How much rain does a desert ecosystem receive each year?

Average annual precipitation in desert ecosystems varies significantly, ranging from near zero in some coastal regions to approximately 600 millimetres (24 inches) in others. However, most recognized deserts receive less than 400 millimetres of rainfall annually. Some authorities define true deserts as having a mean annual precipitation of 250 millimetres or less, while areas receiving between 250 and 400 millimetres are often classified as semideserts.1

Rainfall in these environments is typically erratic and meagre, often occurring over only 15 to 20 days per year. In some instances, deserts may experience no rain for several consecutive years. When precipitation does occur, it can be heavy and localized, sometimes evaporating before it even reaches the ground. In certain coastal desert areas, fog and dew serve as critical, supplementary sources of moisture.1