The Weather Notebook: Making Clouds by Fying in Airplanes

Making Clouds by Fying in Airplanes
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Dave Thurlow, Host

Lingering Contrail
Hi, I'm Dave Thurlow from the Mount Washington Observatory and this is The Weather Notebook.

You don't have to live near an airport to recognize the telltale sign of an airplane cutting across a clear blue sky. The sight of those cloud-like lines of exhaust lingering behind a jet is an increasingly common one over the United States.

Millions of civilian and military flights crisscross the country every year and when they hit the upper atmosphere, many form those cloud-like condensation trails. If the weather conditions are right, the contrails linger and can become indistinguishable from natural cirrus clouds.

Satellite images taken of airplane contrails indicate that they can develop into extensive cirrus cloud formations. In one experiment, scientists created a 60-mile long contrail off the California coast. Over six hours it spread out to cover 4000 square miles before dissipating.

Since World War Two, researchers have noticed an increased number of cloudy days in the Northern Hemisphere. One NASA study found five percent more cirrus clouds over the United States in 1996 than in 1971. The reason? More planes, more contrails.

Cirrus clouds tend to reflect the sun's heat away from the earth during the day and trap heat at night. What the overall impact the manmade cirrus clouds will have is uncertain. But one thing is known. Commercial air traffic is expected to double in the next fifteen years.

Thanks today to Weather Notebook writer George Homsy. The Weather Notebook is produced by the Mount Washington Observatory. Funding is provided by Subaru, the beauty of all wheel drive and by the National Science Foundation.