Measuring rainfall

The news says a place got 30 mm of rain in a day. What does that mean, and how is it measured?

With a rain gauge. Rain falls into a funnel and collects in a cylinder with a scale. The scale shows how deep the water is. If it is 5 mm deep, the place got 5 mm of rainfall.

So rainfall is measured as a depth, not an amount of water: how deep it would be if none ran off or soaked in. That's why the same number works for a small garden or a whole district. The wide funnel pours rain into a narrow tube, so a little rain makes a taller column that's easier to read.

You can make one at home: a container, a funnel on top, and a straight tube or jar marked in millimetres.

Using it properly:

• put it in an open place, away from walls and trees (or it will catch too little)
• stand it on a flat surface so it can't tilt or fall over
• read it at the same time every day for a month, so each reading is exactly one day
• if snow falls in, let it melt before you read it

Then work out the average rain for each week and see how it changes.