Polar stratospheric clouds (PSCs), also known as mother-of-pearl or nacreous clouds, have been observed for many years. As their name suggests, the clouds form in cold polar stratospheric winters where, despite the dryness of the stratosphere, the temperature drops low enough for condensation to occur. PSCs are believed to be made of nitric acid and ice. In other parts of the world the stratosphere is too warm for these clouds to form, which is one reason why the "ozone hole" is confined to the Antarctic region.