Clouds 101 Special: Nacreous Clouds

9/19/2012


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.

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8 comments

  1. Love all the pics!
    Xoxoxo

    Carolina

    www.the-world-c.blogspot.com

    Join my GIVEAWAY here http://the-world-c.blogspot.it/2012/09/look-of-day-like-modern-diva_18.html

    ReplyDelete
    Replies
    1. Hey sweetie! :)
      I checked out the giveaway and there's a bunch to be done to enter.
      Will have to take some time to do all but will definitely participate! :)

      Kisses

      Delete
  2. Predivno! Zaista, nemam što drugo reći! Osim, sad imam izrazitu želju posjetiti Norvešku! Ideš sa mnom? :)
    Super mi je što svoje znanje dijeliš s nama, i što ti blog tako dobiva još veću draž i nikako nije monoton. Samo naprijed, ljubim te!

    ReplyDelete
    Replies
    1. Idemo obavezno!
      Zazelila sam se hladnijeg vremena i hladnijih krajeva, jos uz ovakve oblake bilo bi savrseno! :))
      Mene je jako iznenadilo da su postovi o oblacima medju najcitanijima i dobijam dosta upita o njima. To me cini bas sretnom jer iako jako volim ove zenskaste teme, ipak je moja ljubav prema prirodi jaca.
      Hvala ti :)

      Delete
  3. Almost Spooky like Celestial Manifestations

    ReplyDelete
    Replies
    1. I always think how stunning clouds in general are, in all of their variations but I can definitely sense the eerie undertone whilst watching at Nacreous clouds :)

      Delete
  4. I see clouds like this although not quite as grand, where I live in Boulder, Colorado. We are right at the base of the rocky mountains. We have very high winds here. Do you know if they occur in Colorado?

    ReplyDelete
    Replies
    1. Lucky you! I would love to see them in the flesh once :)
      Although rare, I do think they can be seen above Colorado so it is highly likely that you have indeed spotted them yourself.
      Do share a photo if you have one - you can always find me via my Facebook page and I would be honored to share your photo (all credits included) with the readers of my blog!
      Regards,

      Selma

      Delete

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