Three great deserts isolate Najd from north, east, and south as the Red Sea escarpment does from the west. In the north, the An Nafud--sometimes called the Great Nafud because An Nafud is the term for desert--covers about 55,000 square kilometers at an elevation of about 1,000 meters. Longitudinal dunes--scores of kilometers in length and as much as ninety meters high, and separated by valleys as much as sixteen kilometers wide--characterize the An Nafud. Iron oxide gives the sand a red tint, particularly when the sun is low. Within the area are several watering places, and winter rains bring up short-lived but succulent grasses that permit nomadic herding during the winter and spring.

Stretching more than 125 kilometers south from the An Nafud in a narrow arc is the Ad Dahna, a narrow band of sand mountains also called the river of sand. Like the An Nafud, its sand tends to be reddish, particularly in the north, where it shares with the An Nafud the longitudinal structure of sand dunes. The Ad Dahna also furnishes the beduin with winter and spring pasture, although water is scarcer than in the An Nafud.

The southern portion of the Ad Dahna curves westward following the arc of the Jabal Tuwayq. At its southern end, it merges with the Rub al Khali, one of the truly forbidding sand deserts in the world and, until the 1950s, one of the least explored. The topography of this huge area, covering more than 550,000 square kilometers, is varied. In the west, the elevation is about 600 meters, and the sand is fine and soft in the east, the elevation drops to about 180 meters, and much of the surface is covered by relatively stable sand sheets and salt flats. In places, particularly in the east, longitudinal sand dunes prevail elsewhere sand mountains as much as 300 meters in height form complex patterns. Most of the area is totally waterless and uninhabited except for a few wandering beduin tribes.

Data as of December 1992