Philip Eden wrote:

"Stuart Brooks" wrote :

For many years figures from Met Office sites were based on
0900-2100 max and 2100-0900 min (these are still calculated from
NCM reports) precisely to avoid singleton cold nights counting
twice. Average differences between monthly averages worked out
from 09-21/21-09 stations and 0909 stations were as much as 0.7C
for December minima, though depending on the character of the
month the figure could be even higher.

The standardisation on 0909 was presumably to make it easy for once
a day stations, though there is no ideal time of day for a reset
that would be valid all year.

In the Monthly Weather Report, MO synoptic (and auxiliary synoptic)
stations were tabulated with 09-21-09 terminal hours from 1945 to
1983 inclusive, but used 09-09 from 1984 until it was so rudely
terminated in 1993.

Before 1945, synoptic stations (or whatever they were called then)
used terminal hours of 18 and 07 (or in some instances 06), but
climatological stations have always been predominately 09-09 though
there were more exceptions in earlier years (some 07-07, 06-06,
18-18, for instance).

Clearly, 09-09 has to be accepted for conventional practice, although
my opinion is that 09-21-09 gives a better feel for the climate of a
particular place. However, there are are still disadvantages with
09-21-09 .... you have to make special arrangements (or be very
careful) when tabulating highest maxes and lowest mins for the
month because these can occur at night (for maxes) and by day
(for mins). Also, a night minimum can easily occur between
sunset and 21z ... something I've noticed very frequently here at
Whipsnade during this particular winter.

Philip

Also here at Chalfont St Giles, Philip. Last night, for example, the
overnight "low" of 11.7 occurred at 1905z.

--
Norman Lynagh
Chalfont St Giles, Buckinghamshire
85m a.s.l.
(remove "thisbit" twice to e-mail)