I am not sure if you are asking for the exact formula, but in case you are,
it is:

Z' = R*T/g*ln(p1/p2)

Where Z' is the thickness of the layer in metres bounded by pressure
surfaces p1 and p2.
R is the universal gas constant (287) for dry air, T is the mean temperature
of the layer (in deg K),
and g is the gravitational acceleration standard (9.80665).

Also, for pressure levels where p1/p2 is close to unity, the approximate
form is probably close enough:

Z' = R*T/g*p'/p

where p' is the pressure difference between top and bottom of the layer, and
p is the mean pressure.

For a mean temperature of zero C, your factor would be 0.80 for Z' in DAM,
and for 20C it would be 0.86. (pressure difference of 1 mbar at 1000 mbar).







--
Bernard Burton
Wokingham, Berkshire, UK.

Satellite images at:
www.btinternet.com/~wokingham.weather/wwp.html
"Mr Blowman" S@not here.karoo.co.uk wrote in message
...
To calculate thickness values in DAM I have always used the eqution
(1000mB - SLP ) * 0.87 )+ 500 mB height. Is this correct as I cannot
explain the 0.87 constant factor.


Regards


Sean B