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The cloud with a dangerous secret
Atmospheric beauty is becoming more common - and that is bad news for
Thursday August 3, 2006
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 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
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