Atmospheric beauty is becoming more common - and that is bad news for the environment

This rare "nacreous" cloud formation, appearing 12 miles above in the stratosphere, was recently photographed by a scientist at Australia's Mawson station in Antarctica. Nacreous are one of the most beautiful of all cloud formations, but they are also the most destructive to our atmosphere. Their presence encourages the chemical reactions that break down the ozone layer, which acts as an essential shield protecting us from the most harmful of the sun's rays.

Also known as "mother-of-pearl clouds", nacreous clouds exhibit spectacular iridescent pastel colours, caused by the sunlight diffracting as it passes around their tiny ice crystals.

Much less dramatic examples of the same optical effect can sometimes be seen as sunlight passes through patches of lower, common cloud like the delicate wisps of ice-crystal streaks, known as cirrus.

These examples of lower-cloud iridescence are, however, nowhere near as dramatic as the glorious displays of nacreous clouds.

Since they form much higher than common clouds - at altitudes of between 12 and 15 miles - nacreous clouds are most apparent around sunrise and sunset when their colours stand out against the darkened sky. The stratosphere is extremely dry, compared with the cloud-filled lower atmosphere, so the air needs to be very cold indeed for any ice crystals to be able to form there.

Nacreous clouds only appear when stratospheric temperatures are below -83C. This happens more frequently during winter in the Antarctic than the Arctic. This is why this type of cloud is more commonly observed in the southern hemisphere. It is also why the ozone layer is so much more depleted over the south pole, compared with the north pole.

Besides causing beautiful opalescent colours, nacreous clouds' tiny ice crystals also act to encourage chemical reactions between ozone and the chlorine and bromine that we have introduced into the atmosphere by our use of substances such as chlorofluorocarbons (CFCs) in aerosols and fridge mechanisms. Without the presence of stratospheric clouds, reactions leading to the destruction of the ozone layer are negligible. The nacreous cloud's ice particles behave as nuclei on to which the ozone breakdown can take place.

Since the use of CFCs has been phased out over most of the world, you might think that the appearance of nacreous clouds should become less of a concern.

The only problem is that the CFC gases we have introduced are expected to hang around in our atmosphere for another 50 years.

During that time, they will continue to react with the protective ozone whenever these clouds appear. This is why it is worrying that nacreous clouds are appearing more frequently over larger regions of the world.

The most beautiful clouds of all were once only commonly visible to those at the highest latitudes, such as Mawson station, in Antarctica. Now, across much of Britain, we too can watch the legacy of our troubled relationship with the atmosphere played out in glorious, mesmerising Technicolor.

Though nacreous clouds are officially known as "polar stratospheric clouds", they are now common over Scotland, and have been observed as far south as the Midlands.

Just as temperatures down on the ground can vary considerably from one winter to the next, so can those up in the stratosphere.

More nacreous clouds appear during colder winters, which lead to a greater subsequent depletion of ozone. The general increase in observations of these clouds is considered by some scientists as linked to man's contribution to global warming.

Everyone is now familiar with the way all the CO2 we have introduced into the atmosphere since the industrial revolution has a tendency to trap the sun's heat into the lower atmosphere.

Perhaps less commonly understood is that, by keeping more of the heat below, increased levels of greenhouse gases also tend to cool the atmosphere above. Lower average winter temperatures in the upper atmosphere might explain why nacreous clouds seem to be appearing with increased frequency and distribution.

· Gavin Pretor-Pinney is author of The Cloudspotter's Guide, Sceptre, £12.99 and founder of the Cloud Appreciation Society