Re: Why is it so cold?
- From: "Alan Gardiner" <alan.gardiner@xxxxxxxxxx>
- Date: Thu, 02 Feb 2006 15:13:52 GMT
"Tudor Hughes" <tudorhgh@xxxxxxx> wrote in message
news:1138890485.118882.229840@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
The temperature here is -1.5°C (1400 Z) having risen only
slightly from an overnight min of -2.1°C. No frost is visible of
course, there having been a full cover of St with no radiation and the
ground is not frozen at all. In fact from the recent soundings I'd say
the ground is heating the air and not vice versa because there is a
marked lapse rate for about 1500 ft below the big inversion and there
have been reports of light snow, probably due to radiative cooling of
the top of the cloud. But how has this shallow surface layer got so
cold? Is any dynamic effect involved?
Tudor Hughes, Warlingham, NE Surrey, 556 ft.
This is what Will Hand said yesterday.
"OK here we go!
Due to the low cloud top and fairly thin cloud and time of year (low sun
angle)
the short wave radiation from the sun could not overcome the longwave
radiation
from the cloud top, hence the cloud top cools. Weak convective overturning
within the cloud eventually cools the cloud base as heat is re-distributed
from
base to top and then out into space and possibly giving light drizzle or
snow
grains (as in Leeds). Cooling the cloud base chills the air as longwave
radiative fluxes from cloud base to ground decrease. (Cloud acts as less of
a
blanket).
Isn't physics wonderful!
Will."
Alan
.
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