Sunday, 20 January 2008

Types of rainfall

By the end of this lesson you should have been able to name and explain the three types of rainfall. You should also have drawn an annotated diagram to show why rain falls in each of the three cases.

We get rain because of rising air. As air rises, it cools. As cool air can't hold as much water vapour as warm air, the water vapour condenses and turns into water droplets. These fall to earth as precipitation.

In class, we used an animation to show why convectional, relief and frontal rainfall occur. You can see the animation here.

Convectional Rainfall

This is very common in areas where ground is heated by the hot sun, such as the Tropics. We do get some of this type of rainfall in Cambridge in the summer - in fact, its the reason why last year's school sports day was cancelled!

The sun heats up the ground, which heats the air above it. This then rises and cools, and the water vapour condenses into water droplets, forming clouds. The droplets falls as rain under the influence of gravity.

This video shows convectional rainfall in Costa Rica.

Relief Rainfall

With relief rainfall, it is the presence of hills or mountains that leads to the warm aid rising. Its particularly common on the west coast of the UK, since the prevailing winds come from the west and have to rise to pass over hills and mountains in areas such as the Lake District and Snowdonia.

The winds pick up moisture from the sea as they pass over it, and this makes the air moist. As the air rises to pass over the higher land, it cools and the water vapour condenses, forming clouds. The droplets then fall as rain, sleet, hail, or snow, depending on the atmospheric conditions.

On the leeward side of the mountain (the side not facing the wind), it is much drier. This area is said to be in the 'rainshadow'. There is little rain here as the air is descending and warming up.

This video clip shows relief rainfall on New Zealand's south island.

Frontal Rainfall

Frontal rainfall in the UK is associated with depressions. It happens when two air masses (a warm one and a cold one) meet. The warm air is pushed upwards over the cold air, and, as it rises, it cools down. The water vapour condenses to form clouds, which give rain.

This video shows frontal rainfall.

Test Yourself!

Download this PowerPoint, then move the objects into the correct place on each slide and custom animate to help you explain the rainfall processes.

Note for teachers:
I'm not sure who wrote this activity - but I think it may be someone from Manchester. Please let me know if it's you so that I can add an acknowledgement.

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