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