Reply #60: Yup. [View All]

[AZCat]

I was just pointing out that even if a map of relative humidities shows low relative humidities for a particular location that doesn't mean that condensation won't occur because local phenomena (such as the passage of the aircraft) can change the local temperature or pressure. The genesis of vapor trails is different than from a contrail, but the process is the same. Whether the amount of water in the air increases or the capacity of the air to hold water decreases, water is precipitated out and a visible trail occurs.

As a quick aside, the vapor trail is a good example of an adiabatic process, one that occurs either so quickly or so slowly that there isn't any (significant) heat transfer from
the system to the surroundings. Another good example is a champagne bottle - when the cork is popped, the gas inside expands so rapidly that there isn't time to exchange heat with the surroundings, and the temperature sometimes drops enough for vapor to appear - not that different from a vapor trail at a wingtip of an aircraft.

The "shock egg" is pretty cool. There are a couple photos out there showing it.