The Tibesti climate is substantially less dry than that of the surrounding Sahara Desert. Rainfall events are more intense and more frequent, but they are also highly variable from year to year. Indeed, the Tibesti can see seven to eight years pass between storms. In the south of the range, this variation is largely due to oscillations of the Intertropical Convergence Zone (ITCZ), which steadily moves northward toward Northern Chad from November until August, accompanied by the humid monsoonal air. Normally, the ITCZ repels the Harmattan, a dry trade wind that blows south-west from the Sahara Desert, and brings rainfall to southern Tibesti. However, sometimes the front retires early, before reaching the Tibesti, resulting one or more years of drought. In the northern Tibesti, where the monsoon has little influence, storms are caused by Sahara-Sudanese weather systems. For example, between 1957 and 1968, Bardaï saw an average of of precipitation annually, although some years were dry while others saw of rainfall. In general, the range receives a little less than of rainfall per year. However, at elevations above , average precipitation exceeds per year. When the rainfall coincides with low temperatures, it can fall as snow. This occurs, on average, once every seven years.