An inch of rain?

[The Medway Handyman]

When they say "an inch of rain fell in two hours" or whatever, how do they measure it?

I mean, an inch of rain in a 12" dia bucket is a lot different to an inch of rain in a 12' dia pool.

[Jeff Gaines]

It's still an inch of rain :-)

[ARWadsworth]

Up in Yorkshire we have mm of rain:-)

[Ronald Raygun]

They use a funnel to feed a graduated container of a much smaller cross section than that of the funnel aperture. This amplifies the depth so that it's easier to measure down to 1mm or less.

Don't think of an inch of rain as being a linear measure, but as a ratio of volume to area. It just means that 25.4 litres of rainwater fell per square metre of ground.

[Mike Humphrey]

No it isn't - it's one inch of rain over whatever area you're measuring.

If the pool and the bucket are out in the same rain, they will fill to the same depth. The pool will have more water in it in total, but that doesn't matter.

Mike

[zaax]

you mean mmm is it raining (again)?

[Huge]

No it isn't. :o)

(Assuming it's raining over the whole area, anyway.)

[Tim Streater]

Why do you think that?

[Andy Cap]

Think of it as putting a 1" screed on a floor. It doesn't make any difference how big the floor is.

[ARWadsworth]

That's Scotland.

[Colin Stamp]

The standard automated method is to use a tipping-bucket raingauge.

At the top, there's a funnel. Inside there's a pivoted "bucket", divided into two. It's shaped and arranged so that it can only rest tipped one way or the other. Only the side that's tipped upwards will be under the spout of the funnel. It will hold a small amount of water before it becomes heavy enough to make the bucket tip over the other way. The previously full side of the bucket then drains onto the ground whilst the other side now comes to rest under the spout. The process repeats as long as it keeps raining.

Some mechanism is included to count the tips of the bucket. Often it's a magnet on the underside of the bucket, sweeping past a reed switch. The signal from the switch goes off to a counter or a data-logger. Tips vary in size according to the gauge. The ones we used to use normally had

0.2mm or 0.5mm tips.

Posh raingauges have various extra features like heaters to prevent freezing and condensation. The edges of the funnel are often razor-sharp so that raindrops falling on the edge get neatly chopped in half and only the part inside the funnel area gets counted.

Cheers,

Colin.

[Ronald Raygun]

Duh. That's why they don't say "a pint of rain fell in two hours".

A unit of volume is meaningless when measuring rainfall, unless you relate it to the area over which it fell. Think of a pint of rain falling over a 6in x 6in square. That'll be about an inch.

[The Medway Handyman]

Because - if you pored say a pint of water into a 12" dia bucket & it filled to a depth of 1" - pouring a pint of water into a 12' dia pool isn't going to fill it to the same depth - the depth would hardly be measurable.

[Mungo "Two Sheds" Toadfoot]

... My word, that makes it sound a lot, doesn't it??

Si

[MuddyMike]

It makes no difference. So long as the receptacle has vertical straight sides an inch of rain will raise the water level inside by one inch.

Proper rain gauges have a funnel directing the water into a slim measuring tube where an inch can raise the level in the tube by several inches but is calibrated to correct the measurement. It just make reading off small amounts easier.

Mike

[Andrew Mawson]

Dave,

Unless you are leg pulling, I'm rather worried that you are asking this question - tell me you are jesting.

AWEM

[Harry Bloomfield]

Ronald Raygun explained on 01/02/2011 :

It means exactly the same thing, no matter what the area - a square foot, a square yard, a square mile, or a square kilometre. It needn't even be a square, a circular area is exactly the same. In effect, if you were to place a flat bottomed and straight sided container on a surface outside, it would fill up with 1" of water - except they don't measure it that way any longer.

Nowadays they pipe the output of a suitable funnel into a pair of side by side carefully balanced tipping buckets, a bit like a see saw. As one (high) side fills, it tips and the empty bucket comes under the flow whilst the other is allowed to drain. For each 'tipping motion', a pulse is sent to the weather data receiver, to enable it to log the rain fall.

[pete]

It's also 65,000 tonnes of rain in a square mile. Just goes to show what a force nature really is.

[TheOldFellow]

No, that's Cumbria. R.

[Unbeliever]

Hey drama queen, wait for your nail varnish to dry, and then try googling for "how is rain measured in inches" (without the quotes dumbo) - and you will find out!

And you could even try "rain gauge" - and get a result.

Bloody hell, do you want your hand held on everything you do? Just like a failed sales rep!

Doh! *eg*