In lieu of a genius, I'll attempt to answer 1)
The pressure lapse rate is proportional to air density (which in turn is proportional to pressure and inversely to temperature).
At sea level, the density is 1.225 kg/m^3, so the vertical column of air responsible for a pressure of 1mb (=100N/m^2 or 10.2 kgf/m^2) is <10.2/1.225>m = 8.3m or about 27'.
As you go up in altitude the density decreases, so the lapse rate decreases proportionately, or the change in altitude for a 1mBar drop in pressure will increase in inverse proportion.
At FL200 1mBar will be about 50' in altitude.
Last edited by Mark 1; 14th Dec 2005 at 14:23.