the Ahaggar and Tassili n'Ajjer highlands, some parts of which reach more than . Surrounding the Ahaggar are sandstone plateaus, cut into deep gorges by ancient rivers, and to the west a desert of pebbles stretches to the Mali frontier. The desert consists of readily distinguishable northern and southern sectors, the northern sector extending southward a little less than half the distance to the Niger and Mali frontiers. The north, less arid than the south, supports most of the few persons who live in the region and contains most of the desert's oases. Sand dunes are the most prominent features of this area's topography, but between the desert areas of the Grand Erg Oriental and the Grand Erg Occidental (Great Western Erg) and extending north to the Atlas Saharien are plateaus, including the Tademaït and a complex limestone structure called the M'zab where the Mozabite Berbers have settled. The southern zone of the Sahara is almost totally arid and is inhabited only by the Tuareg nomads and, recently, by oil camp workers. Barren rock predominates, but in some parts of Ahaggar and Tassili n'Ajjer alluvial deposits permit garden farming. Climate and hydrology Northern Algeria is in the temperate zone and enjoys a mild, Mediterranean climate. It lies within approximately the same latitudes as southern California and has somewhat similar climatic conditions. Its broken topography, however, provides sharp local contrasts in both prevailing temperatures and incidence of rainfall. Year-to-year variations in climatic conditions are also common. This area, the most inhabited in Algeria, is commonly referred to as the Tell. In the Tell, temperatures in summer average between and in winter drop to . Winters are not cold, but the humidity is high and houses are seldom adequately heated. In eastern Algeria, the average temperatures are somewhat lower, and on the steppes of the High Plateaus winter temperatures hover only a few degrees above freezing. A prominent feature of the climate in this region is the sirocco, a dusty, choking south wind blowing off the desert, sometimes at gale force. This wind also occasionally reaches into the coastal Tell. In Algeria only a relatively small corner of the Sahara lies across the Tropic of Cancer in the torrid zone, but even in winter, midday desert temperatures can be very hot. After sunset, however, the clear, dry air permits rapid loss of heat, and the nights are cool to chilly. Enormous daily