Nacreous Clouds

Copied from an old 1966 slide. Previously posted some years ago, now with a better quality image. View large on black.

The Bureau of Meteorology had asked us to look for these colourful thin clouds and, in August, we were surprised and delighted to see them behind the meteorological office. They are Nacreous Clouds, also known as "Mother of Pearl" clouds because of their colours or, more currently, as "Polar Stratospheric Clouds". They are made of ice crystals (which give their colour), are about 20-25 km high, and form when temperatures there drop below -80 C. They had always been considered quite rare and I believe our sighting was one of the earliest confirmed at Mawson in Antarctica. There have been very numerous sightings since, significantly becoming as late as October, because of climate change effects.

Ozone normally forms in the early spring in the polar stratosphere and absorbs ultraviolet radiation as the sun returns, in the process warming the stratosphere. In the 1970s it was found that Chlorofluorocarbon gases (once used in spray cans and old refrigerators) mix through the atmosphere and break up the newly formed ozone, causing the polar stratosphere to remain colder in spring. This is the "ozone hole". Although chlorofluorocarbon gas use has (largely) been banned, it will take years for the gases already released to break down, for the ozone hole to heal totally, and for the stratosphere to return to normal. Yes, this is relevant to climate change: see this link.