|Jet contrails (a word combination of 'condensation trails') are human-caused clouds that form through condensation of
water vapor in the exhaust of jet engines to form ice crystals.
Water vapor is a natural
by-product of the burning of any kind of fuel, and the amount produced by jet engines are sometimes larger
than the cold, thin air of the upper troposphere can hold in vapor form.
As a result, some of the vapor condenses as water droplets, which then rapidly freeze into ice particles very similar
to high-altitude cirrus clouds.
If the air where the jet is flying has low relative humidity, then a contrail will not form because the extra water vapor produced by the jet engines is
not large enough to produce condensation. But if the if air has high relative humidity, then a contrail
forms, using up the excess water vapor that the air can not hold. Under these conditions, due to the large amount of jet aircraft traffic in some regions, the sky can be filled with jet contrails.
The contrails tend to spread out and become wider over time since the wind speed and direction changes somewhat over the vertical depth of the contrail.