Contrail

[nikolay-spb]

Hi all.

Why contrail doesn't form on high altitudes?

In game it forms somewhere on 8km, and stop to form above 14km.

In real world, contrail appears somewhere at 8km and disappears if aircraft (or rocket) is flying above ~15km.

As far as i know, contrail is formed when jet exhaust over-saturate surrounding air, and then as it cools, it turns into solid state (crystals). There is also a lot to read about that in internet.

But why it disappears on high altitude i cant find. My guess is that because of low pressure, ice crystals cannot form.

What do you think?

Edited April 21, 2016 by nikolay-spb

[SargeRho]

Contrail formation depends on the humidity of the surrounding air. They won't from in bone-dry air, while you get gargantuan contrails in very humid air.

[Snark]

My guess would be that when the ambient pressure is low enough, you don't get vapor deposition into solid crystals.

It's worth reading about cirrus clouds, since contrails are essentially just man-made cirrus clouds (and, indeed, the range of altitudes where cirrus clouds form is similar to the numbers quoted above for contrails).  The cirrus article linked above doesn't come right out and state directly that this is the reason, though the following quote seems like it may be impying it:

"Cirrus clouds are formed when water vapor undergoes deposition at high altitudes where the atmospheric pressure ranges from 600 mbar at 4,000 m (13,000 ft) above sea level to 200 mbar at 12,000 m (39,000 ft) above sea level."

[wizzlebippi]

The engine exhaust also needs to have the correct amount of soot and water vapor for contrails to form.  Stealth aircraft have contrail detectors to help them avoid throttle settings that could help the enemy spot them.

[K^2]

These are typical altitudes where relative humidity peaks. It all depends on weather conditions et cetera, of course, but generally, humidity levels off at about 10% once you reach ~15km, while temperature and pressure continue to drop, leading to a very fast drop of absolute humidity. All of this means that you have a sweet spot for condensation trails at about 10km, where air is both cold enough and humid enough to facilitate their formation.