Nacreous clouds near McMurdo, Ross Island, Antarctica on August 28, 2004. These polar stratospheric clouds at 80,000 feet are the highest of all clouds. They only occur in the polar regions when the stratospheric temperature dips below 100 degrees below zero Fahrenheit. They are also the site of chemical reactions that break down ozone in the upper atmosphere and contribute to the creation of the ozone hole above Antarctica. (Zenobia Evans/National Science Foundation)
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