air is generally drier as altitude increases, particularly above a few thousand feet, because the atmosphere becomes thinner and holds less moisture due to lower atmospheric pressure and adiabatic cooling. While relative humidity might increase at certain altitudes where air cools and can hold less water vapor, the absolute amount of water vapor (absolute humidity) decreases significantly with elevation.
That is correct---but irrelevant. The relevant measure is relative humidity, which determines whether water vapor freezes or remains vapor. And that can be higher at higher altitudes, notwithstanding a lower absolute humidity. This is a result of the fact that high altitude air cannot sustain high water vapor levels---thus resulting in the formation of ice crystals (contrails).
air is generally drier as altitude increases, particularly above a few thousand feet, because the atmosphere becomes thinner and holds less moisture due to lower atmospheric pressure and adiabatic cooling. While relative humidity might increase at certain altitudes where air cools and can hold less water vapor, the absolute amount of water vapor (absolute humidity) decreases significantly with elevation.
That is correct---but irrelevant. The relevant measure is relative humidity, which determines whether water vapor freezes or remains vapor. And that can be higher at higher altitudes, notwithstanding a lower absolute humidity. This is a result of the fact that high altitude air cannot sustain high water vapor levels---thus resulting in the formation of ice crystals (contrails).