Expeditioners at Mawson station have been treated to a display of beautiful and rare ‘nacreous’ clouds.
On 3 June 2009, station doctor, Glenn Browning, took these photos of the clouds, which form in the stratosphere about 20km above the ground, under very cold conditions. Atmospheric scientist, Dr Andrew Klekociuk, says the clouds only occur at high polar latitudes in winter, at temperatures less than −80ºC. They can only be seen when the sun is just below the horizon. Their iridescent colouring, often described as ‘mother-of-pearl', is caused by sunlight passing through ice crystals in the atmosphere. Unfortunately, the clouds promote chemical changes in the atmosphere that destroy ozone.
The clouds, which are also known as ‘polar stratospheric clouds', have been observed at Mawson station over the past few years, including a spectacular sighting by meteorologist, Renae Baker, in 2006.
One of the small clouds in the images has a faint tail. This small cloud is produced in a cold and localised region of an atmospheric lee wave, where air has been forced to cool in response to flow over mountains in the Mawson region. The tail indicates where the main cloud has seeded further nacreous cloud development downwind of the lee wave, causing the wave to have have a longer-lasting and more widespread effect on the stratosphere.
The satellite images of the nacreous clouds were taken by the MODIS sensor on the Terra and Aqua satellites on 3 June 2009. The Terra images were taken at 0514 Universal Time, or 11:14am Mawson time — about 30 minutes before Dr Glenn Browning took his photos. The streaks running through the images are background noise.
The infrared image more clearly illustrates the clouds Glenn photographed. The bright clouds north of Mawson correspond to the bright cloud band in Glenn’s images. The clouds that appear to the left (west) of Mawson in both the Terra satellite images were not as well illuminated by the sun and were not visible from the station.