From Skeletons on the Zahara: A True Story of Survival, by Dean King (Little, Brown, 2004), Kindle pp. 94-95:
What they looked out on, in 1815, had never been scientifically explored and was almost too mind-boggling to imagine. They faced the western edge of the world’s largest desert. Occupying a third of Africa, it stretches more than three thousand miles east to the Red Sea and twelve hundred miles from the Sahel—the fringe of savanna in the south—to the Atlas Mountains in the north, mountains that snare almost all the moisture traveling down on the northeast winds. Relative-humidity levels, rarely above an abrasive 30 percent, are often as low as a lethal 5 percent, dry enough to kill bacteria and mummify corpses. On the coast, the heat of the Sahara clashes with the cold waters of the Atlantic, often creating heavy fogbanks that envelop the shore, and on many days the irifi, a powerful, searing wind, shrouds the region in a melancholy ocher veil of dust.
The Sahara was not always like this. From 5500 to 2500 B.C., it was relatively fertile, wet and inviting. Up until Roman times, antelope, elephants, rhinoceroses, and giraffes roamed a savanna densely studded with acacia, while crocodiles and hippopotamuses wallowed in lush rivers. Ostriches, gazelles, and antelope still persisted in 1815, but by then the Saharan climate was arguably the most extreme on earth. Its temperature could sizzle at more than 120 degrees Fahrenheit in the shade, the ground temperature soaring 50 degrees higher in the sun; at night, the thermometer could plunge as much as 85 degrees. These conditions, combined with frequent windstorms and less than five inches of average annual rainfall, made sustained life virtually impossible in many parts. As flora and fauna died off or adapted, the land itself deteriorated. While only about a tenth of the Sahara is covered in barren sand dunes, or erg, almost equally formidable are its stepped plains of wind-stripped rock covered in boulders, stones, and dust—the lower elevations generally known as reg and higher ones as hammada.