Thursday, 17 July 2008

How to make a model rain gauge - with on air commentary

Download Paul's BBC Radio interview with live rain gauge construction!

Last week was wet, and not just outside of the studio. We thought it might be fun and interesting with school holidays coming up to do something on how to make your own weather observing instruments. So, I embarked on a brave attempt, Blue Peter-style, to build my own rain gauge and water barometer live on air – with mixed success.

It started well with the simple rain gauge, but the barometer proved a bit more tricky, and I seemed to have some problems with maintaining pressure. But despite my poor attempt, and the stick that I had from colleague afterwards, it is easy to do and if you are interested then there are some very simple instructions (with helpful pictures) on our website at

This week something less taxing hopefully!

Friday, 4 July 2008

How does the rain gauge work?

Download Paul's BBC Radio interview on rainfall

There are lots of different types of rain gauges, weighing gauges, tipping bucket gauges, siphon gauges, etc. but the ‘standard rain gauge’ used by the Met Office (as a reference gauge) is a copper cylinder with a knife-edged brass rim of 127 mm (5 inches) diameter, which is set in the ground with the top of the gauge 300 mm above ground level . Inside is a glass bottle contained within a removable overflow can. The top, cylindrical part of the rain gauge contains a funnel that directs the rain into the glass bottle.

Last week I was asked how come one gauge will measure the same depth of rainfall as another gauge that has a completely different diameter – surely the one with the bigger diameter will measure more rainfall because the whole at the top of the gauge is bigger. Well yes and no!

Gauges are ‘calibrated’ to make a measurement of a standard cubic volume of water. That is, the measuring scale in the gauge is designed to show you the depth of water in mm as if it had fallen through, let’s say for the sake of explanation, a volume of 1 m high onto an area of 1 m square. What that means is if you had two gauges of different diameters they might collect different amounts of water in them, but if you compared the measuring scale from the two gauges you would notice the two scales would not match each other – they would both be calibrated differently.

Here’s what I mean as an example. Let’s say you had two gauges, both the same height, but the first gauge was twice the area of the second one. And let’s say that the first gauge is a quarter full. If we pour the water from the first gauge into the second gauge, the second gauge would then be half full. That’s because as the second gauge is only half the volume, the water will go twice as far up in the gauge.

In order that we make sure we have the same depth reading in both gauges we would therefore need to calibrate the second gauge so that the measuring scale was twice as wide. I think this picture helps to explain.

Why not try making your own rain gauge with just a 2 litre plastic bottle, it’s lots of fun and you don’t need to worry about calculating areas and volumes!.