A smile in the sky

Circumzenithal arc over Ridgecrest, CA on 30 July 2014. Photo courtesy of Marian Murdoch.

Circumzenithal arc over Ridgecrest, CA on 30 July 2014. Photo courtesy of Marian Murdoch.

What appears to be a curved rainbow in the sky is actually what’s called a circumzenithal arc and is quite different from the more familiar rainbows that grace the sky after a rainy day. The thin, wispy cirrus clouds in this picture occur at high levels in the atmosphere where temperatures are way below freezing. At these cold temperatures, these clouds are therefore made up of ice crystals. Ice crystals can come in many shapes and sizes, as seen in this diagram.

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What controls the type of ice crystal that forms? Well, that depends on the temperature and moisture content of the environment it forms and grows in. For the circumzenithal arc to form, the ice crystals need to be plate-like. These plates have horizontal faces and shorter vertical side faces. Sunlight enters these faces and is bent within the crystal; this is called refraction. A simple diagram will help to visualize this concept:

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The separation of the lines in this picture represents the sunlight being separated into the colors of the visible spectrum (ROYGBIV), such as what you see when light passes through a prism. For the circumzenithal arc to form, the sun’s rays enter the uppermost horizontal face and exists through one of the vertical side faces. For the light to enter these crystals at these angles (nearly parallel), the sun has to be lower than about 32 degrees above the horizon.

So why is it called a circumzenithal arc? Well, the word “zenith” refers to directly overhead. If this arc were a complete circle, the center would be directly above you. The arc is seen directly above the sun.

How does it differ from a circumhorizon arc, which is typically seen below the sun? Circumhorizon arcs also form as light is refracted through plate-like ice crystals, but the sun’s rays first enter the vertical side face of the crystals and exit out of the bottommost horizontal face. For this to happen, the sun has to be at a much higher angle in the sky (~58 degrees above the horizon).

Sun pillar in PA

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Pj Johnson
Location: Lancaster County, Pennsylvania
Date: 5 July 2014

Patches of cirrus clouds not only make for a colorful sunset as the shorter wavelengths of the sun’s rays (the blues) are scattered away, leaving fiery views, but they can also create other optical phenomena. In this picture, you can see a small, yellow vertical swath of the light above where the sun is setting behind the hills. This is called sun pillar.

A sun pillar isn’t actually a vertical beam of light, but is the collection of light from many ice crystals in the cirrus clouds. Plate-like crystals (like those shown below) gently rock side to side as they slowly fall. The sun’s light is reflected off the horizontal face of these crystals, combining to create this pillar. These can most often be seen at sunrise or sunset when the sun is low in the sky.

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Mares’ tails over a national forest

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Janet Ealem
Location: Shasta-Trinity National Forest, CA
Date: 7 July 2014

At heights more than 20,000 ft (6,000 m) above the surface, cirrus clouds are made up primarily of ice crystals. These small, light, delicate crystals are easily carried by the winds, which tend to be stronger the higher you go up in the atmosphere. The winds can twist, spread, and generally distort the shape of the cirrus clouds as the wind speed and direction vary between different levels. The ends of these clouds can appear to curl as the crystals fall into different winds below, giving them the name Cirrus uncinus, meaning “curly hooks.” These clouds are also commonly given the nickname Mares’ tails, which you can see the resemblance below.

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Did you know that the nickname “mares’ tail” is also given to the invasive plant Equisetum  (also referred to as horsetail)? I can see the resemblance. I enjoy the similar patterns that appear in nature. 🙂

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Back to the cirrus clouds, it sure looked like a pleasant day to be in the national park. These clouds typically indicate fair weather, although of they start to cover more of the sky and thicken, it can be a sign of increasing moisture ahead of a warm front and possibly less pleasant weather in the next day or so.

Fallstreak hole over NY

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Daniel Linek
Location: Oneonta, NY
Date: October 2011

The sky over Oneonta on this autumn evening was covered with an altocumulus stratiformis cloud layer. These mid-level clouds (typically located between 6,500 and 20,000 feet or 2,000 to 6,000 meters above the ground), are primarily made up of water droplets, but at these levels in the atmosphere, where temperatures are below freezing, some of this water remains in liquid form: this is called supercooled water. In order for these supercooled droplets to freeze, the temperature has to either be below -40 degrees (Celsius or Fahrenheit) or there needs to be something other than water (a tiny particle) to serve as a nucleus to freeze upon. Once this freezing gets started, a process that gives off heat, nearby supercooled droplets evaporate at the expense of the growing ice crystals creating a hole in the cloud layer. As these ice crystals grow then begin to fall, you can see them in the center of the hole, giving this phenomenon the name Fallstreak Hole (also referred to as a “Hole punch cloud”).

But what kicks off the freezing process? While this could happen naturally, research has found that aircraft flying through this supercooled cloud layer can set off freezing. As air flows around airplane propellor tips and over jet wings, it can cool in a localized area, at times to temperatures colder than the -40-degree threshold required for spontaneous freezing of drops. This sets off the freezing process, which, as previously mentioned, gives off heat. Computer simulations (described in a research article in Science) indicated that this process can induce vertical motions in the atmosphere than can least more than an hour, leading to the expansion of the hole.

Notice how the hole in this particular picture looks more like a line. This is due to the lower angle through which the aircraft flew through the clouds.

Reference: Heymsfield et al., 2011: Formation and Spread of Aircraft-Induced Holes in Clouds. Science, Vol. 333 no. 6038, pages 77-81.

Cumulus under Cirrus before the heat returned

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Sheila Martin-Lynch
Location: Redding, CA
Date: 26 June 2014

Early on this, moist onshore flow found its way into this valley, providing moisture for shallow cumulus clouds to form. Upper-level cirrus clouds above indicated moisture associated with a disturbance moving through the area, responsible for light showers prior to this photo. But, as high pressure nudged its way back into the area, the lack of instability limited the growth of the “fair-weather” cumulus humilis clouds, eventually clearing out the sky for the return of hot, dry conditions for the weekend.

A halo among the fibrous cirrus

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Hanne Pernille Ryel
Location: Oslo, Norway
Date: 6 September 2013

Cirrus clouds appeared wispy and fibrous across the late summer sky as strong upper-level winds (more than 6 km or 20000 ft above the ground) carried the ice crystals. These ice crystals, specifically those with a columnar shape, bent the sun’s rays to form part of a 22-degree halo around the sun. The sun’s light enters one side of this columnar crystal, is refracted (bent) as it enters and bent again as it exits another side of the crystals resulting in a total bending of 22 degrees from how it enters, with a separation of the light into the colors of the visible spectrum (ROYGBIV).

Here is a diagram showing this refraction through a column ice crystal from the University of Illinois Urbana-Champaign’s online weather guide (http://ww2010.atmos.uiuc.edu/(Gh)/home.rxml)

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Beautiful mix of clouds in Norway

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Hanne Pernille Ryel
Location: Bjørnemyr, Norway
Date: 27 June 2014

Wow, what a mix of clouds over Norway in this picture! At the highest levels, cirrus clouds smear the sky as the wind carries the ice crystals, while mid-level altocumulus clouds spread out across the sky with their darker appearance creating a great contrast against the cumulus clouds being illuminated by the setting sun. These clouds block the sun to create upward-directed crepuscular rays! Crepuscular means “referring to twilight”, which is appropriate as it forms in the waning daylight hours over Norway. This mixture of clouds indicates moisture at many levels and if, especially, the mid-level cloud layer continues to increase in coverage across the sky, it could mean more inclement weather in store.