Martin Rowley wrote:
"Hugh Newbury" wrote in message
...
... but presumably the standard gauge is at ground level?
Yes, standard height and level.

... I wondered if the two units are experiencing the same 'air
envelope' during precipitation - but I don't know your exposure etc.,
so unless there are obstructions nearby to generate a distinct
distortion to the air-flow, I would have thought that wouldn't have
accounted for some of the larger differences.

Also, have you had time to perform a check on the AWS gauge by
passing a measured volume of water through it?
Martin, I hadn't thought of that. Just tried it, and it seems to be
working OK, but only when I pour in a small amount very very slowly:
eg in 0.1mm doses into the tipping bucket set at 0.2mm. I'm not
convinced it would work with the sort of heavy rain showers we've
been having: I recently had a shower of 40.0mm/hr for a few minutes
which I'm sure would have swamped the tipping bucket. This is
perhaps the cause of the wide differences between the AWS and Std
gauge here last month.

... Yes, looking at your data on the web site, it does look as if
during intense rainfall (particularly if accompanied by a gusty wind -
as has been the case recently), then the differences are the largest.

I wouldn't have expected such large differences though: for example,
at Copley (see thread elsewhere), the difference is less than a couple
of mm over the month as a whole, and they have some pretty challenging
conditions.

We really need Stephen Burt to weigh in on this as he has a lot of
experience of comparative trials of rain gauges.

Martin, It occurs to me that maybe the answer might be to restrict the
outlet from the AWS to its tipping bucket. Small as it is, possibly it's
still too large to cope with a deluge. The weight of the rain in the
bowl would force the water out more quickly. Certainly my experiments
today seem to suggest this. I'll think about it for a day or two.

Thanks, all, for your ideas and help. (Only what I would expect from the
people in this NG.)

Hugh

--

Hugh Newbury

www.evershot-weather.org