Science in Antarctica

Polar Stratospheric Clouds

One afternoon in the first week of September people all over McMurdo were eagerly peering out their windows, predicting the formation of these exotic clouds, and anticipating the beauty that awaited as the sun began to set. The clouds were a special spring treat for those who had overwintered at McMurdo in nearly absolute darkness. Toward evening, as the clouds began to gather, photographers hauled out their equipment and some drove to the top of Arrival Heights, a big hill overlooking the sea ice, to get a better view. The darkening evening sky was full of wisps of colored clouds, lit brilliantly from below by a setting sun. "Floating rainbows," one person called them. The clouds are nicknamed "nacreous" or "mother-of-pearl" clouds because they shine and glint like the inside of a seashell.

Although polar stratoshperic clouds are strikingly beautiful, they are also agents of destruction. It is within these clouds that the process of ozone depletion takes place. One scientist called them "sad" clouds--they cause chemical reactions that convert inert forms of chlorine to reactive forms, which destroy ozone.

Polar stratospheric clouds are the highest of all clouds. They form in the stratosphere--80,000 feet high. Regular clouds mostly form at lower altitudes, in the troposhere. Polar stratospheric clouds only occur in the polar regions--the Antarctic and the Artic--but because Antarctica is colder, they form more readily here. The clouds are also quite rare, occurring only in the Antarctic late winter and early spring. This time of year in Antarctica there are both very cold temperatures and sunlight, which are key ingredients in the formation of the clouds. The temperature must fall below minus 83 degrees Celsius (about -118 F) for the clouds to form. Regular clouds are made of water. Polar stratospheric clouds are water-ice, and some also contain frozen nitric acid and sulphuric acid. Scientists think there are at least three different kinds of polar stratospheric clouds: those that form from rapidly cooling water-ice and are called nacreous clouds, those that are made of nitric acid, and those that are formed from slowly cooling water-ice. The rapidly-forming water-ice nacreous clouds contain a large number of small ice crystals, which is what makes them so visible, and so beautiful--the cyrstals diffract sunlight.

Ruth Hill and Russ Bixby, both of whom work at McMurdo, took me to the top of Arrival heights to see the clouds. Both of them were breathless with excitment, and told me about another time when the sky was even more stupendously beautiful. The sea was orange peach and the mist over the ice was blue, Russ said. "It was like a navy blue fog." The peaks of the Transantarctic mountains were poking out above the navy blue mist, and there were serpentine patterns in the sky. The sky itself was a deep midnight blue, and against it, Russ said, the clouds looked like "a stained glass window into heaven."

Click HERE for an artist's version of the nacreous clouds.

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