In northernmost Sudan, northerly winds prevail for most of the year, and rainfall is rare. To the south of this the seasons are characterized by the north-south oscillation of the boundary between moist southerly air and dry northerly air. In winter the north winds of the tropical air mass blow across Sudan. These winds are relatively cool and dry and usually bring no rain. Sometime around May, the moist southerly air of the southern maritime air mass moves northward across the country. Because of this, central and southern Sudan have rainy seasons, the total lengths of which vary according to their latitude.