Matt Thompson took this amazing photo of a Nacreous (pearl-like) clouds above Antarctica. From the website:
The above photo shows metallic nacreous clouds above McMurdo Station in Antarctica. Nacreous means pearlescene 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 tri-hydrate, 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.
Link - via Living the Scientific Life and sarahcpwilliams