Thursday, December 3, 2009

Contrails - An Environmental Hazard

Vapour trails caused by jet aircraft over Britain can cause clouds covering 20,000 square miles, according to Met Office research. Photo courtesy: Telegraph

An analysis of the contrails left by one large military aircraft circling over the North Sea showed the creation of a thin cloud layer that, at its height, covered an area of more than 20,000 sq. mi.

Contrails (short for "condensation trails") or vapour trails are basically artificial clouds. They are the visible trails of condensed water vapour made by the exhaust of aircraft engines. As the hot exhaust gases cool in the surrounding air they may form a cloud of microscopic water droplets. If the air is cold enough, this trail will comprise tiny ice crystals.

Met Office (shortened from Meteorological Office; but, now the official name in itself) research suggests that for the millions of people who live under busy flight paths the news is grim. The collective impact of hundreds of vapour trails in areas surrounding airports can create a cloud layer that reduces the sunlight for those unfortunate enough to live there.

Contrails sometimes disperse within minutes; but, can also be present in the sky for many hours. They can also act as a catalyst for the formation of further wispy cirrus cloud.

Globally, the amount of sunlight reduction from vapour trails is less than 1%; but, this figure can rise to 10% in areas that are busy air traffic corridors.

In 2003, Patrick Minnis, a researcher with NASA, said contrails “already have substantial regional effects where air traffic is heavy” and that the impact “may become globally significant” because of the growth in air travel.

The Met Office analysis, not incredibly substantial, was based on observations of a single military aircraft circling over the North Sea on a sunny day earlier this year.

Researchers had expected the wind to disperse all contrails quickly; but, instead they attracted more clouds. As they were blown southwards, they continued to grow and eventually formed a hazy, high-level blanket of cirrus clouds.

Jim Haywood, the Met Office’s aerosol research manager, told the newspaper: “At its peak the resulting cirrus cloud covered an area of more than 20,000 square miles.”

He added: “Such clouds are normally short-lived; but, depending on atmospheric conditions, they can last much longer.”

Mr. Haywood said aviation-induced cirrus clouds have a warming effect on the planet as they trap heat and bounce it back to earth.

“Studies show that, overall, the warming effect is stronger so aviation-induced clouds are helping to warm the planet,” he said.

Via Telegraph

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