Nacreous Clouds

The crazy cloud invasion (photo by B-nelson).
Over the past couple weeks, we have seen some spectacular cloud formations. They are some of the best I've seen in my life. They are mostly caused by the rising sun illuminating the nacreous clouds against a contrasting sky. Unfortunately, getting a picture is just about impossible. The brightest parts of the clouds require a short exposure, the regular clouds a longer one, and the ground the longest one. The human eye is the best camera ever and none of these pictures truly do the experience justice.


Probably my best picture because it has the blue sky, bright clouds, and a little foreground.
We don't get these clouds in the middle of the day. They mostly appear at 'sunset'. They are supposed to appear at sunrise too, but I haven't seen any yet. I'll keep my eyes open. The rest of this blog is quoted and paraphrased from an explanation by Bec in the WWTP about nacreous clouds. She's knows a lot more about this stuff than I could ever know.


Clouds over McMurdo Sound (there should be blue sky at the top).
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).


Nacreous clouds over McMurdo Sound.
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.


Nacreous Clouds over Arrival Heights.
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 because chlorine gases are able to react in the clouds and produce ClO which eventually goes onto react with the ozone. It can destroy thousands of ozone atoms. In November/December, the warmer, ozone rich air will flood into the region and the hole will disappear.


More crazy clouds that are missing the bluer skies.