British rainfall

Depressions pass over Britain frequently. They form in the Atlantic and move east across the country, bringing changeable weather.

Why does most rain fall in the west?

Which types of rainfall commonly affect Britain?

Relief rainfall

High ground forces cool air to condense and creates rain
  1. Prevailing winds bring warm, moist air to the western British Isles.
  2. Air is forced to rise over high areas.
  3. Air cools and condenses.
  4. Clouds form and it rains.
  5. Air descends on the other side of the mountains.
  6. It warms up and therefore becomes drier.

Frontal rainfall

Frontal rainfall happens where warm fronts meet cold fronts

The British Isles are affected by a number of different air masses. When warm and cold air meet, a depression forms:

  1. When a cold polar air mass meets a warm tropical air mass they do not mix - they form fronts.
  2. The colder air mass is heavier than the warmer air mass, therefore the lighter, warmer air rises over the top of the heavier, colder air.
  3. As the warm air is forced to rise it cools. Also, the warm air is in contact with the cold air along the fronts, and this also cools.
  4. Condensation occurs and clouds form.
  5. Rain occurs along the front.

In the UK, depressions often follow a similar pattern. First, a warm front passes over, bringing rain and then warmer air. Then a cold front follows, bringing more rain and cooler air.

On synoptic maps, warm fronts are shown by a red line with red semi-circles. Cold fronts are shown by a blue line with blue triangles.

Convectional rainfall

When the land warms up, it heats the air above it. This causes the air to expand and rise. As the air rises it cools and condenses. If this process continues then rain will fall. This type of rainfall is very common in tropical areas but also in areas such as South East England during warm sunny spells.

Convectional rainfall is common where the sun heats the land near the sea
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