(09:00UTC 01/08/09)
15.8 C
Max 18.3 C, min 13.2 C
Dewpoint 15.1 C
RH 96%
Wind 0.5 kts SSE
1006.7 hPa steady
Ppn (standard gauge) 7.9mm last 24hrs
Ppn (standard gauge) 167.6mm this month
Ppn (AWS) 6.2mm last 24 hrs
Ppn (AWS) 126.1mm this month

Misty, damp, light grey overall, cool, slight breeze

Hugh

--

Hugh Newbury

www.evershot-weather.org

On Sat, 01 Aug 2009 10:05:19 +0100, Hugh Newbury
wrote:

Ppn (standard gauge) 7.9mm last 24hrs
Ppn (standard gauge) 167.6mm this month
Ppn (AWS) 6.2mm last 24 hrs
Ppn (AWS) 126.1mm this month

Will be very interested to learn where the two rainfall gadgets are
positioned in view of some VERY close rainfall totals by other
correspondents for last month

R

Robin Nicholson wrote:
On Sat, 01 Aug 2009 10:05:19 +0100, Hugh Newbury
wrote:

Ppn (standard gauge) 7.9mm last 24hrs
Ppn (standard gauge) 167.6mm this month
Ppn (AWS) 6.2mm last 24 hrs
Ppn (AWS) 126.1mm this month

Will be very interested to learn where the two rainfall gadgets are
positioned in view of some VERY close rainfall totals by other
correspondents for last month

Exactly 2m apart. The AWS's tipping bucket rim is 1.80m above the ground.

Any comments welcome.

Hugh

--

Hugh Newbury

www.evershot-weather.org

"Hugh Newbury" wrote ...

Will be very interested to learn where the two rainfall gadgets are
positioned in view of some VERY close rainfall totals by other
correspondents for last month

Exactly 2m apart. The AWS's tipping bucket rim is 1.80m above the
ground.


.... but presumably the standard gauge is at ground level?

Also, have you had time to perform a check on the AWS gauge by passing
a measured volume of water through it?

Martin.

--
Martin Rowley
West Moors, East Dorset (UK): 17m (56ft) amsl
Lat: 50.82N Long: 01.88W
NGR: SU 082 023

Martin Rowley wrote:
"Hugh Newbury" wrote ...
Will be very interested to learn where the two rainfall gadgets are
positioned in view of some VERY close rainfall totals by other
correspondents for last month
Exactly 2m apart. The AWS's tipping bucket rim is 1.80m above the
ground.


... but presumably the standard gauge is at ground level?

Yes, standard height and level.

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.

Thanks for the ideas.

Hugh

--

Hugh Newbury

www.evershot-weather.org

In case it's of any interest, there's a publicly-viewable section on
VP/VP2 rain gauge calibration and faults on a page in our VP
knowledgebase. See:

http://vp-kb.wikispaces.com/Sensor+Errors3

I'll be adding another paragraph to this, probably in the next few
days (if I get time), dealing with height vs calibration issues and
the fact that the drip test - good though it is - won't necessarily
give a calibration result that's exactly in line with a ground-mounted
reference gauge.

John Dann
www.weatherstations.co.uk

"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.

--
Martin Rowley
West Moors, East Dorset (UK): 17m (56ft) amsl
Lat: 50.82N Long: 01.88W
NGR: SU 082 023

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

On Sun, 02 Aug 2009 21:17:24 +0100, Hugh Newbury
wrote:

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.


No I'd suggest the reverse actually. The problem with very high rain
rates AIUI is that the high flow rate causes some losses while the
buckets are in the process of tipping (and also, I suspect, increased
losses due to splashing out of the buckets etc). In addition, a
smaller hole would block more easily.

JGD

John Dann wrote:
On Sun, 02 Aug 2009 21:17:24 +0100, Hugh Newbury
wrote:

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.


No I'd suggest the reverse actually. The problem with very high rain
xxxxxxxxxxxxxxxxxxx
rates AIUI is that the high flow rate causes some losses while the
xxxxxxxxxxxxxx buckets are in
the process of tipping (and also, I suspect, increased
losses due to splashing out of the buckets etc). In addition, a
smaller hole would block more easily.

So you're suggesting an even higher flow rate? I really don't follow
your argument there.

I think I'll try some experiments of my own and report back.

Hugh

--

Hugh Newbury

www.evershot-weather.org