Contrails (short for condensation trails) are condensed water vapor created when the hot exhaust of the jet hits the cold surrounding air, and the water vapor in the exhaust condenses into water droplets and/or ice crystals. They are an ordinary result of normal jet aircraft flying at high altitude, and they are normally very faint, and very short-lived.
Aerosol-Chemtrails on the other hand, a coined name used to distinguish them from normal contrails, are streams of synthetic chemicals and other man-made compounds, intentionally dispensed in long lines across the sky, often criss-crossing or going from horizon to horizon, typically from specially-modified aircraft with internal storage tanks and externally-mounted dispensing nozzles or orifices.
These lines in the sky persist and do not evaporate like normal contrails, and over a period of 15 to 30 minutes (depending on atmospheric conditions) disperse to form a cloudy haze across the sky. As the planes continue spraying, this eventually develops into a man-made hazy cloud cover which, among other things, alters solar radiation absorption in a localized area.
After confronting the reality of aerosol operations, it is a natural question to ask:
why? Who would want to alter the very air that we breathe? The evidence is now
clear that the aerosol operations are a covert activity, an operation that will never
ask for your consent, and that is being conducted on you and your family regardless
of your concerns. The reasons why do not appear to be restricted to a single
purpose. From all data gathered at this point, there could be said to be seven main
reasons why aerosol operations are being conducted.
1. Modification and control of weather and the environment
2. Electromagnetic operations
3. Military operations
4. Biological operations
5. Planetary and geophysical change
6. Sophisticated surveillance systems
7. Ionic disturbance detection
source : carnicominstitute.org.