Rainfall per square meter

Hello!

Please excuse me if this has been asked before…

I have a VP2 and WD and publish my local weather to the web. A viewer has asked me how to correlate the rainfall shown from my weather station to rain per meter squared.

I have never really given this much thought. Do I assume that the values given by my VP2 rain gauge (which is metric) are already adjusted to give a rain per m2?

Also, when comparing my rainfall data to someone else’s, how do I ensure that I’m comparing like for like? Do rain gauges all have a standard diameter / surface collection area?

Yours very confused!

Dan

Think of 10mm of rain being 10mm of water falling over the whole area where that was recorded

Rain gauges normally report rainfall as a depth, so this is independent of the area. If you want to quote rainfall as a volume per square metre, then simply multiply the depth by the required area (in suitable units).

So 10mm of rain per sq metre = 1cm x (100cm x 100cm) = 10,000cm3/m2 or in liquid measure 10 litres/m2

The unit ‘mm’ is equivalent to unit ‘litres per sq. metre’.
Most rain gauges have a standard 10cm diameter, or thereabouts. No need to quote this when comparing gauges unless yours is significantly different.

Sorry, I am still confused :frowning:

Davis quotes the VP2 rain gauge as having a surface area of 214cm2.

Does this mean that if the console or WD shows 0.2mm of rain, 0.2mm has fallen over 1m2 of land, or do I have to do the following:

(100cmx100cm)/214cm2 = 46.7

then

46.7 x 0.2 (rain shown on console or WD) = 9.34mm

Hence 9.34mm has fallen over 1 square meter?

Many thanks.

Regards

Dan

No, the rainfall of 0.2mm is across ANY area. It is a depth of water that is independent of the area.

0.2mm depth of water is the same if the catchment area you use is 1cm square or 100m square, the rain that fell would still be 0.2mm deep. (ignoring localised and other effects :wink:)

Okay, I understand now! Many thanks!

Cheers

Dan