Noctilucent Clouds

Noctilucent Clouds (NLC) occur at extremely high altitude, about 50 miles (80 to 85 km), and glow at night. They form in the cold (below -200 F), summer polar mesopause and are believed to be microscopic ice crystals.

Noctilucent Clouds are seen to have characteristic shapes or forms:
Type IVeil: very tenuous, lacking in structure
Type IIBands: long streaks, parallel or crossing at small angles
Type IIIWaves: closely spaced herring-bone structure resembling sand ripples on a beach
Type IVWhirls: large-scale looped or twisted structures
More detail on form classification and diagrams of the four basic forms.

The twilight conditions which render NLC visible, impose a latitude restriction on their visibility. They are, therefore, most often seen from locations which lie between Latitude 50 and 60 degrees in both hemispheres (though they are occasionally reported beyond this latitude band). Through most of June and July (from the northern hemisphere) these locations never attain true darkness and twilight skies persist all night. The latitude range takes in Canada, Northern Europe and Russia. Very few populated land masses exist at the corresponding latitudes in the southern hemisphere.

Noctilucent clouds appear to be an extension of a more or less persistent feature of the summer polar mesosphere which is not visible to the naked eye at higher latitudes due to daylight conditions. These clouds, observed by instruments aboard satellites, are referred to as polar mesospheric clouds (PMC's).

A few useful internet resources:
NLC Observer's Homepage
Australian Antarctic Division NLC page
Pekka Parviainen's Polar Image site.
*Strange Clouds
*Climatology and Climate Change
Aeronomy of Ice in the Mesosphere (AIM) home page, A current satellite mission launched in 2007
AIM mission song and YouTube video with lyrics

Some nice videos on YouTube:
Denmark (11:25 pm - 4:00 am, July 11/12, 2011; latitude 56.2 degrees North)
Russia (after sunset)
British Columbia (before sunrise)
Norway (after sunset with regular clouds below)
Scotland (with aurora beginning around the middle of the clip)
Northern Europe from International Space Station

HISTORY OF NLC Observations and Theories

June 8, 1885 First reported observation (Bad Kissingen, Germany); possible earlier accounts
summer 1885 Several other reports across northern Europe and Russia
late 1880's Connection with volcanic dust proposed
late 1880's First photos by Otto Jesse
1906 Foerster proposes clouds are made of cosmic dust
1912 Wegener proposes clouds are made of water ice
1926 Malzev disproves volcanic dust hypothesis
1933 Humphreys proposes ice or ice-covered cosmic dust
July 20, 1933 First North American photos (Meanook, Alberta)
1957 Systematic European observations begin
1961 Hesstvedt provides quantitative theory for ice
1962 Systematic North American observations begin
1962 First rocket launch into a noctilucent cloud
mid 1960's Observed in Southern Hemisphere
1960's Measurements of mesospheric temperatures and water vapor lend support to ice hypothesis
1969 Witt proposes formation on ions
1972 Orbiting Geophysical Observatory (OGO-6) satellite detects persistent summer daytime clouds over poles
1981 Solar Mesospheric Explorer (SME) satellite confirms summer polar mesospheric cloud layer
1989 Future increases in frequency/brightness of clouds predicted
2001HALOE (Halogen Occultation Experiment) confirms primary component of PMC's is water ice
2012AIM detects "meteor smoke" in NLC's