It's more than just an academic exercise: the number of instances of 'X'
(whatever its called) is related to the departure of the 12hr average
extremes from the 24hr average extremes. For example, this month
(January) using Hurn data, the current 24hr average minimum is 2.9
against 4.0 for the 18-06 mean & the 24hr average maximum is 10.9 vs.
10.4 for the 06-18Z. We've now had 9 such events so far this month.

In a more 'typical' month, with no, or just one or two, events then the
difference is close to zero, and not usually more than 0.2degC.

Martin.

--
West Moors / East Dorset
Lat: 50deg 49.25'N, Long: 01deg 53.05'W
Height (amsl): 17 m (56 feet)
COL category: C1 overall

Martin,

I managed to 'dig out' an old copy of a Met Office 'Climatological
Memoranda' dealing with mean and extreme temperatures over the UK. It
has a section dealing with the differences between 24 hour (09-09) max
& min temperatures and 12 hour (09-21) day max & (21-09) night min.
The results, following an investigation of 38 stations between 1957
and 1970, show that, on average, in January and December the 24 hour
Max is 0.3 to to 0.4C higher than the 12 hour day max. The differences
for minima show that 24 hour minima were typically 0.7C lower than the
12 hour (21-09) value during the same two months.

Dick