On 3 Jul, 17:43, Paul Hyett wrote:

Why *do* AWS's under-record, BTW?

TBRs rather than AWSs (although most AWSs use TBRs) - mostly due to
evaporation from the open bucket surface, some frictional losses,
tendency to stick in tipped position in very heavy rainfall.

Many of today's cheaper shiny plastic models also tend to lead to the
formation of drops in the funnel rather than allow the water to drain
into the tipping bucket mechanism, further increasing evaporative
losses. (A rub-down with fine emery paper will often solve that
problem.)

Those factors, together with poor calibration and often a poor
exposure, mean they are very much more likely to under-read than over-
read.

Stephen
Stratfield Mortimer, Berkshire