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.
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
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.
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.
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
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
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
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!