This subject comes up regularly, and the received wisdom
is that AWSs provide tremendous detail about the character
of rainfall events, but - especially for those guages in an
elevated position - catch less than a standard manually-read
gauge (127mm diameter copper gauge with its rim 30cm
above the ground.)

The Eden mantra is: "Always install a standard gauge
alongside your AWS".

Here are some comparisons (Manual/AWS):
At Luton the two sites are 1.2km distant so there will
occasionally be some geographical difference between
them, but the land is pretty flat and the difference in
altitude is only 2m. The AWS gauge is 1.6m above
the ground:

July 2007 1.04 (significant geog. difference on one day)
August 1.12
Septmbr 1.24 (significant geog. difference on one day)
October 1.14
Novmbr 1.15
Decmbr 1.08
Jan 2008 1.13
Mean 1.13

At Chesham, the two gauges are on the same site, but
the AWS gauge is 1.6m above the ground:
Oct 7 - Nov 6 1.04
Nov 7 - Dec 2 1.07
Dec 3 - Dec 17 1.07
Dec 18- Jan 4 1.06
Jan 5-28 1.05
Mean 1.06

Philip Eden

"Philip Eden" philipATweatherHYPHENukDOTcom wrote in message
...
This subject comes up regularly, and the received wisdom
is that AWSs provide tremendous detail about the character
of rainfall events, but - especially for those guages in an
elevated position - catch less than a standard manually-read
gauge (127mm diameter copper gauge with its rim 30cm
above the ground.)

The Eden mantra is: "Always install a standard gauge
alongside your AWS".

Here are some comparisons (Manual/AWS):
At Luton the two sites are 1.2km distant so there will
occasionally be some geographical difference between
them, but the land is pretty flat and the difference in
altitude is only 2m. The AWS gauge is 1.6m above
the ground:

July 2007 1.04 (significant geog. difference on one day)
August 1.12
Septmbr 1.24 (significant geog. difference on one day)
October 1.14
Novmbr 1.15
Decmbr 1.08
Jan 2008 1.13
Mean 1.13

At Chesham, the two gauges are on the same site, but
the AWS gauge is 1.6m above the ground:
Oct 7 - Nov 6 1.04
Nov 7 - Dec 2 1.07
Dec 3 - Dec 17 1.07
Dec 18- Jan 4 1.06
Jan 5-28 1.05
Mean 1.06

Philip Eden

This interests me, since I have just installed a 5" copper gauge to back up
my Davis VP2 AWS. So far (only five rain days this month and allowing for
'throw back' of 09Z manual readings) the 5" is measuring about 15% higher.
Obviously such a small sample is pretty meaningless, and could be swamped by
the different 24-hour measuring spans, but I wondered what other observers
are seeing, and how much of the under-reading by the AWS is caused by lost
tips? I haven't calibrated the AWS by pouring water into it, by the way -
perhaps I should.

Steve P
Acton Bridge 38m
http://www.pardoes.com/meteo/weather.htm

Steve Pardoe wrote:

"Philip Eden" philipATweatherHYPHENukDOTcom wrote in message
...
This subject comes up regularly, and the received wisdom
is that AWSs provide tremendous detail about the character
of rainfall events, but - especially for those guages in an
elevated position - catch less than a standard manually-read
gauge (127mm diameter copper gauge with its rim 30cm
above the ground.)

The Eden mantra is: "Always install a standard gauge
alongside your AWS".

Here are some comparisons (Manual/AWS):
At Luton the two sites are 1.2km distant so there will
occasionally be some geographical difference between
them, but the land is pretty flat and the difference in
altitude is only 2m. The AWS gauge is 1.6m above
the ground:

July 2007 1.04 (significant geog. difference on one day)
August 1.12
Septmbr 1.24 (significant geog. difference on one day)
October 1.14
Novmbr 1.15
Decmbr 1.08
Jan 2008 1.13
Mean 1.13

At Chesham, the two gauges are on the same site, but
the AWS gauge is 1.6m above the ground:
Oct 7 - Nov 6 1.04
Nov 7 - Dec 2 1.07
Dec 3 - Dec 17 1.07
Dec 18- Jan 4 1.06
Jan 5-28 1.05
Mean 1.06

Philip Eden

This interests me, since I have just installed a 5" copper gauge to
back up my Davis VP2 AWS. So far (only five rain days this month and
allowing for 'throw back' of 09Z manual readings) the 5" is measuring
about 15% higher. Obviously such a small sample is pretty
meaningless, and could be swamped by the different 24-hour measuring
spans, but I wondered what other observers are seeing, and how much
of the under-reading by the AWS is caused by lost tips? I haven't
calibrated the AWS by pouring water into it, by the way - perhaps I
should.

Steve P
Acton Bridge 38m
http://www.pardoes.com/meteo/weather.htm

My standard 5" gauge averages about 15 percent higher than the Davis
VP. The rim of the 5" gauge is about 9 inches above ground. The rim of
the Davis gauge is about 6 feet above ground.

Norman
--
Norman Lynagh
Chalfont St Giles, Buckinghamshire
85m a.s.l.
(remove "thisbit" twice to e-mail)

I have a standard 5 " gauge a few metres from my Vantage Pro 2, and up to 0900 today I have
recorded 95.3mm in the 5" and 93.8 via the VP2 tipping bucket.
This 1.5 mm diffence has been about the same since the rains of mid January, so it would seem
that some tips may be lost during high rain rates.
However, this difference is only about 1.5%. Your 15% difference suggest a more fundamental
problem!

Roy Avis
Bracknell Weather/Webcam updated every minute plus Latest Statistics at:
http://homepage.ntlworld.com/roy.avis/WDLive/index.html
++++++++++++++++++++++++++++++++++++++++++++++++++ +++++++++++++++++++++++++++++++++++
PC and all mail in/out check by latest version of Norton AntiVirus/Internet Security

"Steve Pardoe" wrote in message
...
"Philip Eden" philipATweatherHYPHENukDOTcom wrote in message
...
This subject comes up regularly, and the received wisdom
is that AWSs provide tremendous detail about the character
of rainfall events, but - especially for those guages in an
elevated position - catch less than a standard manually-read
gauge (127mm diameter copper gauge with its rim 30cm
above the ground.)

The Eden mantra is: "Always install a standard gauge
alongside your AWS".

Here are some comparisons (Manual/AWS):
At Luton the two sites are 1.2km distant so there will
occasionally be some geographical difference between
them, but the land is pretty flat and the difference in
altitude is only 2m. The AWS gauge is 1.6m above
the ground:

July 2007 1.04 (significant geog. difference on one day)
August 1.12
Septmbr 1.24 (significant geog. difference on one day)
October 1.14
Novmbr 1.15
Decmbr 1.08
Jan 2008 1.13
Mean 1.13

At Chesham, the two gauges are on the same site, but
the AWS gauge is 1.6m above the ground:
Oct 7 - Nov 6 1.04
Nov 7 - Dec 2 1.07
Dec 3 - Dec 17 1.07
Dec 18- Jan 4 1.06
Jan 5-28 1.05
Mean 1.06

Philip Eden

This interests me, since I have just installed a 5" copper gauge to back up my Davis VP2 AWS.
So far (only five rain days this month and allowing for 'throw back' of 09Z manual readings)
the 5" is measuring about 15% higher. Obviously such a small sample is pretty meaningless, and
could be swamped by the different 24-hour measuring spans, but I wondered what other observers
are seeing, and how much of the under-reading by the AWS is caused by lost tips? I haven't
calibrated the AWS by pouring water into it, by the way - perhaps I should.

Steve P
Acton Bridge 38m
http://www.pardoes.com/meteo/weather.htm

This may be taken as a silly question - but I presume you have inserted the Metric Measurement
Adapter into the tipping bucket mechanism on your VP2?? They come (or mine did) with a 0.01"
tip as standard. I you don't insert it, each tip will be worth 0.254 mm but your console thinks
it is only 0.2 mm.

--
Roy Avis
Bracknell Weather/Webcam updated every minute plus Latest Statistics at:
http://homepage.ntlworld.com/roy.avis/WDLive/index.html
++++++++++++++++++++++++++++++++++++++++++++++++++ +++++++++++++++++++++++++++++++++++

PC and all mail in/out check by latest version of Norton AntiVirus/Internet Security
"Steve Pardoe" wrote in message
...
"Philip Eden" philipATweatherHYPHENukDOTcom wrote in message
...
This subject comes up regularly, and the received wisdom
is that AWSs provide tremendous detail about the character
of rainfall events, but - especially for those guages in an
elevated position - catch less than a standard manually-read
gauge (127mm diameter copper gauge with its rim 30cm
above the ground.)

The Eden mantra is: "Always install a standard gauge
alongside your AWS".

Here are some comparisons (Manual/AWS):
At Luton the two sites are 1.2km distant so there will
occasionally be some geographical difference between
them, but the land is pretty flat and the difference in
altitude is only 2m. The AWS gauge is 1.6m above
the ground:

July 2007 1.04 (significant geog. difference on one day)
August 1.12
Septmbr 1.24 (significant geog. difference on one day)
October 1.14
Novmbr 1.15
Decmbr 1.08
Jan 2008 1.13
Mean 1.13

At Chesham, the two gauges are on the same site, but
the AWS gauge is 1.6m above the ground:
Oct 7 - Nov 6 1.04
Nov 7 - Dec 2 1.07
Dec 3 - Dec 17 1.07
Dec 18- Jan 4 1.06
Jan 5-28 1.05
Mean 1.06

Philip Eden

This interests me, since I have just installed a 5" copper gauge to back up my Davis VP2 AWS.
So far (only five rain days this month and allowing for 'throw back' of 09Z manual readings)
the 5" is measuring about 15% higher. Obviously such a small sample is pretty meaningless, and
could be swamped by the different 24-hour measuring spans, but I wondered what other observers
are seeing, and how much of the under-reading by the AWS is caused by lost tips? I haven't
calibrated the AWS by pouring water into it, by the way - perhaps I should.

Steve P
Acton Bridge 38m
http://www.pardoes.com/meteo/weather.htm

"Roy Avis" wrote :

I have a standard 5 " gauge a few metres from my Vantage Pro 2, and up to
0900 today I have recorded 95.3mm in the 5" and 93.8 via the VP2 tipping
bucket.
This 1.5 mm diffence has been about the same since the rains of mid
January, so it would seem that some tips may be lost during high rain
rates.
However, this difference is only about 1.5%. Your 15% difference suggest
a more fundamental problem!

Roy, most of the difference comes from the different exposure of
the rain-gauges. If you are achieving only a 1.5% shortfall in an
elevated gauge you are doing exceptionally well. I'd be interested
to see if you maintain that figure. Most users who have made a
systematic check over a long period report a loss between 5
and 15%, the variation indicating differences in the effects of
turbulent air flow around the elevated AWS gauge.

Philip

Thanks, Philip, I didn't realise that it was so widespread a problem.
I would like to point out that I only measure rain in the 5" one a week. Do you think that,
besides the turbulence problem, there could be some systematic errors in reading small amounts
in a 5" measuring glass?
--
Roy Avis
+++++++++++++++++
"Philip Eden" philipATweatherHYPHENukDOTcom wrote in message
...
"Roy Avis" wrote :

I have a standard 5 " gauge a few metres from my Vantage Pro 2, and up to 0900 today I have
recorded 95.3mm in the 5" and 93.8 via the VP2 tipping bucket.
This 1.5 mm diffence has been about the same since the rains of mid January, so it would seem
that some tips may be lost during high rain rates.
However, this difference is only about 1.5%. Your 15% difference suggest a more fundamental
problem!

Roy, most of the difference comes from the different exposure of
the rain-gauges. If you are achieving only a 1.5% shortfall in an
elevated gauge you are doing exceptionally well. I'd be interested
to see if you maintain that figure. Most users who have made a
systematic check over a long period report a loss between 5
and 15%, the variation indicating differences in the effects of
turbulent air flow around the elevated AWS gauge.

Philip

"Roy Avis" wrote in message
...
This may be taken as a silly question - but I presume you have inserted
the Metric Measurement Adapter into the tipping bucket mechanism on your
VP2?? They come (or mine did) with a 0.01" tip as standard. I you don't
insert it, each tip will be worth 0.254 mm but your console thinks it is
only 0.2 mm.

For a novice like me, there are no silly questions (possibly some silly
answers) but I have indeed installed the metric adapter, thanks.

As it happens the 5" gauge is in a good, unsheltered position, whereas the
AWS is more of a compromise (between where I'd like to have it, feasible
cable lengths, and my wife's opinion of its appearance).

Steve P

Philip

Here are rainfall readings (Jan 2008 and Feb 2008 so far) for my
bog-standard rain-gauge with the rim 30 cm from the ground and my Davis AWS
rainfall gauge with the rim 150 cm from the ground. The differences are
quite amazing.

Jan 2008
Standard: 107.0 mm
Davis AWS: 78.4 mm

Feb 2008 (1st - 10th)
Standard: 26.2 mm
Davis AWS: 19.2 mm

The largest differences occur when windy weather accompanies the rainfall.
The Davis tipping-bucket collector is also in a slightly more exposed
position. I have noted (but don't have the records to hand) that during
rainfall events with little or no wind the difference is negligible.

Because of the exposure differences I am going to move the tipping bucket
gauge sometime soon to a site next to the standard gauge and at the same
height.
_______________
Nick G
Otter Valley, Devon
83 m amsl
http://www.ottervalley.co.uk

"Roy Avis" wrote in message
...
Thanks, Philip, I didn't realise that it was so widespread a problem.
I would like to point out that I only measure rain in the 5" one a week.
Do you think that, besides the turbulence problem, there could be some
systematic errors in reading small amounts in a 5" measuring glass?
--
Probably, but not very much ... I think Bernard may have explored
this idea. You can also lose a little by evaporation, but for most of
the year in the UK the losses from a standard gauge/bottle would
be very small in a week. During hot and sunny weather, though,
such a loss would probably be significant. (This has probably been
measured in the past but I can't lay my hands on any references just
for the moment).

A standard gauge actually catches marginally less than the
true rainfall of a particular location (calculated using gauges
flush with the ground, surrounded by an anti-splash grid) but
only by, I believe, a couple of percentage points at all but the
most exposed sites.

Philip