"Adam Lea" wrote ...
snip
Out of interest, typically what surface temperatures, daytime and
nighttime would you expect for, say, a 510dam thickness assuming
clear skies? Can't remember experiencing a thickness that low
before.


.... In the January 1987 severe cold spell, when thickness values
dropped below 500 dam (probably, briefly, around 495dam), then
widespread day maxima below -6degC were recorded across southern
Britain .. (12th).

I developed a diagram he-
http://weatherfaqs.org.uk/node/155

which attempted to relate theoretical maxima to total thickness; this
was based on various papers written back in the 1950s & 1960s when
this method of forecasting extremes was all that was used ... things
have moved on, and in particular, we now use algorithms based on
vertical slices of the atmosphere below 1500 m, and in these extreme
cases (i.e. very low thickness values), the lowest few tens of metres
of troposphere often becomes 'uncoupled' from the general air mass and
both maxima and minima can depart wildly from any relationship
developed on a layer meaned over 5km. See also the notes at the bottom
of the referenced/linked page.

On the continent last winter (09/10), a cold pool [defined by the
500-1000 hPa thickness] of ~510dam drifted across southern Poland and
parts of Germany (Saxony, Thuringia etc.) 18th/19th December 2009:
remembering that this area is in the middle of a continent, and was
already well-chilled/snow-covered, the typical day maxima were -14 and
night minima -17 (over the lower ground). In our part of the world,
given the more 'maritime' character of our weather, I wouldn't expect
anything like such low values, at least not for the maxima; minima, as
pointed out above, behave completely differently, especially over
snow-covered surfaces.

Martin.


--
Martin Rowley
West Moors, East Dorset (UK): 17m (56ft) amsl
Lat: 50.82N Long: 01.88W
NGR: SU 082 023