For example, N'Djamena records a maximum annual average rainfall of 580 millimeters, while Ouaddaï Prefecture receives just a bit less. During the hot season, in April and May, maximum temperatures frequently rise above 40°C. In the southern part of the Sahel, rainfall is sufficient to permit crop production on unirrigated land, and millet and sorghum are grown. Agriculture is also common in the marshlands east of Lake Chad and near swamps or wells. Many farmers in the region combine subsistence agriculture with the raising of cattle, sheep, goats, and poultry.
The Saharan region covers roughly the northern third of the country, including Borkou-Ennedi-Tibesti Prefecture along with the northern parts of Kanem, Batha, and Biltine prefectures. Much of this area receives only traces of rain during the entire year; at Faya Largeau, for example, annual rainfall averages less than three centimeters. Scattered small oases and occasional wells provide water for a few date palms or small plots of millet and garden crops. In much of the north, the average daily maximum temperature is about 32° C during January, the coolest month of the year, and about 45° C during May, the hottest month. On occasion, strong winds from the northeast produce violent sandstorms. In northern Biltine Prefecture, a region called the Mortcha plays a major role in animal husbandry. Dry for nine months of the year, it receives 350 millimeters or more of rain, mostly during July and August. A carpet of green springs from the desert during this brief wet season, attracting herders from throughout the region who come to pasture their cattle and camels. Because very few wells and springs have water throughout the year, the herders leave with the end of the rains, turning over the land to the antelopes, gazelles, and ostriches that can survive with little groundwater.
The soudanian region is predominantly savanna, or plains covered with a mixture of tropical or subtropical grasses and woodlands. The growth is lush during the rainy season but turns brown and dormant during the five-month dry season between November and March. Over a large part of the region, however, natural vegetation has yielded to agriculture.
You can read more regarding this subject on the following websites:
Geography of Chad - Wikipedia
Chad Country Studies index
Country Studies main page