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Frontal Precipitation

Geography: KS3, topic 5.1.8

Frontal Precipitation

Air can rise in one of three main ways: convection precipitation, frontal precipitation, relief or orographic precipitation.

Ocean air masses

Ocean air masses

  • If a large area (i.e. an area of many hundreds or thousands of square kilometres) remains above a uniform land or sea surface for a sufficiently long time, it will acquire the temperature and moisture of that area.
Air masses

Air masses

  • An air mass will be cold or warm, humid or dry depending on its origin and its path.
  • The British Isles are located where six main air masses meet.
Air masses in Britain

Air masses in Britain

  • The five main air masses affecting the British Isles are:

  • Tropical continental

  • Tropical maritime

  • Polar continental

  • Polar maritime

  • Arctic maritime

  • The sixth one is Returning Polar maritime

Air fronts

Air fronts

  • When two air masses meet, (e.g. a cold polar air mass meets a warm tropical air mass) the boundary is a front and the air does not mix immediately.
  • There are three types of front:
  • Warm fronts
  • Cold fronts
  • Occluded fronts.
Air fronts cont.

Air fronts cont.

  • At a front, the colder air is heavier than the warmer air.
  • The lighter warmer air will rise above the other colder air.
  • The lighter warmer air expands and cools as it rises.
  • Cooler air cannot hold as much water vapour as a gas as warmer air.
Water vapour

Water vapour

  • The air becomes saturated with gaseous water vapour.
  • The gaseous water vapour condenses to liquid water droplets.
  • Eventually the weight of the liquid water droplets or ice is enough for gravity to return these water droplets / ice to the surface as precipitation.
Depressions

Depressions

  • Depressions form along a front between two air masses.
  • They affect the UK. They form in the Atlantic at the boundary of colder polar air and warmer tropical air.
  • They move across the UK from west to east.
  • In a depression, the air between the warm and cold fronts (the warm sector) is forced to rise, and it expands and cools and precipitation occurs.

Test yourself on Frontal Precipitation

  • Which is NOT one of the main air masses that affects the British Isles?

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