Some of these dunes are 30-40 feet high. Not as impressive as other parts of the desert, but plenty high when you're walking up them. The sand was softer than any I've seen in my life, and am afraid I've damaged my camera by using it uncovered.

Taken from: http://www.calacademy.org/exhibits/africa/exhibit/sahara/

Its parched, forbidding landscape took shape over thousands of years, but even today, the Sahara is constantly changing. Why is this desert so important to Africans? How do people survive its harsh, dry climate? And is the Sahara getting bigger, or smaller?

By studying satellite photos, some scientists have come to believe that the Sahara regularly shrinks and grows. In the early 1980s, the Sahara's southern edge expanded into the Sahel, a dry band that separates the desert from the savanna. But by the mid-1980s this area was green and wet again.

The Sahara receives less than three inches of rain a year; Chicago's annual precipitation, its combined rainfall, snow and sleet, is 33.34 inches (84.68 cm). Even in the Sahara's wettest areas, it may rain twice one week and not rain again for years.

For centuries caravaneers have traveled through the Sahara desert. Even though there are many oases in the Sahara, the desert is so immense that travelers may go for days to reach them. 

Oases make trade possible between the ports of North Africa and savanna markets further south. Without these wet rest stops for humans and animals, crossing the desert would be almost impossible.

As the world's biggest desert, the Sahara covers a third of the African continent-an area about the size of the United States.

What makes the Sahara a desert?

The Sahara is one of the hottest places on Earth. Even though temperatures there may rise to 136 F (57.7 C), its dryness, not heat, that makes a place like the Sahara a desert. The frozen continent of Antarctica is so dry that some scientists consider it a desert, too.

As the world's largest desert, the Sahara receives less than three inches (7.6 cm) of rain a year. Even in its wettest areas, rain may arrive twice in one week, then not return for years.