Southern and western Mali have a Sudanese climate with a short rainy season from June to September. Rainfall averages 140 cm (55 in) at Sikasso in the far south. To the north is the Sahelian zone, a semiarid region along the southern border of the Sahara. At Gao, in Mali's northeast Sahel, rainfall is about 23 cm (9 in) a year. Actual year-to-year rainfall, however, is extremely erratic. In the Sahelian zone there are considerable variations of temperature, especially in April, May, and June, the period of maximum heat, and in December, when the hot, dry harmattan blows. Continuing north, one gradually enters into a Saharan climate, marked by the virtual absence of rain and an extremely dry atmosphere. Over 40% of the country is, in fact, desert, and unsuitable for agriculture. The year is divided into three main seasons varying in length according to latitude: October–January, a cool and dry season; February–May, a hot and dry season; and June–September, a season of rains characterized by lower temperatures and an increase in humidity. Between 1968 and 1974, Mali, with neighboring Sahel states, experienced the worst drought in 60 years. Drought returned during 1982–85, and there is continuing concern over the southward advance of the desert.