Extreme conditions create rare Antarctic clouds

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Extreme Conditions Create Rare Antarctic Clouds

SYDNEY (Aug. 1) - 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 112 Fahrenheit.

Meteorologist Renae Baker said a weather balloon in the vicinity of the clouds in the stratosphere about 12 miles above the Earth's surface measured temperatures as low as 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 5 and 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 143 mph

Questions:

1. What are nacreous clouds?

2. When do they form?

3. Are nacreous clouds signs of global warming?

Polar stratospheric clouds (nacreous clouds) are very common over Antarctica during winter and early spring, when stratospheric temperatures get low enough to form them. They are particularly common over the Antarctic Peninsula and western Weddell Sea, where terrain forcing by the peninsula can sometimes lift extra water vapor into the lower stratosphere. Although they are common, it's not common to see them from the ground because of the extensive lower cloud cover. The fact that they occur frequently during winter and spring is what gives rise to the ozone "hole": these cloud particles are the substrates on which the catastrophic ozone depletion chemistry gets started.

A sighting or two of polar stratospheric clouds isn't an indication of global warming. However, one known consequence of global warming is that the stratosphere will cool as the troposphere warms. Therefore polar stratospheric clouds might become more persistent. However, if the Montreal Protocol holds, the ozone layer is expected to recover from CFC contamination within about 50 years as the CFCs are removed from the stratosphere by the Brewer-Dobson circulation. So in the long run an increase in polar stratospheric cloud amount wouldn't lead to more ozone depletion over Antarctica.

Questions:

1. Are nacreous clouds common?

2. Why are they not seen very frequently?

3. What is one known consequence of global warming?

4. Will an increase in these clouds lead to global warming?

5. Did you get a different perspective on the nacreous cloud sightings by reading the comments made by the Dan Lubin from the University of California, San Diego?