It's probably been discussed before, but...

The general idea is that rain gauges should be as flush with the ground as
possible to ensure that the catch is affected as little as possible by the
wind. Just to test this I put three idenitical TBR (Youngs) gauges at
rim-heights of 40cm, 100cm and 160cm above the ground. From 12/11/07 to
29/01/08 they collected 210.4, 213.2 and 216.5 mm respectively. Um...

Chris

On 29 Jan, 15:13, "Chris Kidd" wrote:
It's probably been discussed before, but...

I don't think it has, but here's my thoughts....

I think that the RG with the rim flush with the ground should be in a
pit with a coarse mesh surrouding it also flush with the ground like a
huge grid - this has the effect of reducing wind turbulence IIRC.

Just remember some uni experiments to show this on Plynlimon.

Steve J

On Tue, 29 Jan 2008 07:58:15 -0800 (PST), Steve J
wrote:

On 29 Jan, 15:13, "Chris Kidd" wrote:
It's probably been discussed before, but...

I don't think it has, but here's my thoughts....

I think that the RG with the rim flush with the ground should be in a
pit with a coarse mesh surrouding it also flush with the ground like a
huge grid - this has the effect of reducing wind turbulence IIRC.

Just remember some uni experiments to show this on Plynlimon.

But as that would tend to reduce splashing into the guage in heavy
rain, wouldn't it tend to increase the 'error' as shown by Chris's
results? Hmmm....

--
Dave

In article , says...
It's probably been discussed before, but...

The general idea is that rain gauges should be as flush with the ground as
possible to ensure that the catch is affected as little as possible by the
wind. Just to test this I put three idenitical TBR (Youngs) gauges at
rim-heights of 40cm, 100cm and 160cm above the ground. From 12/11/07 to
29/01/08 they collected 210.4, 213.2 and 216.5 mm respectively. Um...


I'm not sure what the question here is. The variability in precip over
even very small distances could explain this. We had a student here a
few years ago that was involved in a project where they put out two
identical rain gauges attached to each other and found differences of a
couple of percent, IIRC, in what landed in each gauge.

You might try putting them out at the same height in the same locations
as you used this time and see what variability there is.

Harold

--
Harold Brooks
NOAA/National Severe Storms Laboratory

On Jan 29, 3:13�pm, "Chris Kidd" wrote:
It's probably been discussed before, but...

The general idea is that rain gauges should be as flush with the ground as
possible to ensure that the catch is affected as little as possible by the
wind. Just to test this I put three idenitical TBR (Youngs) gauges at
rim-heights of 40cm, 100cm and 160cm above the ground. From 12/11/07 to
29/01/08 they collected 210.4, 213.2 and 216.5 mm respectively. Um...

Chris

Do the 3 gauges record identical amounts if they are all placed at the
same height above ground? Could the differences just be due to
instrument calibration?

Dick Lovett

Chris Kidd wrote:
It's probably been discussed before, but...

The general idea is that rain gauges should be as flush with the ground as
possible to ensure that the catch is affected as little as possible by the
wind. Just to test this I put three idenitical TBR (Youngs) gauges at
rim-heights of 40cm, 100cm and 160cm above the ground. From 12/11/07 to
29/01/08 they collected 210.4, 213.2 and 216.5 mm respectively. Um...

Speaking as someone with a science background (and not necessarily in
climatology or whatever), I would be performing the same test with the
same gauges but changing the order of heights. Repeating this over
several observations (of, say one month) and trying all combinations
will minimise the effect of any difference in calibration that the
gauges have. You may also have localised effects, so trying the
experiment in many different locations and analysing the statistics over
all places will help to minimise that effect over time.

What you can't do is to do it once and take one set of results as
verbatim. The variability of precipitation could make accurate
statistical analysis difficult though.

--
Jonathan Stott
Canterbury Weather: http://www.canterburyweather.co.uk/
Reverse my e-mail address to reply by e-mail

Thanks for the responses:

Articles by Ian Strangeway (formerly CEH), amongst others, suggest that
gauges should be flush with the ground - with a grid, as Steve suggests to
reduce splash-in. Failing that the gauges should be as aerodynamic as
possible.

The gauges here are the Young's 0.1 TBRs, and therefore are not terribly
aeordynamic. They are spaced 2m apart - so unlikely to be local effects
affecting the totals. Yes, the sensible thing would be rearrange them to
take into account any individual gauges inaccuracies... .

At the end of the day, as I've found elsewhere gauges close together can be
10-20% out quite easily: I believe the NWS recommend three gauges for each
site now...

Chris



"Jonathan Stott" wrote in message
...
Chris Kidd wrote:
It's probably been discussed before, but...

The general idea is that rain gauges should be as flush with the ground
as possible to ensure that the catch is affected as little as possible by
the wind. Just to test this I put three idenitical TBR (Youngs) gauges at
rim-heights of 40cm, 100cm and 160cm above the ground. From 12/11/07 to
29/01/08 they collected 210.4, 213.2 and 216.5 mm respectively. Um...

Speaking as someone with a science background (and not necessarily in
climatology or whatever), I would be performing the same test with the
same gauges but changing the order of heights. Repeating this over several
observations (of, say one month) and trying all combinations will minimise
the effect of any difference in calibration that the gauges have. You may
also have localised effects, so trying the experiment in many different
locations and analysing the statistics over all places will help to
minimise that effect over time.

What you can't do is to do it once and take one set of results as
verbatim. The variability of precipitation could make accurate statistical
analysis difficult though.

--
Jonathan Stott
Canterbury Weather: http://www.canterburyweather.co.uk/
Reverse my e-mail address to reply by e-mail

Just wondering.

Is rainfall such a uniform happening that you would really expect to get
exactly the same values in any rain gauges placed close to each other,
no matter how they were sited?

--
Mike LONGWORTH, Yateley, Hampshire, UK

Mike - the problem is, ultimately, are gauges really representative of the
rain falling over a particular area - is the sampling area of the gauge
sufficient to overcome the spatial variability of the rainfall? Although we
expect rainfall to vary over, for example, a few km, what is the variability
we can expect over a few m?

Chris



"Mike LONGWORTH" wrote in message
...
Just wondering.

Is rainfall such a uniform happening that you would really expect to get
exactly the same values in any rain gauges placed close to each other, no
matter how they were sited?

--
Mike LONGWORTH, Yateley, Hampshire, UK

On 30 Jan, 12:29, "Chris Kidd" wrote:

Is rainfall such a uniform happening that you would really expect to get
exactly the same values in any rain gauges placed close to each other, no
matter how they were sited?

I've got three gauges placed in a triangle about 1.5 m on a side, in
an open site (it's a MetO and EA registered rainfall site). They are a
standard 'five inch' MkII storage gauge and two Didcot Instruments
TBRs. The rim of the standard gauge is 30 cm above short grass, the
two TBRs are a little taller so their rims are at 45 cm.

All three are normally within 2 per cent of each other, the TBRs
tending to be slightly lower owing mainly to evaporation in the
bucket, etc. All have been recording for many years and (apart from
known transient mechanical problems, blockages etc) have never been
more than 4-5 per cent different in monthly totals.

When I made a slight site move in 2005 I operated another five-inch
gauge 10 m away from the current checkgauge for a 10 week period. The
two gauges differed by just 1 per cent over that period, which is well
within the tolerance you'd expect between adjacent gauges.

Based on my own observations, I've not found any significant evidence
of metre-scale rainfall variations between identically-exposed
instruments. If rainfall really was this variable, it would be
difficult to compile consistent and representative long-term averages
where gauges move slightly. Based on a number of years' experience
within the MetO rainfall branch, I know that is certainly not the case
and where significant differences occur between old/new sites these
could almost always be ascribed to significant site/exposure changes
or to changes in observational practices (such as a daily-read gauge
being replaced by a weekly-read one). I'd look very closely at any
real evidence of this supposed micro-gradient.

I don't dispute the variation with height, of course, and would refer
interested parties to the numerous multi-year side-by-side trials
conducted by the British Rainfall Organisation and the Met Office
between the 1870s and the 1930s and published in full in British
Rainfall - available from the National Meteorological Library.

--
Stephen Burt
Stratfield Mortimer, Berkshire