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!.


Andrew said...

Very interesting Paul. I bought a 5 inch standard met office type raingauge today and found your blog after searching for information on them. I was a meteorological technician with the New Zealand Meteorological Service in the 1980's. I am interested to see that typically a glass bottle is placed within the overflow can and catches the rain via the funnel. The N.Z. Met Office gauges did not use such a glass bottle (and still don't as far as I know) but instead let the rain collect in the overflow can.
Andrew, New Zealand.

Paul H said...

Hi Andrew – thanks for the note and good to hear you’ve got hold of one of the 5 inch gauges. I had a good visit to the New Zealand Met service a few years ago although that was in early 2000. Also had the chance this January when I was at the AMS Annual Meeting to catch up again with Neil Gordon who I hadn’t seen in a while.