Well, I cannot tell you the exact number, but here is a site that shows a live radar of all the aircraft in the US flying. From the airlines to the general aviation aircraft.
http://flightaware.com/live/
Gas comes out of airplanes, which causes air pollution, and sometimes changes the sky color.
The sky
A formation
The annual average number of airplanes in the air can vary greatly depending on the time of day, day of the week, and season. However, on average, there are estimated to be around 9,700 planes airborne at any given time globally.
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'.
The engines create smoke .
There where three plains that where ordered out of the sky on 9/11.
Some people have drawn things in the sky with airplanes, some gas or smoke but its only temporary.
-- stars -- meteors -- airplanes.
exhaust
there are 1,ooo,oo1 airplanes