A basic rainfall gauge is made up of a container. What type works well? Encourage students to try a variety of containers. They will need one that is stable, is large enough to cope with the predicted rainfall, and is preferably transparent (although this is not essential, as the rain could be measured by pouring it into another container).
The second important thing to consider is where to place the rain gauge.
Encourage students to try to account for why the amount of rain in different positions varies, for example, less rain is collected in sheltered areas.
They also need to think about whether rain will splash into the gauge from the surrounding ground.
They could experiment with a watering can, to find how high rain splashes on different types of ground, and work out how high the top of the gauge needs to be.
Another thing they could think about is evaporation. Half fill a jar with water and mark the level. Leave it out of doors for a day in different weather conditions and measure the level of the water again.
Your students will be able to see that in certain weather conditions a large amount of water can evaporate (for example, on dry, sunny days).
Encourage them to consider how they might protect the water they collect in their rain gauge from evaporation. A funnel works well.
Once they have designed a rain gauge, encourage them to take measurements over a week.
To take accurate measurements, they will need to measure the height of rain in a container with the same cross section as the funnel. A plastic water bottle works well. Cut off the top of the bottle, invert it and place it back into the bottle. The children can design a readable scale to help them take measurements.
Sophie Duncan is project manager for science at the BBC www.bbc.co.uksn