Earth rainfall climatology Is the study of rainfall, a sub-field of meteorology. Formally, a wider study includes water falling as ice crystals, i.e. hail, sleet, snow (parts of the hydrological cycle known as precipitation). The aim of rainfall climatology is to measure, understand and predict rain distribution across different regions of planet Earth, a factor of air pressure, humidity, topography, cloud type and raindrop size, via direct measurement and remote sensing data acquisition. Current technologies accurately predict rainfall 3–4 days in advance using numerical weather prediction. Geostationary orbiting satellites gather IR and visual wavelength data to measure realtime localised rainfall by estimating cloud albedo, water content, and the corresponding probability of rain. Geographic distribution of rain is largely governed by climate type, topography and habitat humidity. In mountainous areas, heavy precipitation is possible where the upslope flow is maximized within windward sides of the terrain at elevation. On the leeward side of mountains, desert climates can exist due to the dry air caused by compressional heating. The movement of the monsoon trough, or Intertropical Convergence Zone, brings rainy seasons to savannah climes. The urban heat island effect leads to increased rainfall, both in amounts and intensity, downwind of cities. Warming may also cause changes in the precipitation pattern globally, including wetter conditions at high latitudes and in some wet tropical areas. Precipitation is a major component of the water cycle, and is responsible for depositing most of the fresh water on the planet. Approximately of water falls as precipitation each year; of it over the oceans. Given the Earth's surface area, that means the globally averaged annual precipitation is . Climate classification systems such as the Köppen climate classification system use average annual rainfall to help differentiate between differing climate regimes. Most of Australia is semi-arid or desert, making it the world's driest continent. Australia's rainfall is mainly regulated by the movement of the alien monsoon trough during the summer rainy season, with lesser amounts falling during the winter and spring in its southernmost sections. Almost whole North Africa is semi-arid, arid or hyper-arid, containing the Sahara Desert which is the largest hot desert in the world, while central Africa (known as Sub-Saharan Africa) sees an annual rainy season regulated by the movement of the Intertropical Convergence Zone or monsoon trough, though the Sahel Belt located at the south of the Sahara Desert knows an extremely intense and a nearly