The Earth Story

Nacreous (‘mother of pearl’) cloud Nacreous clouds, also known as polar stratospheric clouds (PSCs), appear high in the atmosphere, some 15 to 30km (10 to 20 miles) above the Earth, generally in latitudes higher than 50°, particularly in the northern hemisphere. They form in the freezing temperatures of the lower stratosphere, often below -80°C (-112°F), and are usually a mixture of nitric acid and ice crystals, sourced from parcels of moist air that are forced up through the tropopause by the same orographic oscillations that are responsible for producing high lenticular wave clouds.

Noctilucent clouds over Russia. Read more about these clouds here: https://tmblr.co/Zyv2Js1m_BIS-

Polar stratospheric clouds

Formed at icy temperatures (-85 C) high up in the stratosphere (some 25km above the ground), an outbreak of these beautiful iridescences may be starting in the boreal regions of the world, exemplified by this shot taken in Norway a couple of days ago. They are also called nacreous or mother of pearl clouds, and display a wonderful spread of shifting colours as the sunlight gets diffracted by the tiny ice crystals forming the nebulosity. They are best seen in twilight, when the angle of the solar rays produces the best effect.

Chemically they are very complex, and play a role in the destruction of stratospheric ozone caused by chlorine and bromine products such as CFC's, that we have released into the air over the past decades. The ice particles provide a physical support to a series of complicated chemical interactions and remove nitric acid from the air, changing the cycles of these elements in such a manner as to promote ozone destruction, resulting in the inevitable increase in cancer causing UVB rays reaching the surface. There are several types, depending on their composition, measured using LIDAR (laser radar, put simply). Type 1 contain water and nitric/sulphuric acid and are the main contributors to ozone destruction, while type 2 are made mainly of plain water.

Loz

Image credit: Ivar Marthinusen

http://www.antarctica.gov.au/about-antarctica/environment/atmosphere/polar-stratospheric-clouds http://www.seas.harvard.edu/climate/eli/research/equable/psc.html http://www.theozonehole.com/psc.htm

Chomped The ammonite in the photo had a really bad day sometime in the Late Cretaceous before it ended up as part of the late cretaceous formations of South Dakota's Black Hills and then flowed into the collections at the Peabody Museum of Natural History in Yale University. The species is named Placenticeras whitfieldi and it lived in the Great Interior Seaway that turned North America into 2 sub continents during this time of high sea level caused by the split up of Pangaea. Supercontinent breakup is a time of high sea levels as there are many new oceans forming, with hot buoyant rocks in their spreading ridges that float higher on the mantle and basically push the ocean waters onto land. The dark circular marks you see on the shell are puncture marks, from some predator - possibly another ammonite or nautilus. The blue colour iridescence is caused by thin film interference from the nacreous shell, in a manner similar to the gem material ammolite (see https://bit.ly/2H2cNfS) Loz Image credit: Hectonichus

styrac The iridescent, nacreous outer layer of a fragment of ?Hildoceras whorl 🌈

Ammolite

While not the only fossil that is used as a gem (five pointed sea urchins spring to mind), the opal like sheening ammonite shell from the Rocky mountains of Canada and Utah is certainly the most beautiful. I have frequently seen it mislabelled as opal, or opalised ammonite, though it is actually fossilised nacre (0.3-0.8mm thick), similar to that of modern nautilus shells (living relatives of ammonites) or pearls, rather than any kind of replacement with gemmy play of colour silica. Dating from the late Cretaceous, the usually red or green sheen on the nacre is due to a phenomenon called thin film interference, something you all see in the sheen of petrol puddles on water. The thin layers of aragonite forming the shell reflect the light differently, creating waves of interference whose colour depends on the thickness of the plates, which are of similar size to the wavelengths of light. Thicker layers produce reds and greens, and thinner ones the rarer blue and violets. Southern Alberta in the Cretaceous was a shallow sub-tropical inland sea, and the dead shells were preserved from decay in volcanic ash, that gradually altered into protective clays.

The colours are enhanced by polishing, and being a fragile material (Mohs hardness 4.5), thin slices of the shell are often used to make doublets or triplets (much like with opal), where layers of harder material such as dyed black onyx or quartz are used to protect the gem itself. As with opal, the wider the spread of playing colours as the stone is moved in bright light, the greater the value. Many different fracture patterns exist, caused by tectonic compression, with a variety of exotic names. Value is also negatively affected by the presence of cracks. It has also been stabilised with resins when exceptionally fragile. All such treatments are fully acceptable, as long as they are clearly declared.

The main commercial mine is in Southern Alberta, mining mostly fragments from the Bearpaw shale and finding about 50 intact specimens a year, whose export from Canada needs a permit. Nacreous ammonites occur worldwide, but the colours from here are the best, as is the suitability of jewellery use. The same care to avoid exposure to shock and caustic substances applies to ammolite as to pearls, unless it is protected in a triplet gem. Some only have one colour, others the full spread. The specimen in the photo resides in the American Museum of Natural History.

Image credit: James P Fisher III

http://www.gemsociety.org/info/gems/ammolite.htm http://bit.ly/2sJ9ozO http://bit.ly/2o9IXhU

Nacreous clouds vs iridescent clouds

This image shows polar stratospheric clouds (also known as nacreous clouds) over Mc Murdo station in Antarctica. Nacreous clouds typically occur in the stratosphere at heights of 15 to 25km. A rare phenomenon they appear in the Antarctic regions from mid-June to mid-October and the arctic regions during winter. They are usually seen when the sun is between 1 and 6 degrees below the horizon. At this time the stratosphere is illuminated while the troposphere is already in shadow. Thus, the best time to view them is around twilight although sometimes in full moonlight the clouds are also observable. The fact that the pearly white clouds (also nicknamed ‘ mother of pearls’) contrast sharply with the much darker sky adds to their magnificence.

Nacreous clouds are quite often mistaken for iridescent clouds. Iridescent clouds are far more common and appear in the troposphere (from 0-10km height) and all over the planet. They most commonly appear near the sun when droplet size in a cloud is uniform. These droplets in turn are organized in a coronal rings giving it almost the appearance of a rainbow. However, as the image shows nacreous clouds display much more vivid colors and literally outshine iridescent clouds.

-OW-

Image 1: Nacreous clouds seen near McMurdo station on Antarctica. Copyright Alan R Right. Image 2: Irisdescent clouds seen over Florida. Copyright George Quiroga.

References: This link also provides the best times and places to view nacreous clouds in the southern hemisphere: http://www.antarctica.gov.au/…/polar-stratosph…/observations http://www.atoptics.co.uk/droplets/irid1.htm

Awesome Nacreous Clouds over Ireland

Nacreous (‘mother of pearl’) cloud

Nacreous clouds, also known as polar stratospheric clouds (PSCs), appear high in the atmosphere, some 15 to 30km (10 to 20 miles) above the Earth, generally in latitudes higher than 50°, particularly in the northern hemisphere. They form in the freezing temperatures of the lower stratosphere, often below -80°C (-112°F), and are usually a mixture of nitric acid and ice crystals, sourced from parcels of moist air that are forced up through the tropopause by the same orographic oscillations that are responsible for producing high lenticular wave clouds.

The likeliest time to see them is during a winter sunrise or sunset, when most of the sky is dark, leaving them lit by the sun from beneath the horizon. Their iridescent pastel colours can be magically beautiful, an effect heightened by their enormous distance from the viewer.

But there is a dark side to these iridescent wave clouds: their chemical composition assists the production of chlorine atoms, which in turn contributes to the depletion of the ozone layer, 25km (15 miles) above the Earth, where nacreous clouds are mostly found. A single chlorine atom can destroy up to 100,000 ozone molecules (O3)—which is why the large-scale release of chlorofluorocarbons (CFCs) into the atmosphere was to prove such a disaster— so, sadly, these most beautiful and benign-looking clouds turn out to have a powerfully destructive environmental impact.

~ JM Image Credit: http://bit.ly/1LqD32g retrieved on 23/06/15. Labelled for reuse

More Info: Toon, O. B., & Turco, R. P. (1991). Polar stratospheric clouds and ozone depletion. Scientific American, 264(6), 68-74. Stanford, J. L. (1973). On the physics of stratospheric (nacreous) cloud formation. Tellus, 25(5), 479-482. Polar stratospheric cloud: http://bit.ly/1AS5UGI Nacreous Clouds: Stunningly Beautiful, Surprisingly Destructive. Video: http://wxch.nl/1QN87wO

NOCTILUCENT CLOUD - NONNATURAL

Noctilucent clouds are actually crystals of ice that hang around 80 kilometres (50 miles) high in the atmosphere. The ice crystals catch the light of the Sun long after it has set on the horizon. The uppermost parts of the cloud in this image are iridescent (nacreous), which gives them the appearance of mother-of- pearl. Natural nacreous clouds occur at altitudes of 20-25 kilometres. The lower parts of the cloud have a redder colour due to the large amount of dust and water in the lower atmosphere scattering blue light. The cloud’s shape is due to differing wind speed at differing altitudes.

The cloud in this image formed from the exhaust of a missile launched from a distant firing range. Same basic process - ice crystals in the atmosphere, just a different source.

-TEL

Previous post on noctilucent clouds: http://on.fb.me/WJSNQy

http://www.sciencephoto.com/media/161419/view Photograph: Science Photo Library/Rex Features