"Paul Hyett" wrote:

... can be a total PITA, as you can never tell what time of day or night
they will occur!

I've always kept separate 09-09 and 09-21-09 figures, and some
time ago I needed to tabulate them, so I now have 25 years' worth
of records for Luton using both methods. The figures below show
day max minus 24h max, and night min minus 24h min (degC):

Month MeanMin MeanMax
Jan -0.55 +0.29
Feb -0.42 +0.15
Mar -0.20 +0.07
Apr -0.05 +0.04
May -0.02 +0.02
Jun -0.03 +0.04
Jul -0.01 +0.02
Aug -0.01 +0.03
Sep -0.08 +0.03
Oct -0.16 +0.06
Nov -0.47 +0.19
Dec -0.60 +0.34
Year -0.22 +0.11

The biggest difference in mean min was -0.94degC in Jan 2001
The biggest difference in mean max was +0.66degC in Jan 1992

Philip