Deserts cover more than one fifth of the Earth's land, and they are found on every continent. A place that receives less than 250mm of rain per year is considered a desert. Deserts are part of a wider classification of regions called "dry lands." These areas exist under a moisture shortage, which means they can frequently lose more moisture through evaporation than they receive from annual precipitation. Desert are created through many different processes of erosion with the wind been a major contributor of this. These erosion processes include of abrasion, deflation and attrition.