The first Polar Stratospheric Clouds (PSC) for 2007 were detected over Davis, Antarctica, by Australian Antarctic Division LIDAR scientist, Didier Monselesan, in May. PSCs promote chemical changes in the atmosphere that lead to the depletion of the ozone layer over Antarctica each year. They occur only at high polar latitudes in winter, when temperatures in the stratosphere (8–50km above ground) fall below about −85°C. Average temperatures in the Antarctic lower stratosphere were below average at the time of the sighting — as they were in 2006 when persistently low winter temperatures led to record levels of ozone destruction.
Although invisible to the eye, the PSCs were detected by the Davis atmospheric LIDAR. The LIDAR uses pulsed light from a powerful laser to map clouds and measure atmospheric temperatures. The PSCs first appeared on May 28 as a thin layer at an altitude of 22km.
In the accompanying LIDAR image, the sloping wavy features in the cloud layer arise from a rising and falling motion due to atmospheric waves propagating up from the lower atmosphere. Based on the prevailing temperatures and characteristics of the LIDAR measurements, it is likely that the PSCs consisted of micron-diameter particles comprised of an icy mixture of water with nitric acid and sulphuric acid.
A photograph taken by Didier on the same evening shows the green laser beam of the LIDAR, the green arcs of the aurora australis (right side of image) and a waxing moon. Also visible are a few wispy stratus clouds at an altitude of about one kilometre.
ANDREW KLEKOCIUK and DIDIER MONSELESAN, Ice, Ocean, Atmosphere and Climate programme, AAD