Thickness: what is it?
'Thickness' is a measure of how warm or cold a layer of the atmosphere is, usually a layer in the lowest 5 km of the troposphere; high values mean warm air, and low values mean cold air. It would be perfectly feasible to define the average temperature of a layer in the atmosphere by calculating its mean value in degrees C (or Kelvin) between two vertical points, but an easier, practical way to measure this same mean temperature between two levels can be gained by subtracting the lower height value of the appropriate isobaric surface from the upper.
Thus one measure of thickness commonly quoted is: height (500 hPa surface) - height (1000 hPa surface)
The 500-1000 hPa value is used to define 'bulk' airmass mean temperature, and can be seen on several products available on the Web.
For more information see here.
(see also here for typical figures, extremes etc.)