A rare and spectacular cloud formation has been seen at the end of the polar night at Australia’s Mawson station in Antarctica. These so-called nacreous clouds were situated high in the stratosphere, some 20km above the ground, and reveal very cold temperatures in the rarefied atmosphere.

Photographs taken by Renae Baker, a meteorological officer with the Australian Bureau of Meteorology at Mawson, on July 25 show delicate colours produced when the fading light at sunset passed through tiny water-ice crystals blown along on a strong jet of stratospheric air.

“Spectacular is an understatement. The clouds were such a special and welcoming sight now that the sun has just started to return near the end of winter. I am keeping my eyes towards the celestial dome and camera at the ready in the hope of some more.”

Nacreous clouds show colours similar to those on the inside of a Mother-of-Pearl shell. The clouds only occur at high polar latitudes in winter, requiring temperatures less than approximately −80ºC to form.

“Our weather balloon measured temperatures down to −87ºC in the vicinity of the cloud layer. 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 kilometres per hour,” Ms Baker said.

Clouds this spectacular are seldom seen.

Atmospheric scientist Dr. Andrew Klekociuk of the Australian Antarctic Division explains that these clouds are occasionally produced by air rising over the Antarctic and Arctic mountains.

“You have to be in the right part of the world in winter, and have the sun just below your horizon to see them. Our expeditioners at Mawson are in one of those special parts of the world.

“These clouds are more than just a curiosity. They reveal extreme conditions in the atmosphere, and promote chemical changes that lead to destruction of vital stratospheric ozone” Dr Klekociuk said.

Atmospheric studies at Mawson, and at Davis station, some 900km to the east, are helping to unravel details of the clouds and their role in climate.

“We are using instruments on the ground, on balloons and on satellites in an international program to find out what this type of phenomenon tells us about the current and future state of climate,” Dr Klekociuk said.

This study will continue through the forthcoming International Polar Year, which will focus attention on Earth’s polar regions from 2007 to 2009.

Additional information

The stratosphere is the atmospheric layer situated above the Earth’s ‘weather layer’ or troposphere. In Antarctica, the troposphere extends from the surface to a height of about 8-10km. The stratosphere extends from the top of the troposphere to a height of about 50km.

The nacreous clouds seen at Mawson were more than twice the height of clouds normally seen in the polar regions.