Clouds 101 Special: Nacreous Clouds

Unfortunately, I have never had the opportunity to observe
them personally. But these are very, very special special clouds.

Nacreous clouds are rare. They have been observed mainly in
Scotland and Scandinavia, but have also been reported from France and Alaska.
Measurements have indicated that the nacreous clouds observed over southern
Norway occured at an altitude between 21 and 30 kilometres (70.000 and 100.000
feet).

Nacreous
clouds are wave clouds and 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. Nacreous clouds resemble Cirrus or almond-shaped
(cousins of theirs!) Altocumulus. They show 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.

These clouds form in
the stratosphere, between 10 and 30 miles up. Clouds we see every day actually
form in the troposphere, with cirrus clouds being the highest at about 6 miles
up. The next layer in our atmosphere is the stratosphere, which is separated
from the troposphere by the tropopause. The stratosphere is very different to
the troposphere; it is made up of thin dry air in stable conditions. Also in
contrast to the troposphere, the temperature rises from the bottom to the top
in the stratosphere, which is caused by the absorption of heat from the ozone
layer. Also, the stratosphere is very dry and it is rare for moisture to find
it's way into this layer of the atmosphere. The ice crysals that form nacreous
clouds are pushed up into the stratosphere by wave winds that are so strong
they oscillate up through the trophosphere and into the stratosphere layer
above

They form at temperatures of around minus 85ºC, colder than
average lower stratophere temperatures, and are comprised of ice particles
~10µm across. The iridescent colours of nacreous clouds is due to the ice
crystals being of a uniform shape and size and the cloud cover being thin. The
sun has to be at just the right angle below the horizon to cause differaction
and interference with the crystals to produce these beautiful colours. The clouds
must be composed of similar sized crystals to produce the characteristic bright
iridescent colours by diffraction and interference.

Did you know that these beautiful clouds actually harm the
ozone layer? Pretty incredible but true. Nacreous clouds are a type of Polar
Stratospheric Cloud (PSC). PSC's come in two types.

Type1

Nacreous clouds composed of ice crystal with temperature of
~minus 85ºC

Type 2

Less impressive than nacreous clouds, more diffuse and less
bright colours. Sometimes nacreous clouds are embedden in them. These clouds
are slightly warmer (~minus 78ºC) than the first type of PSC's and are composed
of exotic solids or liquid droplets.

Type 2a

Crystalline compounds of water and nitric acid-especially
NAT (nitric acid trihydrate HNO3.3H20)

Type 2b 

Small spherical droplets of a solution of nitric and
sulphuric acids

Type 2c

Small non spherical particles of a metastable nitric acid –
water phase

PSC's were long regarded as curiosities and of no real
consequence. However, type 2 clouds are now known as sites of harmful
destruction of stratospheric ozone over the Antarctic and Arctic. Their
surfaces act as catalysts which convert more benign forms of man-made chlorine
into active free radicals (e.g. chlorine monoxide). During the return of Spring
sunlight these radicals destroy many ozone molecules in a series of chain
reactions. Cloud formation is doubly harmful because it also removes gaseous
nitric acid from the stratosphere which would otherwise combine with chlorine
monoxide, to form less reactive forms of chlorine. Nacreous clouds help accelerate the process of the depletion of the
ozone layer, without the clouds there is little or no damage.

Images Source

Definition of a cloud: A cloud is a visible aggregate of minute particles of water or ice, or of both, in the free air. This aggregate may include larger particles of water or ice and particles, such as those present in fumes, smoke or dust.

Appearance of clouds:

The appearance of a cloud is determined by the nature, sizes, number and distribution in space of its constituent particles. Appearance is best described in terms of dimensions, shape, structure texture, luminance and colour of the cloud.

Classification of clouds:

A classification pf the charcteristic forms of clouds, in terms of “genera”, “species” and “varieties” has been established

(I) Genera

The classification is essentially based on 10 main groups, called genera, which are mutually exclusive, that is to say, a given cloud can belong to one genus only. The genera are; 
Cirrus 
Cirrocumulus 
Cirrostratus 
Altocumulus 
Altostratus 
Nimbostratus 
Stratocumulus 
Stratus 
Cumulus 
Cumulonimbus 

(II) Species

Obsever peculiarities in the shape of clouds and differences in their internal structure have led to the subdivision of most of the cloud genera into species.

(III) Varieties

Clouds may exhibit special characteristics which determine their variety. These characteristics are related to the different arrangements of the could
elements.