photos by per andre hoffmann in antarctica of polar stratospheric clouds, so named because they form fifteen miles above the earth, a hight which, given the curvature of the planet, allows them to be illuminated after sunset by light reflected from below the horizon. 

where the stratosphere is usually much too high for water molecules to remain stable and form clouds, the temperature in the antarctic winter season drops to such an extent that what sparse water molecules are present in the upper atmosphere condense from the pressure drop, forming wide stretches of thin clouds.

unfortunately, these clouds also contain nitric acid, which reacts violently with chlorine released into the atmosphere by industrial processes elsewhere on the planet, causeng holes to form in the ozone layer.  (see also: ann hawthorne

known informally as asperatus clouds, this atmospheric phenomenon gets its name from the latin aspero, which roman poets used to describe the sea as it was roughened by the cold north wind. 

though the cause of their formation remains unknown, it is likely that the undulating and lumpy underside is a result of warmer, moister air from above and colder, dryer air from below meeting at the boundary between the lower and middle atmosphere.

when high level wind passes over rolling terrain, you get the same wavy effect as on the surface of water. but despite their ominous appearance, asperatus clouds tend to dissipate without a storm forming.

photos by (click pic) ken prior and allan gathman in perthshire, scotland; bryan and cherry alexander in qaanaaq, greenland; ti cranium in ohio; robert lurie in cape town, south africa; witta priester in new zealand; jesse klein in wisconsin

photos by ann hawthorne in antarctica of polar stratospheric clouds, so named because they form over eighty thousand feet high, which is high enough, given the curvature of the planet, to be illuminated after sunset by light reflected from below the horizon. it is this iridescence that gives the clouds their informal name, mother of pearl clouds.

where the stratosphere is usually much too high for water molecules to remain stable and form clouds, the temperature in the antarctic winter season drops to such an extent that what sparse water molecules are present in the upper atmosphere condense from the pressure drop, forming wide stretches of thin cloud.

unfortunately, these clouds also contain nitric acid, which reacts violently with chlorine released into the atmosphere by industrial processes elsewhere on the planet. the reaction causes holes to form in the ozone layer, which in turn serves to further cool the upper atmosphere, thus spurring the formation of more stratospheric clouds, and more chlorine radicals, and more holes in the ozone layer.  

photos by mike hollingshead in oklahoma of mammatus clouds, which are formed when air laden with big water droplets is carried to the top of a thunderstorm cloud whose altitude is cold enough to freeze the water droplets. the resulting crystals sink back down towards the earth, collecting at the base of the cloud before they have time to evaporate. mammatus clouds are usually only stable for a few minutes. (see also: previous cloud posts)

images by seb janiak, which he creates by layering several photos on top of each other. (see also: previous cloud posts)

circumhorizontal arcs photographed by (click pic) david england, andy cripe, del zane, todd sackmann and brandon rios. this atmospheric phenomenon, otherwise known as a fire rainbow, is created when light from a sun that is at least 58 degrees above the horizon passes through the hexagonal ice crystals that form cirrus clouds which, because of quick cloud formation, have become horizontally aligned. (see also: previous cloud posts)

photos by (click pic) tim kemple (previously featured) and scotty rogers of the moab monkeys slacklining three thousand feet above rio de janeiro (see also: previous brasil posts)

photos by denis budkov of the 2012 tolbachik eruption in russia’s kamchatka peninsula (see also: previous volcanology posts)

"stormscapes" by nicolaus wegner in wyoming (see also: previous extreme weather posts)