http://www.scientificamerican.com/article.cfm?id=why-do-jets-leave-a-white(That's right. This is where you say "ScientificAmerican" is part of the NWO conspiracy)


Why do jets leave a white trail in the sky?
November 19, 2001

Jenn Stroud Rossmann, an assistant professor of engineering at Harvey Mudd College, provides this explanation:
Jets leave white trails, or contrails, in their wakes for the
same reason you can sometimes see your breath. The hot, humid exhaust
from jet engines mixes with the atmosphere, which at high altitude is of
much lower vapor pressure and temperature than the exhaust gas. The water
vapor contained in the jet exhaust condenses and may freeze, and this
mixing process forms a cloud very similar to the one your hot breath
makes on a cold day.
Jet engine exhaust contains carbon dioxide, oxides of sulfur and
nitrogen, unburned fuel, soot and metal particles, as well as water
vapor. The soot provides condensation sites for the water vapor. Any
particles present in the air provide additional sites.
Depending on a plane's altitude, and the temperature and humidity
of the atmosphere, contrails may vary in their thickness, extent and
duration. The nature and persistence of jet contrails can be used to
predict the weather.
A thin, short-lived contrail indicates low-humidity air at high
altitude, a sign of fair weather, whereas a thick, long-lasting contrail
reflects humid air at high altitudes and can be an early indicator of a
storm.
The mixing gases contained in the contrail rotate with respect to
the ambient air. These regions of rotating flow are called vortices.
(Any sharp surface, such as the tip of a wing, can cause vortical flow
in its wake if it is sufficiently large or the flow is sufficiently
fast.) On occasion, these trailing vortices may interact with one
another.

In one well-known example of this fact, the Crow Instability causes
the vortices to develop symmetric sinusoidal oscillations and eventually
to merge and form vortex rings behind the jet. This instability can be
triggered by turbulence in the surrounding air or by local variation in
air temperature or density, which may itself be the result of the
stratification of the atmosphere. When the contrails are visible and
strong, it is possible to see the white streaks become wavy and then
leave rings floating high in the sky, like smoke rings from a giant
cigar.
Recent research has suggested that the ice clouds contained in contrails cause greenhouse effects and contribute to global warming
as part of the insulating blanket of moisture and gases in the
atmosphere. Researchers in this area seized on the opportunity
presented on September 11 and 12 over the U.S. The complete cessation of
commercial air traffic offered a control sky without contrails for use
in quantifying the environmental effects of contrails.