cloud of the day

All posts tagged cloud of the day

Mirostaw – World Meteorological Organization

Cloud of the Day – Volutus

Volutus, from the Latin volutus, which means rolled, is a species of cloud (yes, they actually come in species, and genera too) that the World Meteorological Organization (WMO) recently added to the latest edition of its venerable cloud atlas (2017.) It’s described as

a long, horizontal, tube-shaped cloud mass, often appearing to roll about a horizontal axis. It usually occurs as a single line and seldom extends from horizon to horizon.

This species is usually found in the stratocumulus genera, and more rarely in the altocumulus genera. In either case, volutus is rare.

The more observant among you might notice a resemblance to a previous Green Comet Cloud of the Day: Roll cloud. You would be right. It’s the same cloud, only now the WMO has officially included it in the cloud atlas, and given it a Latin name in keeping with the rest of the clouds. The atlas has to have a consistent naming regimen if it’s to be useful for scientific research.

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Painting by Edvard Munch – Public Domain

Did Edvard Munch paint nacreous clouds? The hypothesis was put forward by Norwegian scientists at the European Geosciences Union General Assembly in Vienna this spring (2017.) The long-held assumption is that the vividly colored sky in The Scream (1893) is a reflection of the artist’s troubled mind. A facile analysis and attractive mostly for the reason that we like to attribute romantic madness to our artists. More careful research is showing that some paintings that have been thought to contain fanciful imagery are really representing unusual meteorological phenomena, such as the lurid sunsets that happened after the volcano Krakatoa (1883) put huge quantities of aerosols into the atmosphere.

Munch himself said in his diary about the incident that inspired The Scream:

I went along the road with two friends — the sun set

I felt like a breath of sadness —

— The sky suddenly became bloodish red

I stopped, leant against the fence, tired to death — watched over the

Flaming clouds as blood and sword

The city — the blue-black fjord and the city

— My friends went away — I stood there shivering from dread — and

I felt this big, infinite scream through nature

Photo credit – Deven Stross

Check out the links and get the rest of the story. Go have a look at the Green Comet Cloud of the Day post on nacreous clouds and see if they look anything like the sky in Munch’s The Scream. This could be speculation, somewhat like my suggestion that Vincent van Gogh might have been inspired by asperatus clouds to paint some of his skies. It is also quite possible that this hypothesis is accurate and that Edvard Munch really did see nacreaous clouds. Personally, I prefer it to the analyses of pop psychologists.

Photo credit – NASA – PD

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Asperatus-Heimo-Lauhaluoma
Cloud of the Day – Asperatus

Asperatus, also known as undulatus asperatus and altocumulus undulatus asperatus, is biblical. It’s the kind of cloud that makes people think that the world might be at the mercy of supernatural forces. They might fall to their knees and beseech their wrathful gods for mercy. Other people might look at them and say, “So, that’s where van Gogh got it from.”

Photo credit - Ken Prior

Photo credit – Ken Prior

Asperatus, loosely meaning “roughened waves,” is thought to form under the same kind of conditions as mammatocumulus, only with winds strong enough to shear the mammatus bulges into wave-like undulatus forms. This cloud hasn’t yet been officially named and added to the World Meteorological Organization’s definitive International Cloud Atlas. The Atlas was most recently published in 1975. The last time a cloud was added was 1951. The jury is out on whether asperatus will be added to the Atlas, and no one expects it to be soon if it is.

Note: The World Meteorological Organization has added asperatus — renamed asperitas — to the International Cloud Atlas (see this Green Comet post) in its fifth edition published in 2017.

Photo credit - Agathman - CC-BY

Photo credit – Agathman – CC-BY

The Cloud Appreciation Society has been very important in the discovery of this new cloud type, especially its founder, Gavin Pretor-Pinney. The society has thousands of members who send in beautiful photographs of clouds, and it has a nice selection of asperatus. Because of the way their site is set up, I can’t link directly, so you’ll have to search on “asperatus.”

Photo credit: Ave Maria Mõistlik – CC-BY-SA

Asperatus is not a harbinger of stormy weather, more often appearing as the weather abates.

Photo credit - NASA - PD

Photo credit – NASA – PD

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Corona diagram

Corona diagram

All images, except where noted, credit Wiebke Salzmann CC-BY-SA. Click images for larger versions.

Cloud of the Day – Corona

Here is a meteorological phenomenon that is often misnamed “halo.” A corona is similar to a halo in that they both form rings around the Sun and Moon. The Sun’s corona (the one formed in Earth’s atmosphere, not the one around the actual Sun) is hard to see because the Sun is so bright. A corona is a more subtle effect and needs the more muted light of the Moon to really show itself.

Corona-bright-Wiebke-Salzmann-cc-by-saWhile haloes result from the light being refracted by ice crystals high in the atmosphere, coronae are caused by the diffraction of light scattered by particles – water droplets, ice crystals, dust motes, etc – in the lower atmosphere. A corona can also form on a foggy window pane. Haloes have fixed dimensions, calculable from the known refractive index of ice. Coronae come in various sizes due to the variability in the size of the light-scattering particles. Smaller droplets make larger coronae. In addition to the light scattered from the surface of the particle, small contributions to the corona are made by light that reflects directly off the droplet, or passes through it.

Corona around street lamps through an aspirated window pane.

Corona around street lamp through an aspirated window pane.

Artificial corona around LED lamps of different colors, created with lycopodium spores. As can be seen the diffraction rings of red light have a greater radius than those of blue light.

Artificial corona around LED lamps of different colors, created with lycopodium spores. As can be seen the diffraction rings of red light have a greater radius than those of blue light.

Image credit - Florian Marquardt - CC-BY-SA

Interference patterns – Florian Marquardt – CC-BY-SA

A classic corona consists of a bright aureole in the center, with one or more colorful rings around it. For the sharpest coronae, the droplets must be all close to the same size, so the interference pattern in the light can be well defined. It is constructive and destructive interference among the scattered light waves, where they add to make bright regions and subtract to make dark regions, that make the alternating rings of bright and dark.

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Image credit - Michael Glanznig - CC-BY-SA

Image credit – Michael Glanznig – CC-BY-SA

1 22°-halo, 2 Parhelia (Sundog), 3 Sun pillar, 4 Parhelic Circle, 5 Circumzenithal Arc, 6 Tangent arcs and Circumscribed Halo, 7 46°-halo, 8 Subsun

Cloud of the Day – Halo

Image credit - Rudy23 - CC-BY-SA

Image credit – Rudy23

Image credit - Andrzej Barabasz - CC-BY-SA

Image credit – Andrzej Barabasz – CC-BY-SA

A meteorological halo, also called nimbus, icebow and gloriole, is a product of sunlight or moonlight being refracted by ice crystals in the atmosphere. Although a halo can often be seen around an artificial light, such as a street lamp, the real ones form in cirrostratus clouds 5-10 kilometers above the ground. Due to the refractive index of the ice crystals, the halo forms 22 degrees away from the light source. Sometimes a second one can be seen at 46 degrees. As long as the ice crystals extend far enough and the Sun or Moon is at least 22 degrees above the horizon, the halo is always circular.

Public domain

Public domain

Public domain

Public domain

In addition to their beauty, haloes are also used in weather lore. Since the presence of cirrostratus is often a harbinger of lowering cloud, they can be used to forecast approaching weather.

Lunar

Image credit - Humberto Romero - CC-BY-SA

Image credit – Humberto Romero – CC-BY-SA

Image credit - nuno morao - CC-BY-SA

Image credit – nuno morao – CC-BY-SA

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