Temp and Height change per Millibar

[Cron]

I have seen mention of a '4% rule' in meteorology notes relating to the relationship between temp and height change per millibar:
eg 1mb = 27' at sea level, 73' at 30,000.

I would be grateful for some explanation of this. Thanks in advance.

[tom775257]

To find the value of 1mb pressure change at a specific alt in feet

96 x T
-------------
Pressure

Where T is temperature in Kelvin, Pressure is in millibars.

so at MSL (ISA):

96x288K
---------- = 27ft/mb
1013mb

At 20,000ft

96x 248K
--------- = 51ft/mb
466mb

At 30,000ft

96x 228K
---------- = 73ft/mb
300mb

Remember pressure does not lapse in a linear fashion with height.
All the best,
Tom. :ok: