Category Archives: Welcome

A gustnado near St. Louis

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Eileen Lenkman, 7 May 2016, eastern Missouri looking across Mississippi River toward Illinois

On May 7, 2016, a line of storms moved through the midwest. Eileen Lenkman shared a series of photos from eastern Missouri as the storms moved from the NW toward the SE.

In this first photo, the storm can be seen approaching the area.

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Eileen Lenkman, 7 May 2016, eastern Missouri looking toward Illinois

 

Composite radar imagery at this time shows the extent of this precipitating system as it moves toward the St. Louis area.

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In the next photo, the heavily raining portion of the storm appears in Eileen’s view.

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Eileen Lenkman, 7 May 2016, eastern Missouri looking toward Illinois

 

The rain locally cools the air, which spreads out near the ground away from the raining core of the storm. The leading edge of this rain-cooled air is referred to as a gust front and is typically accompanied by strong winds.

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Schematic showing the gust front at edge of rain-cooled air

Not only is there a marked temperature distance across the gust front between the rain-cooled air behind it and the warm, moist air ahead, difference in wind speed and direction behind and ahead of the gust front can create considerable horizontal wind shear across that boundary far out ahead of the raining core of the storm. The warm, moist air is lifted up and over the colder dense air behind the gust front. This upward motion can tilt and vertically stretch the small-scale vortices that can form along the edge of the gust front due to the horizontal wind shear, creating a spinning vortex that can extend upward from the ground; this is casually referred to as a “gustnado.

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This shallow, short-lived vortex may only extend upward 10s of feet above the ground with no apparent connection to the cloud above. Away from the raining core, a debris cloud or dust whirl is seen near the surface. Wind speeds can reach 60-80 mph in these gustnadoes, but they are not considered to be a tornado as they are not associated with any sort of parent rotation in the cloud above.

On 7 May 2016, when the storms were moving near St. Louis, Eileen Lenkman captured one of these gustnadoes on camera.

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Gustnado, Eileen Lenkman, 7 May 2016, eastern Missouri looking toward Illinois

In this photo, the raining core can be seen to the far left of the photo while out to the right the small debris whirl near the surface can be seen at presumably the leading edge of the gust front.

 

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Supernumerary bows over Oahu

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Primary rainbow with supernumerary bows and a faint secondary bow. Photo from Noboru Chikira, 21 Apr 2016, Oahu, Hawaii.

 

No, your vision isn’t blurred. There are actually additional colored bands bordering the bright primary rainbow in this picture from the Hawaiian Island of Oahu. These faint, pastel bands of light are referred to as supernumerary bows. “Supernumerary” means “more numerous” and is an adjective used not only for describing this optical phenomenon, but also for everything from teeth to military officers. To understand how these bands form, we first need to consider what creates a rainbow in the first place.

Rain showers over the tropical Hawaiian Islands, and elsewhere, can lead to vibrant rainbows. How does this occur? First, consider the energy emitted by the sun, which includes waves of energy in the UV, infrared, and visible portions of the electromagnetic spectrum. Most of the sun’s energy is emitted as visible light, which includes a range of colors (ROYGBIV).

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In 1665 Isaac Newton, through his infamous prism experiment, was able to prove that the white light of the sun was actually composed of a color spectrum. The sun’s visible light entered the prism and was refracted (bent) through the prism, with the red (longest wavelength) bending the least and the shorter wavelength violet bending the most.

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A raindrop falling from a cloud can act as a prism, bending and reflecting the light to produce the colors of the rainbow. When the sunlight encounters a raindrop, some of the light is bent as it enters the drop. This refracted light hits the back of the raindrop, is reflected internally within the drop, then is bent (refracted) once again as it exits the drop.

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Because the different colors of the sunlight bends at different angles, the result of one internal reflection means the primary rainbow will have red on the outside and blue on the inside. This process happens to an assortment of raindrops falling from the sky leading to the existence of many rainbows at the same time; however, which one you see (and how much of one you see) depends on your viewpoint relative to the angle of the sun above the horizon behind you.

Sometimes, a second, fainter rainbow can be seen in the sky. This secondary rainbow results from two internal reflections (instead of one) inside the raindrop, leading to the colors appearing in the opposite order as the primary bow. In the following picture, you can see the faint secondary bow in the upper right.

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Primary rainbow with supernumerary bows and a faint secondary bow. Photo from Noboru Chikira, 21 Apr 2016, Oahu, Hawaii.

 

The key to the formation of rainbows, is therefore dependent on the sunlight, raindrops, and how much and how many times the sunlight bends and reflects within the drop. To understand supernumerary bows, the feature of this post, it’s important to remember that rays of light are waves of energy. Think of the ripples and waves that form on a water surface and what happens when they interact with each other. Some can counteract and destroy each other, while others can join to make a bigger wave. A similar description can be applied to waves of light.

The distance between the crests of these waves of energy is referred to as the wavelength (recall red light has a longer wavelength than blue).

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If the waves are in sync with each other, they can constructively interfere to amplify the wave. If they are out of sync (out of phase) with each other, they can destructively interfere to cancel each other out.

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So with this in mind, given the different angles of refraction for the different colors of the sun’s rays when sunlight encounters and is bent through and around a raindrop, there is bright light where the crests of waves are aligned (and therefore constructively interfere); similarly, there is darkness where the waves of light destructively interfere. Each of the bright fringes is a supernumerary bow, created by interference between different portions (colors) of the same light wave.

Why can’t we always see supernumerary bows? Well, their presence depends on the size of the raindrops. Supernumerary bows can only be seen when the sunlight encounters small raindrops that are all nearly the same size. In a typical rain storm, there are drops falling of many different shapes and sizes, which would wash out the colors of the supernumerary bows. Basically each differently sized raindrop would produce differently spaced, overlapping fringes that would blur. There is a sweet spot, though, because as the drops become even smaller, the bow broadens, the colors become less saturated, and eventually there is no longer a vibrant rainbow but a faint cloudbow or fogbow. An excellent description of these bows and their dependence on drop size can be found here: http://www.atoptics.co.uk/rainbows/supdrsz.htm

The first explanation for supernumerary bows was provided by the English physician and scientist Thomas Young in 1804.

Fallstreak holes over Missouri

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Fallstreak Hole, Contrail, Altostratus (Karl Kischel, Williamsburg, Missouri, 18 Feb 2016)

Hole punch clouds have fascinated sky watchers, scientists, and pilots since the 1940s. Nearly circular in appearance, the hole punch cloud can have streams of ice falling from its center (as in Karl’s picture), thereby giving a subset of these clouds the name fallstreak holes. These beautiful, fascinating atmospheric phenomena are created by aircraft penetrating cloud layer. In Williamsburg, it’s not unlikely to see planes ascending and descending due to the nearby busy St. Louis airport.

Fallstreak holes require a specific cloud type: mid-level altostratus or altocumulus clouds that exist between 6,500 and 20,000 ft above ground. At these altitudes, temperatures are well below freezing, but water droplets exist in liquid form at these sub-freezing temperatures, called supercooled water. To freeze, liquid water droplets need either a nucleus to freeze upon (either ice itself or a particle in the air such as dust, bacteria, fungal spores, volcanic ask, etc.) or temperatures to be below -40 degrees C to freeze spontaneously without a nucleus.

So how does an aircraft flying through supercooled water lead to freezing of drops and ultimately a fallstreak hole?  We know that when aircraft fly high in the sky, at very cold temperatures (i.e., below -40 degree C), the water vapor in the jet engine exhaust rapidly freezes to form contrails across the sky, as can be seen in the photo above. But the key for fallstreak holes is the localized cooling that’s created around propellors and wings. Propellors push air outwards, causing the air to expand, which lowers the pressure and therefore cools the air. For jets, lower pressure exists above the wing compare to below, again leading to localized cooling of the air. This can cool the air to temperatures below -40 degree C, even when the aircraft is flying at lower temperatures, causing the supercooled water droplets in the cloud layer to spontaneously freeze where the aircraft passes through.

These ice crystals begin to grow at the expense of the nearby supercooled droplets, referred to as the Bergeron-Findeisen process. The vapor left behind by the evaporated supercooled droplet deposits onto the ice crystals thereby, along with the remaining supercooled droplets freezing on impact, allowing the ice crystals to grow.

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Drawing representing the Bergeron-Findeisen process: Ice crystals growing at the expense of water droplets

 

The freezing processes gives off heat, warming the surrounding environment.

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This warmer air rises, cools, and creates small circulations where downward (subsiding) air compensates for the locally rising air where the ice crystals are growing. The subsiding air warms, creating the hole.

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Computer simulations of a hole punch cloud showing heating (red), cooling (blue), and the corresponding circulations (black arrows). From Muraki et al. (2015).

 

These holes can spread for hours, lasting more than 4 hours at times, and the ice crystals can grow so large that they start to fall as snow, leading to the name fallstreak hole.

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Zoomed in view of the hole punch cloud showing the fall streaks (Karl Kischel, Missouri, 18 Feb 2016)

 

On 18 February 2016, Karl was lucky enough to photograph two of these hole punch clouds over Williamsburg, Missouri.

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Two fallstreak holes, Karl Kischel, Williamsburg, Missouri

 

These fallstreak holes could also be seen on visible satellite imagery! Those in the circle are the same ones Karl was photographing. Notice there’s a third one nearby. In fact, because these are visible from satellites, scientists have used high-resolution satellite data to look at the occurrence of fallstreak holes around major airports in the U.S. They found that they occur 3-5% of the time on average per year, and about 15% of the time during the winter (when we’re most likely to see these altocumulus/altostratus cloud layers).

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GOES Visible Satellite image from 15:15 UTC on 18 February 2016 showing the fallstreak holes.

Besides wanting to know your chances of seeing these beautiful hole punch clouds in the sky, why is it important to know how often they occur? There’s an argument that the increased snow that falls from the holes could mean more de-icing would be required at the airport before takeoff.

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Computer simulation after 60 minutes from when ice was introduced into the cloud layer. From Heymsfield et al. (2011)

 

Thanks for the great pictures, Karl! Enjoy additional photos he took from that day.

A “Bore”ing, “Glory”ous Morning over Oklahoma

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Roll cloud associated with an undular bore over Norman, Oklahoma on 17 January 2016. Credit: Pamela Heinselman

 

On the morning of 17 January 2016, Pamela Heinselman captured a beautiful sight over Norman, Oklahoma: A long, extensive roll cloud associated with an undular bore.

 

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Roll cloud associated with an undular bore over Norman, Oklahoma on 17 January 2016. Credit: Pamela Heinselman

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Roll cloud associated with an undular bore over Norman, Oklahoma on 17 January 2016. Credit: Pamela Heinselman

 

What exactly creates these clouds? An undular bore is a type of gravity wave (meaning gravity is the restoring force) in the atmosphere (particularly the lower levels near the ground) that occurs when a low-level boundary such as outflow from a thunderstorm or a cold front reaches a layer of cold, stable air. This “disturbance” of the air is similar to when you disturb water by throwing a rock in the pond, creating ripples on the water’s surface. The disturbance of the low-level air by the front leads to ripples in the air. If the air is moist, clouds will form where the air is rising along this wave-like disturbance.

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Idealized drawing of an undular bore created by a cold front. From Hartung et al. (2010).

 

Bores have been described in the scientific literature since as far back as 1950. With advancements in our observational capabilities through ground-based and satellite-borne instruments, the formation, properties, and impact of these bores have been studied in great detail since then. The drawing above is adapted from one of these studies, in which a group of scientists described an undular bore associated with a cold front that moved through Oklahoma in 2006, much like the one photographed by Pamela in this blog. In that study, they showed that there was a strong temperature inversion near the surface in the morning, meaning that temperature increased with height indicating stable air ahead of the cold front.

Shown below is data from instruments released on a balloon that tells us how temperature, moisture, winds, and pressure change as you go up in the atmosphere. On the morning of 17 January 2016, not long before Pamela took her photos of the roll cloud, the ballon data showed a strong inversion near the surface with the temperature at the ground below freezing, indicating that the air over Norman, Oklahoma was cold and stable ahead of the cold front.

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This cool, relatively stable air trapped near the surface was lifted ahead of the advancing cold front. Due to the inversion, that lifting air was trapped, leading to the oscillating, wave-like pattern that resulted in the undular bore seen in the photos over Norman. Not only did this bore show up as a roll cloud in photos by Pamela and others, the roll pattern of clouds associated with the bore showed up in visible satellite imagery, looking like ripples in a pond. Notice how far these clouds extend across Oklahoma!

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Visible satellite image at 1437 UTC (8:37 AM) on 17 Jan 2016 showing the undular bore over central Oklahoma

 

When these bores pass over an area, not only can you visibly see them as beautiful, long rows of clouds, they are also associated with shifts in surface observations, particularly gusty shifting winds and rising pressure. Data from a station at Norman shows a sharp increase in pressure along with a quick increase in wind speed and change in direction around the time these photos were taken! The cold front passed through later in the day, leading to another shift in the winds to northerly and decreases in temperature and moisture.

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Surface data from Norman, Oklahoma showing the surface response to the bore.

 

While many studies of bores have occurred in Oklahoma due to many instruments available, they are not unique to this area. Examples from Iowa have shown bores on radar and webcams, as well as examples off the coast of Texas. Perhaps the most well-known example of undular bores throughout the world is a roll-cloud formation in Australia called the Morning Glory.

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The view from a glider of a morning glory cloud in northern Australia (Credit Al Sim – file photo)

 

Triggered by sea breezes near the Gulf of Carpentaria, the length, persistence, and smoothness of this cloud band attracts scientists and gliders (as seen in the photo above) who like to “surf” these atmospheric waves. The Morning Glory Cloud is such a common feature in the Spring in this area that there’s even a festival named after this incredible phenomenon.

Welcome to our new site!

The Community Cloud Atlas began on Facebook, but after the wonderfully overwhelming response to the page, we decided to expand to the blogging world. This site will allow us to better organize and tag the photos submitted, making them more searchable in terms of cloud type, location, time of year, etc. In addition, we can share and explain more of the submitted photos without overwhelming Facebook news feeds.

To submit photos:

– Share them on our Facebook page (http://www.facebook.com/CommunityCloudAtlas)
– Use @TheCloudAtlas or #CommunityCloudAtlas on Twitter
– Email us at CommunityCloudAtlas@gmail.com

Thanks and welcome to our community!

Angela and Nick