Wind Chill: -48F
It has only been a week since the sun made its first appearance, and already it is staying up for 5 hours a day. It will be some time before we can see it from town because it is still very low on the horizon to the north, where the view is blocked by Erebus and the rest of Ross Island. I have the pleasant luck to go up the hill every day and so I have seen it several times so far. It's a big deal. People are ecstatic to go up with me for a glimpse.
Unfortunately, the return of light doesn't make us any less crazy. In fact, it may make it worse initially; some folks are having trouble sleeping again now that the light is back to influence their circadian rhythm. I've had a bit of insomnia myself. It is exciting to see the light again, though, because it means the end is near, and that lifts spirits. If only it could lift the constant fatigue.
The temperature took another drop and we've been steady at -40°F this past week. I suppose that's nothing compared to -90°F at the pole, but damn, it's still cold. The nacreous clouds have ceased for the time being, though I'm not sure why, the cold should help.
Nacreous clouds (otherwise known as Polar Stratospheric Clouds, PSCs, or Polar Nacreous Clouds, PNCs) form in the lower stratosphere, from about 8 to 15 miles up. They require the frigid temperatures that occur at this point in the atmosphere. As you travel up through the stratosphere, the temperature increases. It decreases again in the mesosphere and then increases dramatically in the thermosphere and on into space. "What?!?" you say, "space is hot?" Well, not really. Space itself is cold; it's a void. However, the particles up that high are not protected from solar radiation, which can heat them up to 4500°F. It's really more accurate to say that the particles in space are hot, unless they're in the shade, that is.
So, particles in the the lower stratosphere are generally about -70°F, but PSCs only form below -80°F, and the spectacular Type II PSCs that we see here form at the lowest temperatures, below about -110°F. Temps that low occur in the stratosphere only near the poles, and are rarer in the Arctic. In the Arctic, landmasses contribute to atmospheric weather flow by mixing in warmer air from lower latitudes. In the southern hemisphere, there is comparatively little landmass, so the air over Antarctica becomes much more isolated from the air swirling around the rest of the Earth. This is called the polar vortex. In the austral winter, this separation allows the Antarctic air to cool much more than Arctic air.
So, in that frigid air, PSCs form. Type I PSCs consist of ice as well as nitric acid and sometimes sulfuric acid. Type II PSCs are made of pure ice, so their nacreous, or mother of pearl, color is a result of diffraction of sunlight through ice crystals in the clouds. Because PSCs occur at high altitude, they are able to reflect direct sunlight to areas where the sun is still well below the horizon, making them appear extremely bright. So the up side is: they're amazingly beautiful.
I've done some more timelapse photography, primarily of nacreous clouds. I've put them all on surlyjam in high resolution, and I recommend downloading them from there (note: you'll need to have the XviD codec installed to play them). Here are a couple low-res teasers, though:
By far my favorite is the one from August 15th. The wind at Arrival Heights was blowing steadily over 50 knots, whipping up huge plumes of blowing snow that you can see flowing across the ice.
So that's the up side. The down side is that PSCs play a large role in ozone depletion, especially in the dreaded ozone hole. But before I go into detail, let me add in the disclaimer that it's not the clouds' fault, it's ours. PSCs are recorded in the diaries of early Antarctic winter-over groups over 100 years ago, long before the onset of severe ozone depletion.
First of all, what is ozone? Ozone is just oxygen, in a less common (and less stable) form, O3 instead of O2. It resides primarily in the stratosphere, and does a fantastic job of absorbing ultraviolet radiation from the sun, preventing us (and all the other plants & critters) from adverse effects like skin cancer and death. For example, reduced plankton populations in certain areas are suspected to be a result of increased UV levels.
What is happening to ozone? Most of it boils down to the chlorine (Cl) in chlorofluorocarbons, though other airborne emmisions like hydroxyl (OH), nitric oxide (NO), and atomic bromine (Br) perform the same role. These chemicals act as ozone depletion catalysts, which means they are able to break down ozone without being altered themselves. "One CFC molecule typically degrades around 10,000 ozone molecules before its removal, but this number can sometimes be in the millions."
How do PSCs fit in? Well, the chlorofluorocarbons have to be broken down into atomic chlorine in order to react with ozone. On the surface of the frozen cloud particles, chlorine gases are able to break down in a much greater way than they are normally capable. So the clouds aren't actually depleting ozone, they're speeding up the breakdown of more complex chlorine gasses. One of the more simple resultant gasses is chlorine monoxide (ClO). When the sun returns to the Antarctic, the light breaks ClO apart. Then this happens:
Cl + O3 → ClO + O2
Ozone gets turned into oxygen and ClO. The sunlight breaks apart the ClO, and the process repeats, and repeats, and repeats, and repeats. The single oxygen molecules left floating around are much more likely to become oxygen than ozone. Because the cold Antarctic air is isolated during the austral winter, the ozone hole forms. This is what the hole looked like one week ago, according to NASA sattelite data:
This picture is from a NASA website that has really good ozone hole information, including pictures and movies. Follow this link to see the formation and dissapation of the ozone hole in 2007. It is a great way to see how the hole forms over the winter every year and then blends in with the rest of the atmosphere when the sun returns to warm it up.