"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.

Decrease in pressure with height is 1hPa per 0.87 dam, roughly.