The Sahara is a region which has no season of regular, annual and general rains. The desert, its greatest length extending east and west, lies roughly between 16° and 29° north latitude, which means that it is traversed through the middle by the tropic line. Aside from the few isolated points which attain an altitude of some 10,000 feet, its climate is practically uniform throughout. The most significant factor in the desert climate is the rainfall, since it is the insufficiency of rain that brings the desert into being. The evaporation in such an atmosphere must obviously be very great, especially as the Sahara is one of the regions of the world which is subject to the most intense heat, the thermometer reaching maxima of 122° Fahrenheit with a general average only a little less than this. Perhaps the fundamental characteristic of the desert climate may be considered the dryness of the air.