A very dry area that did not receive any rain for a minimum of 12 straight months or an arid area that receives less than 10 inches of rain every year is called a desert.Other than lack of rain, another reason for aridity is due to high evaporation and transpiration rates.
The ground loses much water through a high evaporation rate. Plants lose their water through a high transpiration rate. Both evaporation and transpiration is summarized as evapotranspiration. The aridity index is P over PET, where P is average yearly rainfall and PET is potential evapotranspiration. The world's deserts either fall into the arid or hyperarid categories with an aridity index of less than 0.20. They include South America's Atacama Desert, North America's Sonoran Desert and Northern Africa's Sahara Desert.