When we consider the dam air values we are talking about the thickness of
the layer between 1000mb and 500mb. It is measured in decimeters (tens of
metres) such that 10dm (or dam) = 100m or the famous 528dm = 5280m (or just
over 5 kilometres).

The thickness of the layer is then the height of the 500mb surface minus the
height of the 1000mb surface.

If we take the example where the surface pressure is 1000mb then the height
of the 1000mb surface above the ground is zero. The thickness can then be
read straight off as the height of the 500mb surface, such as the area
neatly circling Denmark on the map you have linked. He

(Height of 500mb) - (Height of 1000mb) = (thickness of layer)
(520dm) - (0m) = (520dm)

In other parts of the map where the surface pressure is not as convenient we
have to calculate the height of the 1000mb surface off the ground. We do
this using a rough version of the hydrostatic balance equation:

height of 1000mb surface = (surface pressure - 1000mb)*100 / (11.8)

For example, at the high centre on your map over the Atlantic:

height of 1000mb surface = (1040mb - 1000mb)*100 / 11.8
= 4000 / 11.8 = 339m = 34dm

Carrying on to find thickness...

Thickness = Height of 500mb - Height of 1000mb
570dm - 34dm = 536dm

Hope this helps,

Jeremy

PS Alternatively you could find a map that shows the lines of thickness!
They are around somewhere but I'm afraid I don't know where to find them.



"Buzz" wrote in message
...
Can anyone tell me how to calculate dam air values from 500mb heigth
charts,
for instance :-


http://www.wetterzentrale.de/pics/Rukm1441.html


Thanks in advance.


Regards


Sean B