"Alan" wrote in message
...
On 7 Dec, 19:35, "comp.john" wrote:
On 2009-12-07, John Hall wrote:



The 12:00 ECMWF run is now out. The operational run wants to direct the
coldest air from NE to SW just to the east and south of us, but as
that's at days 9 and 10 it might easily change.

How does 850mb temp correspond with surface temp at this time of year?
is -5 @ 850mb 0C at the surface?
--
comp.john

One of the experts on here will probably give you a much better
answers, but assuming no temperature inversion, typically -5 @ 850mb
would equate to about +4 degrees at sea level, for air which has come
of the ocean. If the air comes direct from the continent then things
are more complex, there’s likely to be an inversion this time of year,
but if the air is bone dry, in theory it could be as high as +10C, but
highly unlikey.
============

-5C is normally cold enough for rain to turn to snow at sea level and for
definite snow above 200 metres.
-7C is cold enough for snow at all levels. But as Jon and John have said it
is a lot more complex than that really as it depends on the actual
temperature structure below that level.

Will
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