In message
Norman Lynagh wrote:

On Wed, 30 Dec 2015 11:52:36 +0000, Alan White
wrote:

With 410.4mm so far this month, December 2015 is the wettest month we've
experienced since moving here in August 2002.


This is a factually correct statement and this month is generating lots of
similar statistics and it's very easy to see why, because such statistics are
very easy to produce. Using the calendar month is very easy from a clerical
point of view but it has no particular scientific basis. For recording
purposes
we merely use the calendar months for convenience. Equally legitimate
would be
to assume that the 'clerical' month started on. say, the 15th of each
calendar
month and this would produce different statistics. An added
complication is that
calendar months are not all the same length. A more useful statistic for
comparison purposes would be the running 30-day rainfall totals
starting on any
day of the month.

I've had a trawl through the rainfall measurements for Tideswell for
the period
from 1st Jan 2009 to date. The wettest calendar month (starting at
0900z on the
1st) was December 2011 with 224.4 mm. This, of course, covers a 31-day
period.
(December 2015 will have a much lower total). The highest 30-day total
startiing
on any day of the month was 233.9 mm in the 30 days ending 0900z on 3rd Dec
2015. This is 4.2% higher than the total for the highest calendar month. If I
chose to start the 30-day period at any hour rather than just 0900z
then I might
find a slightly higher 30-day total but it's not so easy for me to do
that as my
daily measurements are made using a standard 5" manual rain gauge.

The next highest 30-day total (09z-09z) was 233.0 mm in the 30 days to 13 Dec
2015 and after that is 232.8 mm in the 30 days to 4 Dec 2015.

It's almost certain that November/December this year will produce
record-breaking rainfall statistics for many locations in the British
Isles but
the statistics will have more legitimacy if they are not constrained to the
calendar months.

I was surprised that that highest 30-day total for Tideswell was only 4.2%
higher than the highest total for a calendar month. I'd be interested
learn how
the statistcs for other locations compare.

I have 66.3mm so far for this month in Leicester. Not much more than
half the rainfall of December 2012. In fact 2012 had 5 months that
were wetter than December 2015. That's calendar months of course.

Martin


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