Contrails, are water condensation from internal combustion engines....normally form, above 20,000 feet for aircraft, or very cold days from automobile engines...
quote:
Condensation from engine exhaust
The main byproducts of hydrocarbon fuel combustion are carbon dioxide and water vapor. At high altitudes this water vapour emerges into a cold environment, and the local increase in water vapor can push the water content of the air past saturation point. The vapour then condenses into tiny water droplets and/or deposits into ice. These millions of tiny water droplets and/or ice crystals form the vapour trail or contrails. The vapor’s need to condense accounts for the contrail forming some way behind the aircraft’s engines. At high altitudes, supercooled water vapor requires a trigger to encourage deposition or condensation. The exhaust particles in the aircraft’s exhaust act as this trigger, causing the trapped vapor to rapidly turn to ice crystals. Exhaust vapour trails or contrails can only occur above 8000 meters (26,000 feet), and only if the temperature there is below -40°C (-40°F).[3]
http://en.wikipedia.org/wiki/Contrail
I’m well aware of what contrails are, thanks. I have studied them comprehensively over many years in the battles with chemtrailers.
It’s important to remember that water vapor is invisible, and that when the ice crystals form at high altitudes it is a sublimation process. That is, the water vapor sublimes directly to ice and then directly back to water vapor. It never goes through the intermediate liquid water stage.
Your wiki article is a bit incomplete. Contrails can actually occur at all altitudes with varying levels of persistence and they can form at various distances from the engine from immediately after to well after. Numerous photos show this.
Here’s a cool WW2 study:
http://naca.central.cranfield.ac.uk/reports/1942/naca-wr-l-474.pdf