One of the many bogus conspiracy theories speaks of so-called chemtrails.
Chemtrails are actually contrails. Contrails (condensation trails) are water vapor caused by both jet engine exhaust and the Bernoulli effect around a plane’s wings as air is rapidly decompressed and cooled, causing water to condense (due to the laws of Thermodynamics having to do with pressure and temperature). Basically, contrails are man-made clouds (water vapor only) and they are often quite beautiful. It is a pity that some people are naive and ignorant about the true nature of this phenomenon. They are living their lives in needless fear instead of marveling at what is a beautiful thing created by a combination of technology and nature. Contrails are created by a plane moving through the atmosphere under the right atmospheric conditions. You will tend to notice them on days where there are already cirrus clouds being formed due to the same atmospheric conditions but it is possible to see them on clear days if the amount of water in the air at high altitude is just right.
There are two ways contrails form. One is from jet engine exhaust that contains water. Have you ever noticed water dripping from an automobile exhaust on a cold day? Same thing. Warm air holds more water than cold air. As air cools, the evaporated water in the air will condense. (That is one of the reasons that rain is more likely as night falls and the atmosphere cools.) One of the byproducts of combustion engines, including jet engines, is water in the exhaust. As the exhaust leaves the jet engine, it cools so the water in the exhaust will condense into water vapor (a cloud). On some days at some altitudes, that exhaust water is fully absorbed by the atmosphere. On other days, it takes a while to be absorbed so the contrail stays visible for a short period of time. On yet other days, the conditions are such that the water vapor forms into man-made clouds. The same logic applies to natural clouds. Some days clouds appear and others they don’t appear, depending on pressure and temperature of the atmosphere.
The other way contrails form is caused by fluctuations in air pressure around the wings of the airplane. Have you ever released air from a bicycle tire by pressing on the valve and noticed that the air released is very cold and sometimes causes moisture to condense on the valve? Same thing. When a plane wing moves through the air at 500 miles per hour it impacts the air it is moving through. The air is compressed then decompressed, which under the right conditions of moisture content in the air can cause that moisture to condense into contrails.
Contrails are explained in more detail in the Wikipedia article on Contrails.
I put gathered a small collection of links to videos that debunk Chemtrails. These videos are all fairly short and to the point. And of some entertainment value.
There is one popular chemtrail conspiracy video that was proven to be a hoax because the pilot who made the original video later admitted it was a hoax. See: Contrail Science
October 17, 2013