I keep seeing messages from people wondering why planes leave a white streak in the sky. Many go for conspiracy theories and say it's chemtrails. In reality we are talking about contrails, and below you have a brief explanation of the white streaks you can see in the sky after the planes pass.
Jet-powered airplanes leave white trails in their wake for the same reason you can sometimes see your breath. The hot, moist exhaust gases from jet engines mix with the atmosphere, which at high altitude has a much lower vapor pressure and temperature than the exhaust gases.
The jet engine removes carbon dioxide, sulfur and nitrogen oxides, unburnt fuel, soot and metal particles, as well as water vapor.
The water vapor contained in the jet exhaust can freeze, and this mixing process forms a cloud much like your warm breath does on a cold winter's day.
Depending on the altitude of an aircraft and the temperature and humidity of the atmosphere, contrails can vary in thickness, extent and duration. The nature and persistence of white trails can be used to predict the weather. A thin, short-lived streak indicates low-moisture air at high altitude, a sign of fair weather, while a thick, long-lived streak reflects moist air at high altitudes and can be an indicator of a thunderstorm.