The Saharan Zone makes up the northern two-thirds of the country. Its southern boundary corresponds to the isohyet (a line on the earth's surface along which the rainfall is the same) that represents annual precipitation of 150 millimeters. Rain usually falls during the hivernage, which lasts from July to September. Often, isolated storms drop large amounts of water in short periods of time. A year, or even several years, may pass without any rain in some locations.

Diurnal variations in temperature in the Saharan Zone may be extreme, although annual variations are minimal. During December and January, temperatures range from an early morning low of 0° C to a midafternoon high of 38° C. During May, June, and July, temperatures range from 16° C in the morning to more than 49° C by afternoon. Throughout the year, the harmattan often causes blinding sandstorms.

The administrative regions (formerly called cercles) of Tiris Zemmour in the north, Adrar in the center, and northern Hodh ech Chargui in the east, which make up most of the Saharan Zone, are vast empty stretches of dunes alternating with granite outcroppings. After a rain, or in the presence of a well, these outcroppings may support vegetation. In the populated Adrar and Tagant plateaus, springs and wells provide water for pasturage and some agriculture. In the western portion of the Saharan Zone, extending toward Nouakchott, rows of sand dunes are aligned from northeast to southwest in ridges from two to twenty kilometers wide. Between these ridges are depressions filled with limestone and clayey sand capable of supporting vegetation after a rain. Dunes in the far north shift with the wind more than those in the south.

The Saharan Zone has little vegetation. Some mountainous areas with a water source support small-leafed and spiny plants and scrub grasses suitable for camels. Because seeds of desert plants can remain dormant for many years, dunes often sprout sparse vegetation after a rain. In depressions between dunes, where the water is nearer the surface, some flora--including acacias, soapberry trees, capers, and swallowwort--may be found. Saline areas have a particular kind of vegetation, mainly chenopods, which are adapted to high salt concentrations in the soil. Cultivation is limited to oases, where date palms are used to shade other crops from the sun.

Data as of June 1988