Keith (Southend)G wrote:

On Dec 24, 6:47*pm, "Norman" wrote:
Keith (Southend)G wrote:
It doesn't look as threatening as the weather we have had the last 5
weeks, but I was just looking at 2m 12z temperatures on GFS and it
barely gets above freezing right through to T+384, in fact it appears
to drag somewhat colder air in from the east / SE *as time goes on.
The yellow *thickness colours lead you to think things are going to
much milder, but I guess the continent is not going to warm up this
time of year so the lower layers are going to be cold, especially
given all the snow over northern Europe.

Keith, are you looking at thickness or 500 mb height? Also, under
anticyclonic conditions the 1000-850 mb thickness will give a much better
guide to the coldness of the lowest layers than the 1000-500 mb thickness.

--
Norman Lynagh
Tideswell, Derbyshire
303m a.s.l.

Hi Norman,

I was looking at the 500HPa
http://www.wetterzentrale.de/pics/Rtavn2401.png

Keith (Southend)
http://www.southendweather.net
"Weather Home & Abroad"

The background colours on that chart show the height of the 500 mb surface, not
the 1000-500 mb thickness. To work out the thickness you have to take account
of the surface pressure (and temperature if you are trying to be precise). In
very round numbers, close to ground level the pressure falls 1mb with roughly
every 8 metres of height. Take, for example, a spot where the surface pressure
is 1035 mb and the height of the 500 mb surface is 552 dam. The height of the
1000 mb surface will be about 28 dam (35 x 8 metres) so the 1000-500 mb
thickness will be 524 dam (552-28).

--
Norman Lynagh
Tideswell, Derbyshire
303m a.s.l.