Polar stratospheric clouds

I thought I would add this as a separate section on polar stratospheric clouds, or nacreous clouds, since we’re starting to get a few sightings and I have already been asked about them.

Most weather (and clouds) take place in the troposphere, so about the first 10 km above the surface, which also includes around 70 % of the air mass of the atmosphere. Above that is the stratosphere, which includes a thin layer of ozone. We measure the ozone column (vertical section of ozone) at Halley using the Dobson spectrophotometer and compare it with an automated system, the SAOZ (Système d’Analyse par Observation Zénithale, and yes, even the software for it is in French). If all the ozone in the atmosphere, the majority of which is in the stratospheric layer, would be brought to ground level (and therefore compressed due to the higher atmospheric pressure at sea level) it would equate to an ozone layer with a thickness of 3 mm, i.e. 300 DU (Dobson units). The ozone layer is important because it absorbs some of the more energetic UV-B radiation, which is generally the more damaging one in terms of skin cancer.

Basically, ozone is formed by high energy radiation (like UV-B) breaking oxygen (O2) creating excited atomic oxygen (O) which binds with O2 forming ozone (O3). Ozone is destroyed again by absorbing the same high energy radiation (stopping some of it from reaching the Earth’s surface in the case of UV-B), which breaks the bonds resulting in more O2 and atomic oxygen. These go on to form another ozone molecule in what is called the Chapman cycle.

During the Antarctic winter low pressure systems around the continent create the circumpolar vortex, which acts like a barrier and allows the air over the continent to cool to much lower temperatures than the surrounding air. Once the lower stratosphere cools to about -78 °C ice clouds form in the lower stratosphere at altitudes between 15 and 25 km. With the release of CFCs until banned by the Montreal Protocol in 1987 long-lived chemicals including chlorine (Cl) were introduced into the atmosphere. Because they are stable and long lived they distribute evenly over the whole globe. These high clouds provide a surface for unreactive chlorine to convert to reactive chlorine of which a single atom can destroy ozone molecules in the order of 100 000s during their approx. 100 year lifetime. This catalytic reaction leads to the spring ozone hole over Antarctica, which we are currently experiencing, where the stratospheric ozone concentration is reduced from 300 DU to around 150 DU.

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The clouds can be iridescent when produced by lee waves, of which there are few at Halley. We normally see the more diffuse type which you can spot on the horizon at dawn or dusk when the sun is between 1 and 6 degrees. The light of the setting sun illuminates these high clouds when the others are already in the shadow of the Earth. I often spot hazy pink clouds on the horizon once the sun is up when the ozone is very low, which are probably more diffuse PSC.

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