ETYM Old Eng. rein, as. regen; akin to OFries. rein, Dutch and German regen, os. and Old High Germ. regan, Icel., Dan., and Swed. regn, Goth. rign, and prob. to Latin rigare to water, to wet; cf. Greek brechein to wet, to rain.
(Homonym: reign, rein).
1. Anything happening rapidly or in quick successive; SYN. pelting.
2. Drops of fresh water that fall as precipitation from clouds; SYN. rainwater.
3. Water falling in drops from vapor condensed in the atmosphere; SYN. rainfall.
Form of precipitation in which separate drops of water fall to the Earth's surface from clouds. The drops are formed by the accumulation of fine droplets that condense from water vapor in the air. The condensation is usually brought about by rising and subsequent cooling of air.
Rain can form in three main ways—frontal (or cyclonic) rainfall, orographic (or relief) rainfall, and convectional rainfall. Frontal rainfall takes place at the boundary, or front, between a mass of warm air from the tropics and a mass of cold air from the poles. The water vapor in the warm air is chilled and condenses to form clouds and rain.
Orographic rainfall occurs when an airstream is forced to rise over a mountain range. The air becomes cooled and precipitation takes place. In the uk, the Pennine hills, which extend southward from Northumbria to Derbyshire in N England, interrupt the path of the prevailing southwesterly winds, causing orographic rainfall. Their presence is partly responsible for the west of the uk being wetter than the east. Convectional rainfall, associated with hot climates, is brought about by rising and abrupt cooling of air that has been warmed by the extreme heat of the ground surface. The water vapor carried by the air condenses and so rain falls heavily. Convectional rainfall is usually accompanied by a thunderstorm, and it can be intensified over urban areas due to higher temperatures (see heat island).