One way or the other we would have noticed or seen white lines in the sky after a plane passes by which is known as Contrails. It is easily visible on a sunny day, when the sky is blue and clear. Let’s find out what Contrail is, and why some planes leave white trails and others do not?
Condensation trails or Contrails are “streaks of condensed water vapor created in the air by an airplane or rocket at high altitudes. It simply means that they are tiny clouds formed from water vapor which freezes around small particles from the aircraft exhaust. Some of the water vapor is from the air itself, while some comes from the aircraft’s own exhaust. Much like a car might make a little white cloud near its exhaust on a cold morning, a plane will often leave a trail if the conditions are right.
They could also be called clouds. The main reason behind their appearance is the temperature difference between hot humid air around a plane’s engine and low temperatures outside the aircraft. Whether or not condensation trails will form mainly depends on height and composition of the surrounding atmosphere. The atmosphere at high altitude is of much lower vapor pressure and temperature than the exhaust gas from a plane’s working engine. Besides water vapor, the exhaust also contains carbon dioxide, oxides of sulfur and nitrogen, unburned fuel, metal particles and soot, which provides condensation sites for the water vapor.
Since during flight at high altitude the temperature is about minus 40 degrees, the vapor condenses, turning into fog or small ice crystals.
Crystals evaporate more slowly than regular water. For this reason, the white plane trails remain in the sky for a very long time. At the same time, the higher the humidity of the climate and the colder it is outside, the longer, thicker and brighter the white stripes are.
Although contrails are all made of the same things and created in the same manner, observant skywatchers will know that they don’t always behave the same. Some aircraft leave long, defined contrails that persist for several hours after the plane has passed; others leave very short trails that disappear fast.
Sometimes, even in the same patch of sky, one plane will leave a trail and one will not, reason being that planes with hotter engines are less likely to form contrails, as the heat of the exhaust prevents ice from forming. Modern planes with more efficient engines burn hotter, but use bypass air to cool the exhaust, making them more likely to leave contrails in a wider range of situations.
The simplest reason why a plane makes a contrail or not is narrowed to the environment in which it is flying. When the plane is in wet air, it makes a contrail, when it’s not, it doesn’t.
On the other hand, in some northern regions where the air temperature reaches minus 50 degrees and below, a contrail trail from an aircraft can form even during takeoff or landing. (sources from simpleflying and aerotimes)