In message . com, Ken
Cook writes

Jonathan Stott wrote:
Am I right in thinking that if the temperature is below freezing at 9am
(when observations for one day end) then it would count as two days with
air frost rather than one?

--
Jonathan Stott
Canterbury Weather: http://www.canterburyweather.co.uk/
Reverse my e-mail address to reply by e-mail

Hi, Jonathan,

If you are working to Met O climatological station rules of 24 hour,
09Z - 09Z obs, then that is correct. If you are working to 00Z - 00Z,
then the temperature at midnight would be the deciding factor, or 18Z -
18Z at 18Z and so on, depending on the obs hour.

It is a misleading reading, especially if the following night is mild,
as this one will be, but unavoidable at the majority of climatological
stations that are visited only once each day at 09Z. As far as the
record books go the coming night will have an air frost whatever
happens and the max for today will probably be at 09Z tomorrow morning
(thrown back)!

A more accurate 12 hour method is used at some synop. sites where the
max refers to 09Z - 21Z and the min 21Z - 09Z.

Even the 12-hour method isn't foolproof. It occasionally happens that
the 24-hour min occurs sometime between 0900z and 2100z and would
therefore be missed by the 12-hour method. Similarly, the 24-hour max
occasionally occurs between 2100z and 0900z.

There's no perfect method. What is important is to have a system and
stick to it but if yopu want your data to be comparable to that from the
majority of other stations then the 0900z-0900z system is the one to
use.

Norman.
(delete "thisbit" twice to e-mail)
--
Norman Lynagh Weather Consultancy
Chalfont St Giles 85m a.s.l.
England