| Contrails are the cones of white vapor seen behind jet planes with altitude over 20,000 feet. They're visible mostly because they are made up of water vapor which has frozen due to the high altitude the jets fly at. Think of contrails as the breath you see on a cold winter's day, if it was moving five orders of magnitude faster and had that much greater volume. Crystallization is made possible by the soot and fuel particles released by the jet along with the carbon dioxide and water that result from combustion. Were the jet exhaust made up solely of water vapor and CO2, crystallization wouldn't happen very much, and the trails' water would be invisibly dispersed into the air.
Since the exhaust is moving at a very high speed and has such a wildly different temperature than the surrounding air, it forms its own turbulent and self-maintaining system. The gases rotate around each other at high velocity, creating the perfect trails we see from the ground. Depending on the ambient temperature and humidity contrails can look very different from day to day, and can be used to (poorly) predict the weather. Thick, fluffy contrails indicate the high-altitude air is humid, which may mean a storm is brewing in the atmosphere. Thin, pale contrails that fade quickly mean the humidity is low, indicating stable weather.
Researchers have suggested that jet exhaust may add significantly to the blanket of moisture in the upper atmosphere, and thus contribute to global warming. Over September 11 and 12 of 2001, these researchers were able to view the effects a complete lack of contrails had on the atmosphere, since commercial air traffic was grounded. Research results from this unprecedented control condition should be published shortly. | 
