August 1, 2006

Extreme Conditions Create Rare Antarctic Clouds

SYDNEY -- Rare, mother-of-pearl colored clouds caused by extreme weather conditions above Antarctica are a possible indication of global warming, Australian scientists said on Tuesday.

Known as nacreous clouds, the spectacular formations showing delicate wisps of colors were photographed in the sky over an Australian meteorological base at Mawson Station on July 25.

Australian Antarctic Division scientist Andrew Klekociuk said such clouds are occasionally produced by air rising over Arctic and Antarctic mountains in high polar latitudes during winter.

"You have to be in the right part of the world in winter, and have the sun just below your horizon to see them," he said.

Nacreous clouds can only form in temperatures lower than minus 80 degrees Celsius (minus 112 Fahrenheit).

Meteorologist Renae Baker said a weather balloon in the vicinity of the clouds in the stratosphere about 20 km (12 miles) above the Earth's surface measured temperatures as low as minus 87 Centigrade (minus 124.6 F).

"That's about as cold as the lowest temperatures ever recorded on the surface of the Earth," Baker, who photographed the clouds, said in a statement.

Klekociuk said the rarely seen clouds, also known as polar stratospheric clouds, were more than just a curiosity.

"They reveal extreme conditions in the atmosphere, and promote chemical changes that lead to destruction of vital stratospheric ozone," he said.

Klekociuk said temperatures in the stratosphere, between 8 and 50 km (5-31 miles) above Earth, would be expected to drop as global warming increases. Data collected over the past 25 years had reflected this, he told Australian Broadcasting Corp. radio.

"Over that time there has been a small decrease in temperature and that change is actually occurring faster than the warming at the surface of the Earth," he said.

The delicate cloud colors are created at sunset when fading light passes through tiny water-ice crystals blown along on strong jets of stratospheric air.

She said winds at the same height were measured blowing at almost 230 km/h (143 mph).