Rising from the ground in an airplane, I fly into the bottom of a cloud. I can explain the bottom of the cloud as air temperature (which decreases with altitude) being decreased to the dewpoint.

I fly further up and the temperature decreases more. The dewpoint also decreases since water in vapor form is being removed as cloud droplets.

Then, when I fly up through the top boundary of the cloud (which is very clear in most of my flights), is the temperature still below the dewpoint? In other words, is water condensing on particulate matter outside the airplane even when above the cloud?

I believe condensation is still occurring above the cloud even though that seems to contradict my being outside of the cloud, so I am hoping someone can also explain this apparent contradiction.


1 Answer 1


Above clouds, you're generally seeing a different layer of air which, typically, isn't cold enough to allow the lower (thus denser) air mass to continue rising. The top of a cloud is often where the relevant air mass stopped rising and cooling, and condensation is not occurring above the cloud. The upper mass can be dryer, so that its temperature is above the dew point.

The temperature generally continues to decrease with altitude (though it can increase), but there is a minimum drop in temperature that's required for an air mass to keep rising, and the upper air mass can be colder but not enough colder for that. The equilibrium temperature profile is described by an adiabatic lapse rate.

  • $\begingroup$ So, you are saying that as I move up through the top of the visible cloud, the temperature usually continues to decrease, but the air abruptly becomes much dryer ("abrupt" because the air must be so dry above the cloud that water droplets do not form even though they form within the cloud at lower/warmer positions)? I don't think the textbook "adiabatic lapse rate" would apply here since the upper boundary of the cloud is usually sharply visible, but the 1D adiabatic model would smooth out the cloud boundary over kilometers. $\endgroup$
    – bobuhito
    Aug 7, 2023 at 23:14
  • $\begingroup$ Well, I guess your answer is more logical than my "condensation is still occurring above the cloud" thought since it avoids the contradiction I mentioned. It was just a gut feeling that the air could not be that dry up there (a rough calculation for -50C shows there must be about 200x less water in the air above the cloud, compared to normal sea-level 50% humidity 20C surface air), but I guess now that it might usually truly be that dry. $\endgroup$
    – bobuhito
    Aug 8, 2023 at 4:38
  • 1
    $\begingroup$ It's not that the boundary is smoothed out according to the lapse rate - it's that when the temperature change drops below the lapse rate, the air mass forming the cloud stops moving, which can create an abrupt change like that (though momentum may carry it a bit higher). The abrupt change is the edge between two separate air masses. $\endgroup$
    – damp_civil
    Aug 8, 2023 at 15:01
  • $\begingroup$ Ok, to help me visualize a typical example, I am thinking of that second air mass (without any cloudiness) as having come from even higher up where it was so cold that nearly all of its water was condensed/frozen out of it (and it therefore does have 200x less water content than normal surface air). $\endgroup$
    – bobuhito
    Aug 8, 2023 at 20:40
  • $\begingroup$ I'm not too familiar with how the background atmospheric conditions form in detail, but presumably that would be plausible, yes. It's also possible that the next layer up is warmer or of similar temperature. $\endgroup$
    – damp_civil
    Aug 14, 2023 at 16:53

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