ABSTRACT
Types of precipitation Precipitation is defined as the condensation of water vapour into liquid water droplets and ice particles that fall to the earth’s surface. Rain is precipitation that is formed by water vapour condensing at temperatures above freezing or by ice crystals thawing before they reach the ground: snow is formed when ice crystals do not thaw. Rain and snow are the most important types of precipita tion in hydrological terms. Hail is created when raindrops are carried further aloft by updrafts within a cloud into air below 0 °C and freeze: although hailstorms are damaging, the hydrological effects are very similar to those of heavy rainfall. Other types of precipitation include dew , where water vapour condenses directly onto a cold surface, and fog-drip (or occult precipitation), where water is “harvested” directly from fog or low-lying clouds by vegetation. Dew is important for some plants but not significant hydrologically, but fog-drip can introduce water into the soil and may, under some circumstances, generate streamflow (as shown by Gurnell (1976) in the New Forest, England). This section focuses on rain and snow. Box 3.1 summarises methods used to measure and estimate precipitation.
