On Fri, 1 Jan 2016 08:17:01 -0800 (PST), Scott W wrote:




Because I'm 09-09 here in Wanstead December was technically not frost free. Temp fell to -0.2C at 0639. Obviously lots of 'what ifs' but I'm just keeping it consistent with the old stats at Greenwich which were 09-09.
The best argument I've seen for 09-09 is that most observers are not around at midnight to read the manual instruments.

Scott, If you are using the standard 09-09 reporting the minimum read at 0900 is
credited to the day on which it is measured. Therefore, the 24-hour minimum
measured at 0900 this morning (1st Jan) is the minimum for 1st Jan. The min of
-0.2 at 0639 this morning therefore 'belongs' to 1st Jan.

In contrast, the 24-hour max temp and the 24-hour rainfall measured at 0900 are
'thrown back' to the previous day. Therefore the 24-hour max temp mearured at
0900 today 'belongs' to 31st Dec, as does the 24-hour rainfall measured at 0900
today.

I'm not saying that this is necessarily the best system but it is the standard
system used at 'official' observing stations and it does enable comparison with
those stations.