20 January 2010

all that jazz



Polar stratospheric clouds are astonishing.  Nine days ago a whole host of them flung themselves into the post sunlit sky and their freakshow left me speechless.  Hence the terse caption dangling from this photo.  But in a week and a half, I have caught my breath, relocated my tongue, and will try to sputter a few words in honor of these dumbfounding beauties.

Despite the flashy examples I've exhibited, not all polar stratospheric clouds (PSCs) are nacreous, or pearly.  In fact, what makes polar strasospheric clouds polar stratospheric clouds is not the color, the shape, or even a specific composition.  PSCs are simply concentrations of solid or liquid aerosols suspended between 15 and 30km during the Arctic or Antarctic winter.  For a PSC to be iridescent, it must be comprised of similarly sized particles whose size is comparable to their wavelength.  The wispy orb-like shape above results from orographic lift, in this case generated by strong tropospheric winds rushing over the Scandinavian Mountains. (As a side note - these mountains are fascinating.  You can read about the Caledonian orogeny here).

Yet despite their pizazz, nacreous clouds are not the most common form of PSCs.  Lately, the lidar at Kiruna has been detecting  a layer of PSCs above us which act nothing like their iridescent kin.  Without forceful winds to whip them into a colorful frenzy, they resemble subtle gray streaks, like soapy residue on a poorly washed window.  On the right-hand side of the photograph below, the sky appears to have a ribbed texture.  These bands are also PSCs, only not nacreous ones.



I must admit, the flare and mystery of a nacreous cloud floods me with admiration, but when I glanced above the trees this weekend while skiing through the woods, I was also amazed to see this warm spectrum of reds and oranges spread before a different kind of polar stratospheric cloud.

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