Sept. 11 2001 gave a unique opportunity to test for the speculated effect of contrails on climate. In the days following the attack, the air over the United States was practically devoid of aircraft, and hence of contrails. An examination of temperature statistics seems to indicate that the overall temperature is not changed, but difference between night and day temperature is decreaded. This is consistent with the effect of a light cloud layer.
Persistent contrails come from the water vapour in the exhaust of fuel-burning engines. It requres certain atmospheric conditions to form and the time the contrail persists can vary from les than a minute to all day, even to growing into a band of clouds.
The conditions for contrails usually exist in altitudes above 10,000ft, but can vary greatly, both with altitude and weather conditions. So you will be able to watch one plane make a contrail that dissapears even as the plane crosses the sky, and another, apparantly in the same altitude, makes one that lasts for hours. Only a few thousand feet of altitude difference can make it.
All traces you see in the air can be explained by the contrail phenomenon. There is no reason to invent chemtrails.
Hans