"John Hall" wrote in message
...
In article ,
Waghorn writes:
OK, so the WMO are a bit slow on this - but why use out of date
averages when talking to the UK public about the UK? Seems plain daft
to me.

Stoopid question-why use 30 yr averages.Why not extend the reference
period decadally as new data
becomes available?

Because our climate isn't constant. There therefore has to be a
trade-off between having a period long enough to iron out most of the
year on year random variations but one short enough to be
representative
of the current climate. Presumably it has been decided that 30 years
is
a reasonable compromise.

.... the following are extracted from the entry against 'Normal' in the
1939 edition of the Meteorological Glossary:

"Normal: the name given to the average value over a period of years of
any meteorological element .....
In the British Isles the present standard period for rainfall normals is
the 35 years 1881-1915; for sunshine and temperature, normals are
calculated for 30-year periods and are revised every 5 years. The period
1901-30 was adopted by the International Meteorological Organisation at
Warsaw in 1935 as a standard period for climatological normals. "

I believe that the WMO (IMO successor) then decided that the 'normals'
should be updated every 10 years.

Martin.