Sometimes they're there, and sometimes not. Can you explain that to me?

google “chemtrails”

ooh wish i could answer you on that one buddy…
uh, good question though…

Google “chemtrails” only if you’re into conspiracy theory stuff.

Okay, according to Wikipedia, contrails are caused by a variety of things.

- Condensation from engine exhaust
At high altitudes, supercooled water vapor requires a trigger to encourage deposition or condensation. The exhaust particles in the aircraft’s exhaust act as this trigger, causing the trapped vapor to rapidly turn to ice crystals. Exhaust vapour trails or contrails usually occur above 8000 metres (26,000 feet), where the temperature is below -40°C (-40°F).

- Condensation from decreases in pressure
The wings of an aircraft cause a drop in air pressure in the vicinity of the wing. This brings with it a drop in temperature, which can cause water to condense out of the air and form a vapour trail or contrail. This effect (the Prandtl-Glauert singularity), is more common on humid days, and can be seen on fighter jets performing high energy maneuvers, during shuttle launches, on the expanding surface “bubble” of nuclear explosions, or on airliners during takeoff and landing. Additionally, the area around a turbo-fan intake will be at a lower pressure than the surrounding air, and may result in a condensation fog forming there during high power settings.

These sorts of vapor trails contrast with the other major type of contrails, which are caused by the combustion of jet fuel. Contrails produced from jet engine exhaust are seen at high altitude, directly behind each jet. With aircraft with four jets, such as the Boeing 747, there are four contrails. In contrast, vapor trails caused by a drop in air pressure are usually seen at low altitude where the ambient humidity is higher, and they follow the wings rather than the jet engines.

- Distrails
Short for dissipation trail. Where an aircraft passes through a cloud, it can clear a path through it; this is known as a distrail. Because the plane’s contrail is not yet visible, (because of its height, contrails usually form at 22,000–28,000 feet, depending on the temperature and other factors) the cloud looks like a tunnel seen horizontally and vertically, assuming the cloud is very thin, looks like it has been divided.

It seems that the altitude of the plane combined with ambient temperature and humidity will determine whether a contrail is created.

Thanks to all three of you, for contributing to my education…

sometimes I eat so fast, my fork gets contrails.

sometimes I fork so fast, my contrails get eaten.

@Alena, et al. Interesting that Congressman Dennis Kucinich is also a conspiracy theorist… enough so to mention chemtrails in a bill he authored in 2001.

http://thomas.loc.gov/cgi-bin/query/z?c107:H.R.2977.IH:

Here’s a pretty good article: http://www.earthisland.org/journal/index.php/eij/article/stolen_skies_the_chemtrail_mystery/