I hope the article below will help to explain what the term means and how its value is arrived at.
The term DAM is used at times but its correct term is 'thickness' between the two levels in the atmosphere. Remember although its often referred to at the 1000-500mb level it can be used between any two levels. For snow forecasting the other most often used is the 1000-850mb values.
DAM heights or total thickness between two levels, usually the 1000mb and 500mb
I hope this may help (!) to show how complex is the relationship but also how relatively easy it is, knowing the two heights, to calculate the ‘thickness’. This can be done for any two heights.
The two most referred to, usually on Net Wx to do with the will it or won’t it snow, are the 1000-500mb and the 1000-850mb heights for ‘thicknesses.
Fortunately this has all been done for us by Paul and Karl with the charts shown below!
DAM is what refers to the 1000-500mb thickness chart.
Its rather complex but there are several ways to work out its value. Below are some of the methods which might help
= height (500 hPa surface) - height (1000 hPa surface) [ for those of you, like me, too old to catch up with all the changes the world brings, millibars = hPa!, so 500 hPa is exactly the same as 500 mb. ]
h(500) = h(1000)+h'(thickness). Or from that
Thickness can be calculated from the heights reported on a radio-sonde ascent, or a thermodynamic diagram can be used to add up the partial thicknesses over successive layers to achieve the net (total) thickness.
An example of the former would be
500 hPa height = 5407 m
1000 hPa height = 23 m
Thickness = 5407-23 = 5384 m (or 538 dam)
Careful note must be made when the height of the 1000 hPa surface is below msl thus: 500 hPa height = 5524 m
1000 hPa height = - 13 m
Thickness = 5524 -(-13) = 5537 m (or 554 dam)
Note the example above when surface pressure is BELOW 1000mb.
Roughly it is taken that 8mb is equivalent to 6DM when forecasters are manually drawing the various upper and surface charts.
If we take the actual msl and 500mb chart from GFS/Extra for 06Z this morning, see below
On the left is the surface isobar chart with the 500mb height; to its right is the ‘thickness’ chart
Notice the differences in values between the left and right charts-obviously the surface values are identical but NOT the ‘thickness’ and 500mb values.
Or to look at how the 00z ascent for Herstmanceux differs in its 500mb height and its 500mb ‘thickness’
In the basic data format the 500mb height was given as 500.0 5490 -22.9 -50.9; i.e. 5490DM; that of the 1000mb height was 1000.0 87 8.2 5.6
The ‘thickness’ is 1000 hPa to 500 hPa thickness: 5403.00
How is that arrived at, see the formula above
100mb height is 87
500mb height is 5490
Therefore 500mb ‘thickness’=5490-87=5403DM
Additional information on atmospheric thickness and it's use is available on the NOAA National Weather Service website: https://www.weather.gov/source/zhu/ZHU_Training_Page/Miscellaneous/Heights_Thicknesses/thickness_temperature.htm