In winter, air temperatures can be very low despite the heat of the sun and temperatures drop very rapidly either when the sun goes down or when there is movement from sunlight to shade, say in a deep gorge or a cave. Increasing aridity means greater difficulty in water availability . Scientists define the problem in terms of water deficits. The region as a whole and the deep Sahara in particular are very serious water deficit areas. Surface waters are lacking almost everywhere except in the case of the Nile in Egypt and Sudan.
Underground water is scarce and often available only at great depths.
They are, however, rare. Since water is the key to sustaining life in deserts, travelers have always to assume that they must be self-sufficient or navigate from one known water source to another. Isolation is another feature of the Sahara. Travelers' tales tend to make light of the matter, hinting that Bedouin Arabs will emerge from the dunes even in the most obscure corner of the desert. This is probably true of the semi-desert and some inland wadi basins but not a correct assumption on which to build a journey in the greater part of the Sahara.