|Jet contrails (a contracion of 'condensation trails') are man-made clouds that form through condensation of
water vapor in the exhaust of jet engines into ice crystals.
Water vapor is a natural
by-product of the burning of petroleum-based fuels, and the amounts 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 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 is air has high relative humidity, then a contrail
forms, using up the excess water vapor that the air can not hold.