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Types of Precipitation

Precipitation is caused when moist air rises; water vapour in the air-cools & condenses & forms clouds. Air cools upwards through the atmosphere because temperature falls with altitude because the Earth's atmosphere is heated from the Earth's surface up over. When the water droplets in clouds grow to a certain size, gravity causes them to fall because of their own weight. There are 3 different types of Precipitation formation;
Relief Rainfall
Convectional Rainfall
Frontal rainfall

Activities

Convectional Rainfall

Convection.gif (4107 bytes)

This type of rainfall occurs in summer months for the UK and brings heavy rainfall and violent thunderstorms to our country.

Short wave radiation arrives at the earth’s surface and is converted to long wave HEAT energy. 

This heat energy heats the air above it.

The now warmed air has more energy and rises through our atmosphere and away from the Earth’s surface in THERMALS which rise as fast as 25m/second.

As it rises, the warm air cools with height at a rate of 1°C per 100m

As the air cools water vapour condenses to form CUMULONIMBUS clouds and eventually it rains over the UK.

These events occur mainly in summer and in the south east of the country.


Relief Rainfall


ReliefRain.gif (12093 bytes)

Relief rainfall is a dominant method of precipitation formation in the UK and relates to the precipitation that is created as air masses are pushed up and over mountainous or upland areas.  Relief rainfall occurs where moist air is forced to rise over a physical barrier such as a mountain range.

Warm air is carried to the West coast of Britain by our prevailing (dominant) winds, the South Westerlies.

This air encounters the high land at the Lake District and the Pennines and it is forced to rise above this barrier.

As it rises, the warm air cools with height at a rate of 1°C per 100m. As the air cools water vapour condenses to form clouds and eventually it rains over Britain's highland areas.

As the air descends to the East coast of Britain it warms slightly and there is less rainfall. This results in a rain shadow on Britain’s east coast

It is for this reason that the West coast of Britain is wetter than the East, Blackpool receives 950mm of rainfall per year, The Pennines 2000mm+, and Newcastle 700mm.

   

Frontal Precipitation – WARM AIR RISING OVER COOLER AIR

 

FrontalRain.gif (4102 bytes)

In this type of rainfall warm moist air encounters colder more dense air at regions we call fronts.  Because the warm air is less dense since it has more energy it is forced to rise above the colder denser air.  As the warm air rises it cools, water droplets within it condense and clouds form.  Watch this animation of Frontal Rainfall.

Frontal air movement animations

Types of rainfall animations from BBC bytesize


 

Activities
1. Complete the 3 flow charts on this word document to explain the processes of Precipitation formation.

2. Describe the pattern of rainfall on the map below. 
3. Think about the 3 methods of precipitation formation.  How can they help to explain the pattern you observe?

4. An exercise showing frontal animations

Find out more - S-cool notes on rainfall types