They call it Project Cloverleaf. At night, giant planes with no pilots roam the sky over the U.S. Instead of a mere vapor trail, they are filling the sky with unknown chemicals designed to darken the earth.

To thousands of people around the U.S., this is not a sci-fi movie or even a conspiracy theory; it is real.

Scott Stevens is a believer.

Stevens, a meteorologist and weatherman with News Channel 6 in Pocatello, said the phenomenon known as "global dimming" could be a clandestine operation by the government to slow the effects of global warming.

While global dimming is a contentious scientific issue, some scientists think thicker cloud cover may be reducing the amount of sunlight reaching the earth.

Stevens, who also runs the Web site www.weatherwars.info, said he believes the cloud cover is generated by airplanes with chemical substances in their fuel to increase and extend the effects of vapor trails. Stevens has appeared on several national radio shows, including "Coast to Coast AM," in an effort to educate the public about chemtrails.

"It's called 'Project Cloverleaf' and it is the primary reason the U.S. has not signed the Kyoto accord, because we already launched a program to try to counteract the CO2 problem and the methane problem," Stevens said. "And that is through distribution of aerosol."

Science has established that vapor trails left by jets can form into clouds, actually growing and spreading. Some scientists think vapor trails might be part of a decrease in sunlight reaching the earth, but they do not believe Project Cloverleaf exists.

While scientists may be skeptical, people around the world have latched onto the unconventional idea. A Google search of "project cloverleaf" brings up about 50,000 hits, many of them conspiracy theory Web sites. A search of "chemtrail" gets about 280,000 hits.

Stevens said it is perfectly logical that jets would produce contrails and said it would be unusual if they did not.

"The question is persistence," he said. "What happens is eight days out of ten, conditions are favorable so that when a plane flies, the contrails persist. I would guess that normal is closer to 3.5 to five days. That means a rough guess is that there is a doubling in the number of days where these contrails can last."

Stevens said many different types of aircraft are spreading the aerosol as part of Project Cloverleaf.

"I think you have the whole range. I think you have some aircraft that have clean fuel. I think you have some aircraft that have some additive within the fuel to create additional cloud cover," Stevens said. "And then you have the black ops planes, and that is all they do. There is the regular condensation trail, and then everything else is for the delivery of aerosols."

Patrick Minnus, a senior research scientist at NASA's Langley Research Center, said that contrails currently account for roughly 3 percent of the cloud cover over the U.S.

Minnus said cloud cover generated by airplanes increased by about 3 percent between 1971 and 1995, but he attributed the increase to additional air traffic, not to a clandestine operation by the government.

"I don't understand it," Minnus said. "Its just one of those conspiracy theories. No amount of logic can debunk it."

The theory may have been hatched after Minnus, David Young and Donald Garber, all researchers and NASA's Langley research center, presented a paper called "Transformation of Contrails into Cirrus during Subsonic Clouds and Contrails Effects Special Study," Minnus said.

In the study, the researchers used a DC-8 airplane to fly an oval pattern off the coast of Texas.

"We just watched the contrail blossom and turn into a huge cirrus cloud," Minnus said.

When they presented the paper in 1998, the Project Cloverleaf theory began to spring up, he said, adding that he did not know if it was coincidence or not.

Minnus said that contrails are formed in air below -39 Celsius when the air is supersaturated with ice.

Due to the physical structure of ice, the humidity level actually has to be higher, about 150 percent humidity level, than it would be for the air to be supersaturated with water.

"The exhaust (jet engine) injects a lot of water into the air," Minnus said.

"The water droplets immediately freeze and you wind up with a contrail."

Minnus said once the contrail is formed in supersaturated air, larger ice particles become nuclei and begin to grow, collecting other ice particles from the surrounding air.

As the particles get heavier, they begin to fall out of the contrail, spreading it vertically, wind shear spreads the contrail horizontally as it continues to collect ice from the atmosphere.

Minnus said in a 1998 paper called "Spreading and Growth of Contrails in a Sheared Environment," that aged contrails can form into clouds almost unidentifiable from natural cirrus clouds.

"Since these persistent contrails have long lifetimes and grow to cover large areas, they have the greatest potential for affecting regional or global climate," Minnus said in the paper.

Stevens' idea of weather modification by the government may seem far-fetched to climate scientists.

But other scientists have considered airplane contrails as a means of counteracting global warming.

A paper published by the National Academy of Sciences in 1992, "Policy Implications of Greenhouse Warming: Mitigation, Adaptation, and the Science Base," speculates that commercial airliners could use contrails or spread dust to combat global warming.

In the geoengineering section of the study (p.459), the paper states, "Cloud stimulation by provision of cloud condensation nuclei appears to be a feasible and low-cost option capable of being used to mitigate any quantity of CO2 equivalent per year."

The paper also says, "These possibilities appear feasible, economical and capable of mitigating the effect of as much CO2 equivalent per year as we care to pay for ... Such systems could probably be put into full effect within a year or two of a decision to do so, and mitigation effects would begin immediately."

Ellsworth Dutton, an NOAA research meteorologist based in Boulder, Colorado, said there is not only no Project Cloverleaf, there is no global dimming.

"The last published data set that showed a downward trend (in solar radiation reaching the earth's surface) was published in 1990," he said.

Dutton said there was a curious decrease in service radiation during the 1970s and 1980s but that trend appeared to have ended.

Stevens says global dimming is not only occurring, but is having an effect.

"We have seen a real result because we have not seen the warming that should have accompanied the increased output from the sun, carbon dioxide, the methane. We have not seen the evaporation off the oceans that should have accompanied the warmth."

Stevens has no doubt about the feasibility of keeping the general population in the dark while operation a huge weather manipulation project.

"There is nothing that isn't possible. The only thing that gets in your way is your will. If you have thousands of scientists, an unlimited budget and political powers telling you to do something, it gets done," Stevens said, adding that people need to be alert to changes.

"There have been some manipulations of the environment for a rather lengthy period of time. We are coming up on thirty years that weather engineering has been attempted, if not accomplished. It is indeed within forty years where we begin to see this slope of warming accelerating," Stevens said. "How much of that is mankind and how much is natural?"