Most of you are aware of contrails (short for condensation trails): clouds that form behind jets flying high in the troposphere or in the lowest stratosphere. The picture below shows a typical example.
Most of you already are pretty expert in producing clouds in cold air: it happens during very cold days--you can see your breath! These clouds are generally made of cloud drops.
Contrails can be produced by both jet aircraft and high-flying propeller driven planes. Contrails were first observed in the 1920s and was a great concern in WWII, since they allowed the enemy to see high-flying bombers. Also contrails could make it difficult for such bombers to stay in formation!
Sometimes contrails are hard to see, while other times they fill the sky as shown in the figure below. Why? The chemtrail folks think this is due to enhanced military or geoengineering activity. The truth is that it all depends on how close the atmosphere is to saturation (100% relative humidity). Sometimes the upper atmosphere is very dry, thus you need to add a lot of moisture from aircraft to
A sky with a lot of cirrus clouds is a good sign there is upward motion and an upper atmosphere at saturation.
If you have a situation where the atmosphere is at or near saturation and a region with a lot of aircraft flying in the upper troposphere, you can have HUGE numbers of contrails. Here is an example from the NASA MODIS satellite. Look carefully and you can see the cirrus clouds as well--just as one might expect.
Contrails, as most clouds, have a noticeable effect on the radiation balance of the planet. On one hand they reflect solar radiation and thus cool. On the other, they absorb and re-emit infrared radiaton, both upwards and downwards. This tends to cause warming. Right now the best estimates are that the warming effect will dominate and thus contrails will heat the planet by a small amount. This fact shows why the chemtrails idea of U.S. government geoengineering to stop global warming is such nonsense---contrails warm the planet and thus would NOT be something you would enhance to stop global warming.