Question submitted by Matt Clarke
Hi there Matt,
Obviously you spend a bit of time watching the sky! Glad to hear it!
Contrails (short for "condensation trails") or vapour trails are visible trails of condensed water vapour made by the exhaust of aircraft engines. As the hot exhaust gases cool in the surrounding air they may precipitate a cloud of microscopic water droplets. If the air is cold enough, this trail will comprise tiny ice crystals.
Depending on atmospheric conditions, contrails may be visible for only a few seconds or minutes, or may persist for many hours. For the contrail to last a significant amount of time, the airmass at 35-40,000 feet must be very stable. This usually occurs when the upper atmosphere is significantly ridging.