section_path: Global climatology
section_name: Global climatology
paragraph_text: Global climatology
  
  Approximately 505,000 km (121,000 cu mi) of water falls as precipitation each year across the globe with 398,000 km (95,000 cu mi) of it over the oceans. Given the Earth's surface area, that means the globally averaged annual precipitation is 990 mm (39 in). Deserts are defined as areas with an average annual precipitation of less than 250 mm (10 in) per year, or as areas where more water is lost by evapotranspiration than falls as precipitation.