Contrails are condensation trails: artificial clouds made by the exhaust of jet aircraft or wingtip vortices which precipitate a stream of tiny ice crystals in moist, frigid upper air.

In 1998, NASA scientists found that by circling a jet off the Pacific coast of the United States they were able to create contrails that eventually coalesced into a cirrus cloud covering 1,400 square miles. Satellite photographs have confirmed that on one occasion jet contrails produced by commercial aircraft over New Mexico formed a cloud covering 13,000 square miles.
..... Click the link for more information.  and ascribed to condensation of water vapor in the aircraft exhaust. Those who use the word chemtrails maintain that the phenomena they observe have an appearance different from those of contrails, and are not consistent with the known properties of contrails. They believe that these trails indicate some kind of chemical spray. Since no government or other organization has acknowledged such spraying, the belief is an example of a conspiracy theory This article is about a movie. For conspiracy theories in general, see Conspiracy theory.

Conspiracy Theory is a one of a handful of 1990s movies starring actor Mel Gibson. It also stars actress Julia Roberts.

Gibson plays a taxi driver who publishes a news letter in which he talks about what he suspects are government conspiracies. One of his subscribers turns out to be a government official, and Gibson is on target about one of his theories, so he and Roberts become endangered.
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Possible conspiratorial explanations include atmospheric and weather modification, biological warfare

Biological warfare, also known as germ warfare, is the use of any organism (bacteria, virus or other disease-causing organism) or toxin found in nature, as a weapon of war. It is meant to incapacitate or kill an adversary.

Biological warfare is a cause for concern because a successful attack could conceivably result in thousands, possibly even millions, of deaths and could cause severe disruptions to societies and economies. However the consensus among military analysts is that except in the context of bioterrorism, biological warfare is militarily of little use.
..... Click the link for more information. , or purposes associated with a New World Order The term New World Order refers to a belief or conspiracy theory among apocalyptic religious and political extremist groups, especially in the United States, that the United Nations has created a secret plan, known as the New World Order (NWO), to rule the world via a totalitarian socialist world government. The belief may stem – at least partly – from the phrase New World Order, which has been used in politics for much of the Twentieth Century. See New World Order (political)
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Chemtrails have been discussed on talk-radio programs hosted by Art Bell Art Bell (born June 17, 1945) is now the weekend host of the radio show Coast to Coast AM.

Bell's local program in Las Vegas, Nevada was originally a political call-in talk radio show. Bell tired of this format, and thought there were too many such programs.

He abandoned political talk and began incorporating his long interest in UFO's, time travel and other oddities into his show, which quickly gained a large following after it was sydicated.
..... Click the link for more information.  and Jeff Rense, who frequently deal with other paranormal and conspiratorial topics. According to an FAQ posted at the Rense website "chemtrails (CTs) look like contrails initially, but are much thicker, extend across the sky and are often laid down in varying patterns of Xs, tick-tack-toe grids, cross-hatched and parallel lines. Instead of quickly dissipating, chemtrails expand and drip feathers and mares' tails. In 30 minutes or less, they open into wispy formations which join together, forming a thin white veil or a 'fake cirrus-type cloud' that persists for hours."

Skeptics

Skepticism (British spelling: Scepticism) can mean:

• Philosophical skepticism - a philosophical position in which people choose to critically examine whether the knowledge and perceptions that they have are actually true, and whether or not one can ever be said to have true knowledge; or
• Scientific skepticism - a scientific, or practical, position in which one does not accept the veracity of claims until the scientific method has been applied to them.