Take the sea-level pressure. Subtract 1000. Multiply what's left by by
0.8. This is the height of the 1000-mb level in decametres. (It could be
negative). Subtract it from the 500 mb height. There's your thickness in
decametres.
The factor of 0.8 is not exact and depends on the pressure and
temperature. 0.8 is exact at 4°C and 1013 mb SLP with dry air. Strictly
speaking it is proportional to the temperature (in K) and inversely to the mean
pressure of the air between sea level and the 1000 mb level i.e. half their
sum). Ignore these niceties; 0.8 will do. Some of the figures quoted in
another thread are too high.

Tudor Hughes, Warlingham, Surrey.

The problem with 1000-500mb thickness is that it is not very reliable
as a snow predictor. I think most professionals would even not rely on
1000-850mb because of the times when warm air between 3000 and 5000ft
is undercut by very cold air.
The wet bulb freezing level is probably the most accurate predictor.

"TudorHgh" wrote in message
...
Take the sea-level pressure. Subtract 1000. Multiply what's left
by by
0.8. This is the height of the 1000-mb level in decametres. (It could be
negative). Subtract it from the 500 mb height. There's your thickness in
decametres.
The factor of 0.8 is not exact and depends on the pressure and
temperature. 0.8 is exact at 4°C and 1013 mb SLP with dry air. Strictly
speaking it is proportional to the temperature (in K) and inversely to the
mean
pressure of the air between sea level and the 1000 mb level i.e. half
their
sum). Ignore these niceties; 0.8 will do. Some of the figures quoted in
another thread are too high.

Tudor Hughes, Warlingham, Surrey.






Thanks Tudor, thats much appreciated.


Merry Christmas


Sean B

I liked this Canadian summary of precipitation-type algorithms.
http://cimms.ou.edu/~cortinas/preprints/canada6.html

BTW I have the feeling that the Americans write about "precipitation
type" when they mean "snow or rain" while the British use more often
"water phase" or "precipitation phase". Is this just due to small
samples or is there a difference in this between Queen's English and
Bush English ?

Elena


wrote:
The problem with 1000-500mb thickness is that it is not very reliable
as a snow predictor. I think most professionals would even not rely on
1000-850mb because of the times when warm air between 3000 and 5000ft
is undercut by very cold air.
The wet bulb freezing level is probably the most accurate predictor.