field and fumaroles on Tarso Voon, Yi Yerra near Emi Koussi and Pic Toussidé. Carbonate deposits in the Trou au Natron and Era Kohor craters are also representative of more recent volcanic activity. The study of fluvial terraces has revealed coarse sand and gravel alternating with terraces of silt, clay and fine sand. This alternation highlights repeated changes in the dominant fluvial or wind patterns in the valleys of Tibesti during the Quaternary Period. The phases of erosion and sendimentation are indicative of the climate alternating between dry and wet conditions, the latter of which fostered vegetation in the Tibesti that was likely significantly denser than that which exists today. Furthermore, the discovery of calcified charophyta (particularly of the family Characeae) and gastropod fossils in Trou au Natron indicates the presence of a lake at least deep during the Late Pleistocene. These phenomena are associated with various changes in climate, most notably during the last glacial maximum, which increased precipitation and reduced evaporation due to lower temperatures. In fact, the Tibesti supplied a considerable amount of water to the Paleolake Chad until the 5th millennium BC. Climate The Tibesti climate is substantially less dry than that of the surrounding Sahara Desert, but rainfall events are highly variable from year to year. 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 humid monsoonal air. Normally, the ITCZ repels the Harmattan, a dry trade wind that blows west or southwest from the Sahara Desert, and brings rainfall to southern Tibesti. However, sometimes the front retires early, before reaching the Tibesti, leaving its southern portion dry. In the arid northern Tibesti, where the monsoon has little influence, storms are caused by episodic Sahara-Sudanese weather systems. For example, between 1957 and 1968, Bardaï, on the northern flank of the range, saw an average of of precipitation annually, yet some years were completely dry while others saw of rainfall. In general, the range receives less than of rainfall per year. However, precipitation increases with altitude; for example Trou au Natron receives annually. When the rainfall coincides with low temperatures, it can fall as snow. This occurs, on average, once every seven years. The average monthly maximum temperature is in the central Tibesti Mountains, while the average monthly minimum is . Lows of