GENERAL CONDITIONS

The main climatic features are the joint result of geographical and aerological factors. The former is expressed by the latitude which gives the territory tropical characteristics, and by the position of West African Finistere which determines different climatic conditions in the coastal region and in the interior. The second one is expressed by the alternation of three flows on the country, whose movements are facilitated by the flatness of the relief.

The first flow is represented by the maritime trade wind coming from the Azores high, from north to northeast. The maritime trade winds are constantly humid, cool and even cold in winter, and marked by a low diurnal thermal amplitude. Its domain is a coastal fringe that diminishes in the south with the rise of the monsoon, but which is maintained for almost the whole year in the north of Cape Verde.

The Harmattan, which is predominantly easterly and is the final branch of the Sahelian continental trade wind, is characterized by a great drought due to its long continental course, and by very marked thermal amplitudes; cool or cold at night, it is hot to torrid during the day. It often carries fine sand and dust particles in suspension, which constitute the “haze”.

The third flow, the monsoon, comes from the trade winds coming from the St. Helena high in the South Atlantic. It benefits from a very long maritime route which makes it particularly humid. It enters the country in the summer period in a southeast-northwest direction and dries out relatively as it penetrates the interior. It is marked by a small thermal amplitude, but with temperatures generally higher than those of the maritime trade winds.

PRECIPITATION

The climatic year is divided into two main seasons by the rainfall criterion. The so-called dry season is really dry only in the interior; whereas on the coast, which benefits from high relative humidity, the season is precisely rainless.

During the dry season, heug rains may occur, associated with episodic invasions of air from temperate regions. These rains are generally low or even insignificant, but exceptionally they can reach high values.

The rainy season or wintering season begins in southeastern Senegal in April with the arrival of the monsoon which gradually invades the country. Rainfall increases slowly at first until August when it peaks; in September the decrease is marked, but then it is very sudden in October. Two phenomena cause precipitation over the country; on the one hand, squall lines and on the other hand, the active part of the meteorological equator, marked by the ascendancy of humid air that cools at altitude and condenses into rain.

The squall lines, improperly called tornadoes, sweep across the territory from east to west (which means that the rain comes from the east) and gradually weaken as they reach the coast; in Dakar, for example, only half of the disturbances reach the coast, having passed through Tambacounda. Rainfall from squall lines is mostly thundery with gusty winds, thunder and lightning. They mark the beginning and the end of the rainy season in the south of the country, but they are the main source of rainfall for almost the entire territory.

Generally speaking, rainfall decreases from south to north: Ziguinchor records 1,250 mm of rain per year, Kaolack 610 mm, while Linguère receives an average of only 414 mm and Podor 220 mm. The number of rainy months varies according to the latitude, but also according to the adopted threshold. If we take as a basis the monthly precipitation above 10 mm (which is very low), the northwest of the country has four rainy months, the south six. With a base of 50 mm per month, the number of rainy months increases to two and five. However, on the basis of 100 mm, the north does not even have a rainy month, while the south still has five months in which rainfall is greater than 100 mm and often much higher, with Ziguinchor, for example, recording 424 mm in August. This latitudinal differentiation confirms the random nature of rainfall in the northern half of the country.

Finally, the climate of Senegal, like that of all Sahelo-Sudanian countries, is characterized by a great variability in rainfall from one year to the next, a variability that is all the more formidable because the annual average is lower; the lower the annual total, the more uncertain and irregular the rainfall and the more serious the deficit. In Ziguinchor, the average of 1,250 mm is the result of rainfall ranging from about 900 mm to slightly more than 1,400 mm from one year to the next; in Linguère, the average of 414 mm covers rainfall ranging from more than 850 mm in an exceptionally rainy year to less than 200 mm in a dry year. This means that the climatic insecurity that weighs on the northern half of the country is not only the result of low rainfall and the shortness of the rainy season; it is above all the result of the inter-annual irregularity of the rains. The drought that has periodically hit the country since 1968 has underlined the seriousness of this situation by its dramatic consequences on the ecological balance and all human activities in the regions located north of the Saloum. The abundance of exceptional rains in 1999 compared to those of the last few decades has given hope to the rural world, which is hoping for a lasting return of good rainfall.

From Marcel Roux and Pascal Sagna, Atlas du Sénégal, Jeune Afrique edition, 2000.