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Rain gauge heights
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 |
Rain gauge heights
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 |
Rain gauge heights
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 |
Rain gauge heights
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 |
Rain gauge heights
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 |
Rain gauge heights
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 |
Rain gauge heights
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 |
Rain gauge heights
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 |
Rain gauge heights
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 |
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