HAMPTON, Va. - The Sept. 11 terrorist attacks spurred the government to ground all commercial air flight for days. Now that action may help researchers better understand global warming.

Researchers at the National Aeronautics and Space Administration's Langley Research Center have been studying contrails, the clouds formed by aircraft emissions. The pollution can form cirrus clouds, which are thin and wispy and form at 20,000 or more feet. They are believed to have the most influence on climate because they spread over large areas and tend to trap sunlight.

The post-Sept. 11 grounding allowed researchers to study just a few such contrails instead of the thousands usually produced by aircraft in flight. The difference has led to some surprises, said Patrick Minnis, a senior research scientist. The eye-opener was that airplanes might affect a larger area than scientists had first suspected.

Researchers were able to track just 10 planes that flew in the eastern part of the country, from about Ohio to Virginia. The crafts' contrails created cirrus clouds that lasted nearly seven hours and stretched over nearly 8,000 square miles. By calculating normal conditions, researchers determined that the normal volume of aircraft would have produced clouds covering about 77,000 square miles.

That kind of coverage was assumed for cirrus clouds across the planet, not just a portion of the United States.

''It was a surprise that it was that big,'' Minnis said.

The data are still preliminary but point to the need for greater understanding, Minnis added.

The contrails work began in 1993, spurred partly by the expected growth of air travel. Aviation travel is projected to increase by 2 percent to 5 percent per year in the next 30 years, Minnis said. That will mean hundreds more aircraft in the skies.

''Contrails represent the largest uncertainty in the climatic effects of aviation,'' Minnis said.

The cirrus clouds already produced are having an effect, Minnis said. On a global level, aircraft-produced clouds have been shown to increase temperatures by 2 percent. On regional levels, it can be as high as 5 percent.

Getting a better understanding of those processes is difficult, however, when so many of them interact. It doesn't help that it is difficult to differentiate between the clouds formed by contrails and ones created naturally.

After terrorists slammed planes into the World Trade Center and the Pentagon, President Bush ordered commercial flights grounded. Between Sept. 11 and Sept. 13, only military aircraft were allowed in the skies. Limited flying was allowed on Sept. 13.

Minnis's team tracked those aircraft, but must analyze data for aircraft that flew on the western side of the United States, as well as for Sept. 12 and 13. Minnis's team will also compare the data to the three days prior to the grounding.

© Copyright 2002 Globe Newspaper Company

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