Photo in the News: Rare Cloud Forms Above Antarctica

Rare nacreous cloud spotted in Antarctica (photo)
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August 8, 2006—It's not just another pretty sunset for scientists working in one of the coldest places on Earth.

Renae Baker, a scientist with the Australian Bureau of Meteorology, snapped this picture of a rare nacreous cloud on July 25 at the country's Antarctic Mawson station.

Nacreous clouds form when the fading light at sunset passes through tiny ice crystals blown along by a strong jet of stratospheric air.

The clouds form only in polar latitudes and at extremely cold temperatures.

"Our weather balloon measured temperatures down to minus 87 degrees Celsius [minus 125 degrees Fahrenheit] in the vicinity of the cloud layer," said Baker on the Web site of the Australian Antarctic Division, a bureau of Australia's environmental department.

"That's about as cold as the lowest temperatures ever recorded on the surface of the Earth. Amazingly, the winds at this height were blowing at nearly 230 kilometers [143 miles] per hour."

The cloud formed at a height of 12.5 miles (20 kilometers) above the ground and was more than twice as tall as clouds normally seen in polar regions. The ice-cold phenomenon could help scientists understand more about atmospheric conditions and climate change.

(Read "Antarctica's Atmosphere Warming Dramatically, Study Finds" [March 30, 2006].)

Nacreous clouds—also known as polar stratospheric clouds—contribute to the formation of the polar ozone holes. The clouds form surfaces where chemical reactions can take place that produce ozone-destroying molecules.

"These clouds are more than just a curiosity," added Andrew Klekociuk, an atmospheric scientist with the Australian Antarctic Division. "They reveal extreme conditions in the atmosphere and promote chemical changes that lead to destruction of vital stratospheric ozone."

—Aalok Mehta

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