# Contrails/Vapor on plane wings? confused

1. Nov 25, 2009

### eyenkay

In the case of a could of vapor forming on the top of a planes wing, I understand that this depends on the relative humidity, and that due to the drop in pressure above the wing (bernoulli..lift..etc), there is a drop in temperature. It is said that the drop in temperature brings the water in the air to its dew point, so as to condensate. All this make sense, and I believe it is the correct explanation, I am just confused since if we were to increase the pressure (compress the gas), this should also lead to the water condensating out.

My question is, since there is both a drop in pressure and in temperature, why is it that the temperature change trumps the pressure change?

Does this have to do with compressibility factor of air since it is not ideal (I think its very close though)

Does this have to do with a change in density of the air above the wing (the gas is at a lower pressure not due to expansion as in the usual case, but due to its speed, from bernoulli) ?

Maybe the fact that since the decompression happens very quickly, we can consider the process adiabatic?

Last edited: Nov 25, 2009