Noctilucent - Old man with his head in the clouds: July 2011

Polar Stratospheric Clouds - Mawson Antarctica

Polar Stratospheric Clouds.
The annual formation of the ozone hole in the southern polar region is associated with the development of polar stratospheric cloud.

Atmosphere temperature (Red) and humidity (Blue) 26th June

Formation of these clouds occurs between Latitude 60 degrees South and the pole and between 10km and 25km above the surface. Temperatures in the stratosphere need to be colder than -75 degrees C. Their movement is west to east. Formation occurs from mid-June to mid-October.

Type I Nacreous Cloud

Two types of Polar Stratospheric cloud can occasionally be observed from Antarctica.

Type I are thin layer clouds made up of small droplets of nitric and sulphuric acid and will form in temperatures colder than – 75 degrees C. They appear as a thin yellowish veil with fine horizontal structure and a bright spot a few degrees above the horizon scattering light from the sun. Orange haze above the horizon indicates scattering from nitric acid particles.

Type I and II Nacreous cloud.

Type II Nacreous Clouds or Mother of Pearl clouds and are made up of ice crystals below -83 degrees C. They have a pearly-white appearance due to scattering of sunlight and may show delicate interference patterns and pink and green colours.

They are best observed when the sun is between 1 and 6 degrees below the horizon during civil twilight. At that time the lower troposphere is in shadow while the stratosphere is still lit by the sun.

So what is the connection with stratospheric clouds and ozone depletion?

Atmospheric layers in the Antarctic winter. © Dr. Andrew Klekociuk, Australian Antarctic Division

Each winter a polar vortex forms in the stratosphere over the Antarctic. The polar vortex is a region where air undergoes little mixing with the rest of the atmosphere, allowing temperatures to drop to around -85ºC and stay low. This occurs when there is no sunshine in the winter months during the polar night. The low temperatures allow water and other chemicals to condense, producing stratospheric clouds.

The polar stratospheric ice clouds provide a surface on which chemical reactions take place that lead to ozone depletion. Compounds called HALO CARBONS are converted to compounds that will catalytically destroy ozone. In spring when the sunlight returns to Antarctica, the destruction of ozone within the polar vortex starts and rapidly accelerates. The thinning of the layer of ozone is what is referred to as the ozone hole. It reaches a maximum in early October and slowly declines so that things have returned to normal by the end of December.

Wind profile 26th June. (1m/s = 3.6km/h)

The ozone depleting compounds contain combinations of the elements chlorine, fluorine, bromine, carbon and hydrogen. The general term for them is HALO CARBONS. Compounds that contain only carbon, chlorine and fluorine are called CHLOROFLUOROCARBONS (CFC). Compounds that contain carbon, bromine, fluorine (and sometimes chlorine) are called HALONS. From the surface stratospheric clouds seem to be stationary however wind speed can be strong. On 26th June speeds were up to 180 km/h 25km high.