Are there any records for rain hours in any particular location in the
same way that sunshine hours are recorded?

Yes, of course; it's quite easy to analyse using AWS data too, and I
did this for a note in COL a couple of years back from which the below
is adapted. (It's easier to see with graphs ... ) Data was for my own
site in Berkshire for the 10 years 1994-2003.

Looking at the figures by month of the year, the percentage of hours
where 0.2 mm or more was recorded range from below 4 per cent for many
hours in July (a little over once per month on average) to 19 per cent
around dawn in November (about six days in an average month). Over the
year as a whole there is a very pronounced bi-diurnal variation in
rainfall frequency, with minima at 00h and 10h GMT and maxima at 04h
and 16h. These coincide exactly with the daily peaks and troughs of
the bi-diurnal curve in barometric pressure. The afternoon peak in
rainfall frequency is larger and broader than the overnight peak, and
is clearly the result of an increase in convective rainfall: there is
a particularly sharp increase in the frequency of rainfall between 12
and 13 GMT. The variation is significant - in the hour commencing 00h
it rained on 315 hours in 10 years, whereas at 16h it rained on 372
occasions, almost 20 per cent more frequently.

When it is considered that these figures relate to the chance of any
part of the hour receiving 0.2 mm of rainfall, and that more often
than not any rain that falls will not be continuous for the whole
hour, it can be seen that in southern England at least the chance of
intercepting measurable rain in a short period at any particular time
of day is quite small - about 3 per cent or once per month on average.
You're also likely to get wet less often in the morning than in the
afternoon.

When considering the diurnal variation in rainfall *quantity*,
however, there is a single daily broad afternoon peak. This is what
would be expected from an increase in convective rainfall at the time
of peak heating; the hour commencing 1500 GMT had, over the 10 year
analysis period, considerably more rainfall than any other hour, and
the rainfall totals for the hours 1300 to 1900 GMT are higher than for
any other hour in the day. There is a very sharp fall-off after 2000,
and little variation overnight - in fact, the overnight peak in
rainfall frequency is not evident at all in the rainfall amounts.

The average *intensity* of rainfall (mm/hr) for all those hours with
rainfall over the 10 year period varies significantly: although
rainfall is almost as frequent at 04h as it is at 16h, it is lighter -
typically 0.8 mm/h compared with over 1.1 mm/h at the afternoon peak,
an increase of almost 50 per cent.

So July mornings look to be about the best bet; November afternoons
I'd take the car, I think.


The figures will vary with location, of course, and the detail will be
different for South Wales, but it does give the general picture to be
expected in our climate zone. HTH.


Stephen Burt
Stratfield Mortimer, Berkshire