These
rare clouds, sometimes called mother-of-pearl clouds, are 15 - 25km (9
-16 miles) high in the stratosphere and well above tropospheric clouds. They are often found downwind of mountain ranges which induce gravity
waves in the lower stratosphere.
Their sheet-like forms slowly undulate and stretch as the waves evolve.
The clouds can also be associated with very high surface winds which
may indicate the presence of, or induce, winds and waves in the stratosphere. They form at temperatures of around minus 85ºC, colder than
average lower stratophere temperatures, and are comprised of ice
particles ~10µm
across. The clouds must be composed of similar sized crystals to
produce the characteristic bright iridescent colours by diffraction
and interference.
They
have iridescent colours but are higher and much rarer than ordinary
iridescent clouds. They are seen mostly but not exclusively in polar
regions and in winter at high latitudes, Scandinavia, Alaska, Northern Canada. Lower level iridescent clouds can be seen anywhere. Nacreous clouds shine brightly in high altitude sunlight up to two
hours after ground level sunset or before dawn. Their unbelievably
bright iridescent colours and slow movement relative to any lower clouds
make them an unmistakable and unforgettable sight
Photo Credits :
Source 1,
Source 2,
Source 3, Source 4, Source 5
