[NAEP] Mother-of-pearl clouds float over Antarctica

[Dawn Wiseman]

How many clouds do you teach about in atmospheric science?

Nacreous clouds, also known as polar stratospheric clouds, only occur 
at high polar latitudes in winter because they require extreme cold 
temperatures in order to form. A meteorological officer   at 
Australia's Mawson station in Antarctica captured one of these rare 
clouds on film last week.

http://edition.cnn.com/2006/TECH/science/08/01/antarctica.clouds.ap/index.html