Why do some planes leave a trail in the sky?
Some time plane in the sky leaves long trails, and it might wonder lot of people, putting them on making their own perspective for the cause.
It is certainly not smoke from the engine and the real term for trails is contrails “condensation trails”.
Contrail forms when the water in jet exhaust mixes with wet cold air, condenses and turns into ice crystals. They normally disappear within minutes if the air is dry, but can develop much longer 300 to 400 meters in length if air has high percentage of humidity. Contrails are indeed a type of cirrus cloud.
Conventional – 28,000 to 40,000 feet, temperatures’ usually having from around -36 to -76degrees is the contrails friendly altitudes to get produce. However, the engines of the airplane can be also the less common reason “contrails” either being produced or not. Some engines will leave a contrail in air were another engine will not.
Let us make vision more clear with coming example (part of a German test to study contrails formation)
An Airbus A340 on the left (maiden flight – 1991), leaving contrails and a Boeing 707 (maiden flight – 1957) not leaving contrails, regardless both flying at 33,000feet. As the exhaust of the newer engines of A340’s is at lower temperature making contrails in wider range comparing to Boeing 707 engines. such effect can be developed with engines at different power setting, precisely if it effects the exhaust temperature.
Source ( Photos and Information) : contrailscience.com and mentalfloss.com