Watch this video explaining how rainfall is measured by the use of the rain gauge. A rain gauge measures the rain over a set period of time usually over a period of 24 hours. However modern rain gauges are self-recording and automatically measure the rain in 5 minute intervals.

Now write what each of the following processes is called:

• The sun absorbs water from lakes, seas and oceans.
• Water vapour is changed into water droplets in clouds.
• Water droplets fall from the clouds.

Now draw a diagram of a rain gauge similar to that seen in the video. Label the diagram with the following:

Metal or plastic cylinder             Funnel             Measuring cylinder marked in millimetres

• Where do you think should we place the rain gauge so that it will provide us with exact and accurate readings?

The figure entitled ‘How does it rain’ available in the resources section explains clearly how rain forms. The process is always the same. The sun heats up the air, the warm moist air rises and cools, condensation takes place, clouds form resulting in rainfall.

Watch carefully the following videos that explain the conditions needed for rain to form.

As already explained rain is formed when air rises and there are three ways how this happens:

• When warm moist air rises over a mountain known as relief or orographic rainfall
• When warm moist air rises by being heated by the sun known as convectional rainfall, and
• When a warm body of air rises over a cold body of air known as frontal or cyclonic rainfall.

Now make a copy of the three diagrams entitled Rain 1, Rain 2 and Rain 3 available in the resources section. Each diagram shows a different type of rainfall.

• Labelly carefully each diagram.
• Under each diagram write an explanation of the process and the conditions required for the rain to occur.

Now that you know how rain forms you can work out the worksheet available in the resource section entitled Rainfall worksheet.