In meteorology, virga is an observable streak or shaft of precipitation that falls from a cloud but evaporates or sublimes before reaching the ground. A shaft of precipitation which does not evaporate before reaching the ground is a precipitation shaft. At high altitudes the precipitation falls mainly as ice crystals before melting and finally evaporating; this is often due to compressional heating, because the air pressure increases closer to the ground. It is very common in the desert and in temperate climates. In North America, it is commonly seen in the Western United States and the Canadian Prairies. It is also very common in the Middle East, Australia and North Africa.