Precipitation

As part of the hydrologic cycle, water vapor is transformed into clouds. Clouds are common and, although in constant motion, usually cover about half of the planet at a time. Even so, precipitation is occurring over a small portion of Earth because most clouds do not produce precipitation. How and why precipitation occurs is vital in understanding the physical geography of Earth, because the precipitated water affects the surface lithosphere, biosphere, and hydrosphere.

Precipitation is triggered by one of two processes. The first is known as the warm cloud process and involves only water droplets (including supercooled water) above tropical locations. Cloud droplets collide and some of them coalesce to become large enough to fall out of the sky as rain. The cold cloud process (also known as the Bergeron process) takes place in middle latitude and polar clouds. The temperatures are below freezing and supercooled droplets and ice crystals coexist at temperatures between 0° and −40°C. The saturation vapor pressure is a bit higher over liquid water than over ice crystals and the net effect is that ice crystals grow by adding mass from the nearby droplets. The enlarged ice crystals become large enough to fall as snow.

The general term for water precipitated out of the sky is “hydrometer.” There are several types of hydrometers and their occurrences are governed by moisture supply, rise of air, and the environmental lapse rate underneath the clouds. Snow starts as snowflakes in clouds and falls through below-freezing air to the surface.