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 or Orographic Rainfall
- Convectional Rainfall
- Frontal rainfall
- Convectional Rainfall
This type of rainfall occurs in summer months and brings heavy rainfall and violent thunderstorms.
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 precipitates.
- Relief or Orographic Rainfall
Relief rainfall is a 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. Relief rainfall occurs very frequently near mountains beside the sea.
Moisture-laden wind blows in from the sea. Because the wind meets a high mountain, it is forced to rise upwards. As it rises upwards, it is cooled and cloud is formed.
The cloud becomes saturated with water vapour and it begins to precipitate on the side of the mountain facing the sea. This side of the mountain is known as the windward side.
The cloud precipitates the most on the windward side of the mountain. By the time the cloud meets the other side, which is called the leeward side, the cloud has already lost most of its moisture so it rains very little there.
This makes leeward sides of a mountain very sheltered from rain and they hardly ever get much rain.
There is a more moist climate on windward sides of slopes whilst there is a more dry, sheltered climate on the leeward side.
- Frontal Precipitation:
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.
When a warm, tropical air mass comes in contact with a cold, polar air mass, the air in the warm front rises over the cold front. The air is cooled and condenses to form stratus cloud.
When the stratus cloud becomes saturated, it begins to precipitate.