Method 1
LOG10 (barometric pressure at higher altitude) =
= LOG10 (barometric pressure at lower altitude) - ((altitude difference in meters between the two altitudes) / (72 * (256.4 + (mean temperature in °C of an air column with a height of the altitude difference in meters))))
Rearranged:
barometric pressure at higher altitude =
= 10 ^ (LOG10 (barometric pressure at lower altitude) - ((altitude difference in meters between the two altitudes) / (72 * (256.4 + (mean temperature in °C of an air column with a height of the altitude difference in meters)))))
If the lower altitude is sea level, then
barometric pressure at given altitude in mm Hg =
= 10 ^(2.88 - ((altitude in meters) / (72 * (256.4 + (mean temperature in °C of an air column with height of that altitude in meters)))))
barometric pressure at given altitude in atmospheres =
= 10 ^((-1) * ((altitude in meters) / (72 * (256.4 + (mean temperature in °C of an air column with height of that altitude in meters)))))
Method 2: Haldane and Priestley
barometric pressure in mm Hg at a given altitude =
= 760 * ((1 - (0.000006873 * (altitude in feet above sea level))) ^ 5.256)
Volume of Gas at a Given Altitude
Since (pressure of gas) * (volume of gas) is a constant:
volume of gas at the altitude =
= (760 / (pressure at altitude in mm Hg)) * (volume at sea level at 1 atmosphere pressure)
