POLAR STRATOSPHERIC CLOUDS

Jan 27, 2020
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Arctic skies are filling with color today--but it's not the aurora borealis. A rare outbreak of polar stratospheric clouds(PSCs) is underway. Jónína Óskarsdóttir photographed the display from Fáskrúðsfjörður, Iceland:

psc_iceland_.jpeg

"We have been seeing these clouds for a couple of days," reports Óskarsdóttir. "In this picture, Mt. Jökultindur is silhouetted by a sky-full of nacreous color*."

Polar stratospheric clouds are rare. Earth's stratosphere is very dry and normally it has no clouds at all. PSCs form when the temperature in the Arctic stratosphere drops to a staggeringly-low -85 C. Then, and only then, can widely-spaced water molecules begin to coalesce into tiny ice crystals. High-altitude sunlight shining through the crystals creates intense iridescent colors often likened to auroras.

NASA models of the polar stratosphere show that temperatures have indeed dropped into the very low range required for Type II PSCs:

temps_strip.jpeg

However, the forecast (yellow) calls for warming as January comes to an end, so now is the time to look. During a typical Arctic winter, PSCs appear no more than two or three times. If you see one, it's definitely worth taking a picture.

more images: from Fredrik Broms of Kvaløya, Norway; from Jónína Óskarsdóttir of Fáskrúðsfjörður, Iceland; from Francois Guilhaume-Bohl of Nellim, Finland

Nacreous adjective:
(en adjective)
  • Of, or resembling nacre (mother of pearl).
  • *{{quote-book, year=2001, author=Neil Gaiman
title=American gods citation , chapter=11 , isbn= , publisher=Harper Collins , location= , editor= , volume= , page=323 , passage=They are covered with a smooth, safe, nacreous layer to let them slip, pearllike, from our souls without real pain.
  • Exhibiting lustrous or rainbow-like colors.
See also:

* opalescent, opaline, pearlescent

See: https://wikidiff.com/color/nacreous
* Nacreous color, nacreous clouds: Clouds resembling Cirrus or Altocumulus lenticularis and showing very marked irisation, similar to that of mother of pearl; the most brilliant colours are observed when the Sun is several degrees below the horizon.

nacreous clouds.jpeg

Ice polar stratospheric clouds, or nacreous clouds, occur mainly at high latitudes during the winter when temperatures in the stratosphere fall below the frost point. They are most common in Antarctica, but have also been observed in the Arctic, Scotland, Scandinavia, Alaska, Canada and the northern Russian Federation. On rare occasions, they have been reported in other parts of northern Europe. Nacreous clouds are often lenticular wave clouds and thus are found downwind of mountain ranges that induce gravity waves in the stratosphere. Their formation may also be associated with severe tropospheric storms.

By day, nacreous clouds often resemble pale Cirrus. After sunset, they are characterized by brilliant iridescent colours, which are more extensive and more intense than the localized irisation, or iridescence, that often appears on the edges of thin tropospheric clouds (for example, Altocumulus lenticularis). Iridescence is most brilliant when the Sun is several degrees below the horizon. Later, with the Sun further below the horizon, the various colours are replaced by a general coloration that changes from orange to pink and contrasts vividly with the darkening sky.

If Cirrus and nacreous clouds coexist after sunset, the high altitude of the nacreous clouds causes them to show bright colours after the Cirrus has already turned grey. Nacreous clouds that are also lenticular will be stationary at the crest of a gravity wave, although the air flows through the cloud. Non-lenticular nacreous clouds may appear to move slowly, due to their distance from the observer, as they appear above the setting Sun.

See: https://cloudatlas.wmo.int/en/nacreous-clouds.html

Gorgeous colors are found in the solar Stratospheric Clouds (PSC). But they only form at a chilly -85C or -121F. Wow, -121F is mighty cold, indeed.
Hartmann352​
 

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