Sunday, August 17, 2008
We've been very fortunate this year as there are Nacreous Clouds
seemingly every day. One of my bosses, Bill calls them the
Death Clouds because their presence signals the death of the
planet's ozone...this is only partly true as will be explained
Seeing Nacreous Clouds is one of the best parts about being
here this time of year. It certainly isn't the balmy
weather. We've had -40 and colder ambient temperatures
and nearly -80 windchills lately. Not good times to be
outside. It's very easy to stop and just look at these
clouds in awe though. These photos that very talented
photographers placed on our common drive are wonderful but
even these do the clouds no justice.
Bec, who worked here a couple of years ago as the Research
Assistant, wrote a brief synopsis how and why Nacreous Clouds
What is a nacreous cloud?
The nacreous color of the cloud is a result of diffraction
of light through ice crystals in the clouds. In themselves,
nacreous clouds are not too uncommon, and can be seen in
mid-high latitudes when the atmosphere is cold enough to
freeze the droplets of water in the clouds. The nacreous
clouds that we see at this time of year in Antarctica though,
are a special variety of nacreous cloud – a type 2 polar
stratospheric cloud (PSC).
PSCs are unique to the polar regions. As their name implies,
they form in the stratosphere, between 10 and 25 km
(6.2 - 15.5 miles). They also only form when stratospheric
temperatures get below about -110F. There are several types
of PSCs, containing some combination of nitric acid, sulfuric
acid and water. The clouds we observed on Monday were type 2
PSCs, which are pure ice – these form in the coldest conditions
(-120 F), and are thus the most uncommon. Because they are so
high up (Mondays were approximately 20-22 km (13 miles),
as determined by the Crary LIDAR), the sunlight is able to
hit them before it hits us here on the ground, giving us
spectacular light shows. The wavy look of them is due to
wave action in the stratosphere, which is often associated
with type 2 PSC formation.
The role of PSCs in ozone depletion:
You may have heard that these beautiful clouds are the
cause of the ozone hole. This is only partly true – there
were PSCs long before there was an ozone hole (sightings
are recorded in the diaries of the first winter-over group).
However they are a key ingredient.
Chlorine is released into the atmosphere in the form of CFCs.
These are very stable, so hang around in the atmosphere until
they end up in the stratosphere, where they are broken down.
In the stratosphere, the chlorine normally forms into one of
two gases which are also very stable (HCl and ClONO2 if you’re
a chemistry type person). These are distributed all over the
planet by normal atmospheric dynamics. In the Antarctic winter,
a wind forms around the continent (called the polar vortex)
that isolates the air over the continent from warmer air
outside. Without the sun hitting that air, it gets colder
and colder until PSCs form. On the surface of the cloud
particles, the stable HCl and ClONO2 gases are able to react
together, in a much greater way than they could have without
the clouds. This releases the chlorine in a very reactive form
(ClO). When the sun returns about this time of year, the ClO
is broken again by the sunlight, and the free Cl atom is able
to catalytically destroy ozone (i.e it destroys the ozone
without being used up itself). Each Cl atom can destroy thousands
and thousands of ozone atoms…. the net result being the ozone hole!
The ozone hole remains confined inside the polar vortex, which
is what gives it its circular shape, until the wind dies down
around November/December. At that time, the warmer, ozone rich
air floods into the region and the hole disappears.
What about the Arctic?
The same process occurs in the Arctic as in the Antarctic – the
difference being that the wind that forms around the Arctic goes
over land instead of over ocean. This means it is a lot more bumpy,
and more outside air is mixed in, keeping it a little warmer. So
there aren’t as many PSCs up there… so not so much chlorine is
released, and ozone depletion is not so great (there IS ozone
depletion in the Arctic, though it is seldom enough to be classified
as a hole).
Isn’t the Ozone Hole over New Zealand?
This is one of the mostly widely held misconceptions in NZ!
The ozone hole is only over the Antarctic. What we do get
though, is some thinning of the ozone layer when the ozone
hole breaks down and all that ozone-poor air rushes out.
As this is in mid-summer in NZ, the resulting sun burns can
be very painful (wear lots of sunscreen!)
Interested in learning more?
There is lots of information on the internet. A great place
to watch the ozone hole forming is http://ozonewatch.gsfc.nasa.gov/
which gives a daily update on the Antarctic ozone hole, from
measurements made from satellite, balloon and ground-based
instruments. There are links from there too.