I work occasionally with a group at NASA that is trying to develop a protocol so that school kids can submit scientifically accurate data on condensation trails left by jet aircraft at high altitude, or contrails.
They have a web-page in progress that gives some idea of what the kids will have to identify. What kind of cloud is it? How wide is it? How long does it last?
This is part of a program that tries to identify clouds in general through volunteer observations. Since it is suspected that contrails have a role to play in cloud formation and therefore regulation of climate, contrail observation is important. There's a fairly good explanation here.
In the commercial flight-free days following the September 11 attacks, it was actually possible to detect a temperature difference over the United States from the lack of contrails. (ScienceNews magazine article, and ARAM X technical abstract.)
Here are some great animations of the hourly density for air traffic above Flight Level 250 on September 3, 2001 vs. the same for traffic above FL250 on September 11, 2001.
Of course, there are other theories about contrails. Many people believe that they are the result of a secret government program to spray chemicals on its citizens. You can begin that web-surfing oddyssey here or here.