On Dec 17, 7:06*pm, "Philip Eden" philipATweatherHYPHENukDOTcom
wrote:
"Pete L" wrote:
Just looking at the figures on the above link.....

Can anybody explain how and why two significant decimal places are
considered? As far as I recollect an ordinary mercury thermometer is
accurate to 0.2 deg. So to suggest global mean temperatures are 14.31
degs seems totally meaningless. I would have though just 14 degs is
enough without trying to invent a greater accuracy.

Pete, the fact is that the 14.31 is not a real temperature. It
involves not only temperature, but also time and space. It is,
perhaps, best to regard it as an index of planetary warmth or
coldness over a given period. Some might even argue that it serves
only to confuse that it looks like a temperature, and that giving it
a unit (i.e. °C) adds to the confusion. The accuracy of individual
thermometers and the precision to which they may be read
are neither here nor there.

Philip

I really don't quite get this. The 14.31, whatever its accuracy,
is as real a temperature as any other mean, eg the mean temperature in
Nether Wallop in May 1955. There is no other way of expressing it.
The accuracy quoted is genuine only if both the errors of the
thermometers and the errors of reading them are symmetrically
distributed about zero. If there is a systematic bias in the
thermometers the final average will be in error by the same amount
whatever the number of readings. On the other hand one could argue
that if there has been a systematic bias in the either the
thermometers or the observers' readings and over the years neither of
these had changed then the quoted figure is OK because its sole
purpose is one of camparison over time. In any case the inaccuracies
introduced by inadequate sampling over the globe must exceed any of
the above errors.

Tudor Hughes, Warlingham, Surrey.