Deserts stir the human imagination. They occupy nearly one-seventh of the world's land surface area, but carry only four percent of the human population. The immense spaces of the desert have inspired the explorers, and breed a heartiness in the people who live there. What is a Desert? Water, or more accurately the lack of water, is what defines a desert. In general terms deserts are dry places. The standard definition is an area that receives less than 10 inches of rain a year, and whose yearly evaporation rate exceeds 10 inches. To gain some perspective the driest desert in the world is the Atacama of South America. Its average yearly rainfall is only 0.4 inches a year, although it's common for several years to go by without rain. Tropical rainforests, however, receive an average of 80 inches a year, and some are deluged by as much as 200 inches. Many factors usually combine to produce a desert, and there are definite patterns as to where they occur. Desert lands cover incredible distances. Many who have driven through deserts in the United States may think they are enormous, but they are only the fifth largest in the world covering about 500,000 square miles. By contrast the great Sahara Desert covers almost 3.5 million square miles, the Australian deserts 1.3 million square miles, the Arabian deserts 1 million square miles, while the deserts of Turkestan have 750,000 square miles. Numerous smaller deserts are also scattered across the globe. All are unique, and have adapted to their own particular environments.