Beautiful, mysterious polar stratospheric clouds

Polar stratospheric clouds (PSCs) play a central role in the formation of the ozone hole in the Antarctic and Arctic. PSCs provide surfaces upon which heterogeneous chemical reactions take place. These reactions lead to the production of free radicals of chlorine in the stratosphere which directly destroy ozone molecules. Despite two decades of research, the climatology of PSCs is not well described, and this impacts on the accuracy of ozone depletion models. The timing and duration of PSC events, their geographic extent and vertical distributions, and their annual variability are not well understood. The AAD's Space and Atmospheric Sciences group encourages people travelling south to keep a lookout for these clouds, and to report any sightings. This information is potentially useful to compare with observations by the Davis lidar, satellite measurements and predictions of atmospheric models.

PSCs form poleward of about 60°S latitude during the winter and early spring in the altitude range 10km to 25km. The clouds are classified into Types I and II according to their particle size and formation temperature. Type II clouds, also known as nacreous or mother-of-pearl clouds, are composed of ice crystals and form when temperatures are below the ice frost point (typically below -83°C).

Type I PSCs are optically much thinner than the Type II clouds, and have a formation threshold temperature 5–8°C above the frost point. These clouds consist mainly of hydrated droplets of nitric acid and sulphuric acid.

The PSC season at the ANARE continental stations typically runs from mid-June to mid-October each year. At the subantarctic sites of Macquarie Island and Heard Island, stratospheric temperatures rarely reach the frost point during winter, but observations are still encouraged.

The best viewing time is when the sun is between about 1 and 6 degrees below the horizon (during civil twilight), when the troposphere is in shadow but the stratosphere is illuminated. This increases the contrast of the PSCs against the background sky, and helps to differentiate against any tropospheric cloud which will appear much darker. The clouds will generally be visible in the twilight arch portion of the sky. It may also be possible to discern the clouds in strong moonlight.

During the PSC season at the ANARE continental stations, the sky is generally covered by a thin yellowish veil of Type I clouds. The veil can be hard to identify, being easily confused with cirrostratus clouds or tropospheric haze. You may notice fine horizontal structures in the veil near the horizon, as well as a bright patch of light a few degrees above the horizon scattered from the sun.

The Type II clouds are a less common phenomenon. There is anecdotal evidence that Type II clouds are more prevalent at Mawson compared with the other stations, possibly due to the influence of the nearby mountains. These clouds look distinctly different to tropospheric clouds. They have an overall pearly-white appearance (due to forward-scattering of sunlight), and may also show some delicate interference colours (pinks and greens). A polarising filter may enhance their visibility. Reports of these clouds are the most valuable as they indicate the occurrence of special atmospheric conditions.

The PSCs will generally be travelling in the stratospheric flow, which is predominantly from west to east, and this may help in identification. For example, at Davis the wind direction in the troposphere generally rotates with altitude from north-easterly near the ground, through southerly to be westerly at the tropopause. The motion of the tropospheric clouds may therefore be quite different to the motion of the PSCs. Type II clouds formed by mountain lee-waves may appear at discrete spacings across the sky, and appear quasi-stationary.

Digital images and reports of PSCs can be emailed to sas_cloud@aad.gov.au with information on the observer's name, location, viewing direction, the date and time (accurate to the nearest few minutes if possible), the focal length setting of the camera lens, and noting if any filters were used. It is also helpful if the horizon is in the photograph, and the location of the camera is roughly known (eg 'clouds photographed while standing on the front steps of the LQ'). Past observations, even a few years old, may be useful.

The clouds and twilight will usually be reasonably bright, so normal camera exposure metering should be adequate. Normal daylight type film is suitable - ISO 400 film will give you more scope for reducing the exposure time.

Further information on observing PSCs can be found at
http://www.aad.gov.au/default.asp?casid=11200

Andrew Klekociuk
Space and Atmospheric Sciences Program, AAD

This page was last modified on 22 September 2003.