Rain Photography

Approximately 505,000 cubic kilometres (121,000 cu mi) of water falls as precipitation each year across the globe with 398,000 cubic kilometres (95,000 cu mi) of it over the oceans. Given the Earth's surface area, that means the globally-averaged annual precipitation is 990 millimetres (39 in). Deserts are defined as areas with an average annual precipitation of less than 250 millimetres (10 in) per year, or as areas where more water is lost by evapotranspiration than falls as precipitation.
Deserts
The northern half of Africa is primarily desert or arid, containing the Sahara. Across Asia, a large annual rainfall minimum, composed primarily of deserts, stretches from the Gobi desert in Mongolia west-southwest through western Pakistan (Balochistan) and Iran into the Arabian desert in Saudi Arabia. Most of Australia is semi-arid or desert, making it the world's driest inhabited continent. In South America, the Andes mountain range blocks Pacific moisture that arrives in that continent, resulting in a desertlike climate just downwind across western Argentina. The drier areas of the United States are regions where the Sonoran desert overspreads the Desert Southwest, the Great Basin and central Wyoming.