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There are three common types of rainfall, all of which occur in the United Kingdom. All have the common theme of air being forced to rise.
As air rises it cools it cannot hold as much moisture as it could when it was warmer. Eventually the rising air reaches a point where it is 100% saturated, in other words it cannot hold any more water. This is called dew point, and it is above this point that condensation occurs.
Condensation is the process by which the water vapour (a gas) held in the air is turned back into water droplets (a liquid), which fall as rain.
Very common in areas where the ground is heated by the hot sun, such as the Tropics. This is why those areas experience heavy rainfalls most afternoons. The United Kingdom does experience some convectional rainfall during the summer, particularly in the South East of the country.
Convectional rainfall occurs when:
- The surface of the earth is heated by the sun.
- The warm surface heats the air above it. Hot air always rises so this newly heated air does so.
- As it rises the air-cools and begins to condensate.
- Further rising and cooling causes a large amount of condensation to occur and rain is formed.
- Convection tends to produce towering cumulo-nimbus clouds, which produce heavy rain and possible thunder and lightning.
The United Kingdom experiences a lot of frontal rainfall, as it is associated with the movement of depressions over the country, which are described in more detail elsewhere in this topic.
Frontal rainfall occurs when:
- Two air masses meet, one a warm air mass and one a cold air mass.
- The lighter, less dense, warm air is forced to rise over the denser, cold air.
- This causes the warm air to cool and begin to condense.
- As the warm air is forced to rise further condensation occurs and rain is formed.
- Frontal rain produces a variety of clouds, which bring moderate to heavy rainfall.
This is also called orographic rainfall, which is very common in the United Kingdom, especially on the West coast since the prevailing weather comes from that direction.
Relief Rainfall occurs when:
- The prevailing winds pick up moisture from the sea as they travel across it, making the air moist.
- The moist air reaches the coast and is forced to rise over mountains and hills.
- This forces the air to cool and condense, forming clouds.
- The air continues to be forced over the mountains and so it drops its moisture as relief rain.
- Once over the top of the mountain the air will usually drop down the other side, warming as it does so. This means it has a greater ability to carry water moisture and so there is little rain on the far side of the mountain. This area is called the rain shadow.
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