First of all, its not smoke. Its Vapor! Since its so cold when you fly at 35,000FT, and the engines are so hot, you get vapor.
The engines create smoke .
Some people have drawn things in the sky with airplanes, some gas or smoke but its only temporary.
The mysterious appearance of black lines in the sky could be caused by phenomena such as contrails from airplanes, smoke from wildfires, or atmospheric conditions affecting the way light is scattered.
The sky
In the sky, you can see clouds, the sun, the moon, stars, and sometimes airplanes.
A formation
The trail left behind in the sky is actually ice, when the air is so cold its below freezing the vapor turns into ice so quickly that it skips the liquid stage and creates a trail in the sky.
Those lines are created by aircraft flying at high altitudes releasing water vapor in the form of contrails, which are frozen water droplets condensing around particles in the exhaust. These contrails are most commonly seen behind jet engines on airplanes.
Sky magazine is published by delta airlines as a service to their customers. I have found no connection between sky online and sky magazine.
Condensation trails or 'con trails'.
Smoke trail is the trail of smoke left behind by an aircraft as it flies in the sky.
There is 4 syllables in "line high is sky". The syllables are line-high-is-sky.