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What is precipitation?

Precipitation is any form of moisture which falls to the earth. This includes rain, snow, hail and sleet.

Precipitation occurs when water vapour cools. When the air reaches saturation point (also known as condensation point and dew point) the water vapour condenses and forms tiny droplets of water. These tiny droplets of water from clouds.

Complex forces cause the water droplets to fall as rainfall.

All rain is the same. It happens as the result of warm, moist air being cooled, leading to condensation and in turn rain. The following examples show three different ways air is cooled causing rainfall.

Relief Rainfall

[relief rainfall]

Stage 1.
Warm wet air is forced to rise over high land.

Stage 2.
As the air rises it cools and condenses. Clouds form and precipitation occurs.

Stage 3.
The drier air descends and warms.

Stage 4.

Any moisure in the air (e.g. cloud) evaporates.

Convectional Rainfall

[convectional rainfall]

Stage 1.

The sun heats the ground and warm air rises.

Stage 2

As the air rises it cools and water vapour condenses to form clouds.

Stage 3.
When the condensation point is reached large cumulonimbus clouds are formed.

Stage 4.

Heavy rain storms occur. These usually include thunder and lightening due to the electrical charge created by unstable conditions.

Frontal Rainfall

[frontal rainfall]

Stage 1.
An area of warm air meets and area of cold air.

Stage 2.
The warm air is forced over the cold air

Stage 3.
Where the air meets the warm air is cooled and water vapour condenses.

Stage 4.
Clouds form and precipitation occurs.

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