rain

The three patterns of rainfall. Frontal (or cyclonic) rain is caused by warm air rising over cold air in a low pressure area. Relief (or orthographic) rainfall occurs when warm, moist air cools as it is forced to rise over hills or mountains. Convectional rainfall is caused when the surface of the Earth has been warmed by the Sun and the air in contact with it rises and meets colder layers. It is usually accompanied by a thunderstorm and is common in tropical regions.

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 vapour 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 vapour 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 southwards from Northumbria to Derbyshire in northern 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 vapour carried by the air condenses, producing heavy rain. Convectional rainfall is usually accompanied by a thunderstorm, and it can be intensified over urban areas due to higher temperatures there (see heat island).