Basic answer is water vapour from the combustion of the hydrocarbon fuel burnt in the engine. The low temp at high altitude causes the vapour to condense - so it is a man made cloud.
All hydrocarbon fuels burn in air (combining with oxygen) to produce water (hydrogen oxide) and carbon dioxide.
At high altitudes a "contrail" or condensation trail is formed due to the combustion from the engines. Whether or not they form depends entirely on atmosphere conditions.
The white trail you see behind an airplane in the sky is called a contrail. It's an artificial cloud made by the exhaust of jet aircraft or from the wingtips.
Contrails
All aircraft engines emit hot air and they are usually flying in very cold air, so the hot air appears as a vapour trail.
Contrails or condensation trails
There are 3 different types of engines for a Nissan X Trail. However, all three engines are 4 cylinders.
The cloud that forms on a jet's wing when it changes direction is called a contrail. This is short for condensation trail, which is formed when the air pressure and temperature changes around the airplane wings cause water vapor to condense and form a visible cloud.
He was a little Indian boy who was in the trail of tears.
Red Cloud
Carbon monoxide and other left overs from combustion. However, the trail you see behind a plane flying high up is called the contrail, and it consists of water vapor that freezes behind the plane.
She died on the Trail of Tears. Samuel Cloud woke up to her cold, dead body.
An alpha particle leaves a short, thick trail in a cloud chamber due to its high ionization capabilities, which means it can ionize many atoms along its path, leading to a dense trail.