The African Sahel, shown in , is a vast dryland area lying between the Sahara desert to the north and more humid savannah to the south. The timing and amount of annual rainfall in the Sahel fluctuates wildly from year to year, with the amount varying between 150 and 600 mm (or about 6 and 24 inches per year; in some years, Sahelian rainfall is similar to what the US desert city of Tucson, Arizona averages, while in other years it is several inches more than San Francisco usually receive). Sahelian rainfall comes only during the 3–5 month period in the summer and early fall, when the intertropical convergence zone is at its northernmost, bringing a low-level southwesterly flow of monsoonal winds that interact with low-level northeasterly winds (Landsea and Gray 1992, 435).