Actually *decameters* and the simbol is *dam*

dm is decimeter, that is 1/10 of a meter.



Or "decametre" and "metre" as we call them on this side of the pond.
It's easy to get the thickness. Take the surface pressure, subtract 1000
and multiply what's left by 0.8. Thus 1020 mb gives you 16, and 980 would give
you -16. Subtract this from the 500 mb height( in decametres). You now have
the thickness to sufficient accuracy. The GFS charts show thickness under the
heading "Mittl. Wolken" (mean cloudiness) but only every 18 dam. As has been
said, thickness lines are useful, but not the be-all and end-all.

Tudor Hughes, Warlingham, Surrey.