In the wunderphoto posted by carlskou
today (see image below), the caption reads: What's that?
Well, I'll answer that question!
That is a Polar Stratospheric Cloud (PSC), also known as a nacreous or mother-of-pearl cloud. These clouds of ice crystals form in the very coldest reaches of the stratosphere over both poles in winter, at an altitude of 25-30 km. These ice-crystal clouds are frequently iridescent, meaning that they diffract the suns's light and form brilliant spots or borders of colors, usually red and green, up to about 30� from the sun.
Polar Stratospheric clouds are fairly rare, because they require extremely cold temperatures that are not often found in the atmosphere. However, residents of Norway, Sweden, Finland, Alaska, northern Canada, and other polar locations can expect to see a lot more of these in the future. Why? Because the stratospere is cooling. The Arctic stratosphere has cooled 3�C in the past 20 years due the combined effects of ozone loss, greenhouse gas accumulation, and natural variability. Winter 2005 temperatures in the Arctic stratosphere were the coldest ever recorded. A large part of this cooling is due to the greenhouse effect. Yes, the surface and lower atmosphere are warmed by the greenhouse effect, but this means that the atmosphere must cool somewhere else to compensate. Much of this cooling happens in the stratosphere. This can best be understood by considering that the Earth is in "radiative equilibrium"--the amount of solar energy coming in is balanced by the amount of energy going out. Surface warming must be balanced by upper-atmosphere cooling, since the amount of solar radiation the Earth receives does not change, and satellite measurements have shown that the amount of heat going out to space from the Earth has not changed, either.
What does all this stratosperic cooling mean for the climate? That is mostly unknown. We do know that PSCs act to greatly accelerate stratosperic ozone destruction, since the chemical reactions that destroy ozone happen much faster on the reactive surfaces that PSCs provide. A recent model study
(Rex et. al.
, 2004), indicates that future Arctic ozone depletion could be much worse than expected, and that each degree Centigrade cooling of the Arctic may result in a 4% decrease in ozone. This ozone loss will occur despite the fact that concentrations of ozone-destroying chlorofluorocarbons (CFCs) are no longer rising, thanks to the international agreements that have phased out CFC use. It's a very good thing that the nations of the world acted quickly and effectively to ban CFC use when they did, or else we would have likely seen an Arctic ozone hole open up each Spring to complement the Antarctic ozone hole.
Tomorrow's topic: Penetrating tornadoes with modified vehicles.