Nacreous Clouds Above Antarctica
December 10, 2003
The above photo shows metallic nacreous clouds above McMurdo Station in Antarctica. Nacreous means pearlescent or pearl-like. These stunning clouds are observed in the Antarctic each Austral spring and fall when upper air temperatures drop low enough for their formation. Nacreous clouds typically occur in the stratosphere, at heights of between 10 and 30 km, where the temperature falls below -80 degrees C. They're likely composed of ice particles with a liquid coating of nitric acid trihydrate, and they appear bright or luminescent because they're sufficiently high to be illuminated by the Sun, long after local sunset. At higher latitudes, the Sun is never far from the horizon from late spring through early fall. The pastel colors of nacreous clouds are attributable to the process of diffraction.
Nacreous clouds are often observed along the Antarctic Peninsula where mountains create lee-waves in the upper atmosphere. The conditions that allow them to form may persist for several months. On occasion, they've been seen as far equatorward as southern England. Stratospheric clouds are associated with ozone depletion since they provide the necessary environment for the chlorine-based catalytic photochemistry that destroys ozone at around 1% per day.