On Sun, 12 Jul 2009 at 08:35:11, Alastair
wrote in uk.sci.weather :

In case there is anyone else, like me

And me...

, who does not understand the
significance of the 564 DAM, I found the following on the Met Office
web site:

Thickness lines
Pressure decreases with altitude, and thickness measures the
difference in height between two standard pressure levels in the
atmosphere. It is proportional to the mean temperature of this layer
of air, so is a useful way of describing the temperature of an
airmass.

Weather charts commonly show contour lines of 1,000-500 hPa thickness,
which represent the depth (in decametres, where 1 dam = 10 m) of the
layer between the 1,000 hPa and 500 hPa pressure levels. Cold, polar
air has low thickness, and values of 528 dam or less frequently bring
snow to the UK. Conversely, warm, tropical air has high thickness, and
values in excess of 564 dam across the UK often indicate a heatwave.
http://www.metoffice.gov.uk/weather/uk/guide/key.html

Thanks, that was very useful.

What happens when 1,000 hPa (or lower) is the sea-level pressure,
though?
--
Paul Hyett, Cheltenham (change 'invalid83261' to 'blueyonder' to email me)