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September-October 2014

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The Extraordinary Sky Part II: Seeking the Atmosphere's Strangest and Most Spectacular Phenomena

The sky above us provides a daily panoply of fascinating events that keep the weather enthusiast busy, day in and day out. But sometimes strange phenomena occur that breach the divide between the weather enthusiast and everyone else, bringing meteorology to the attention of the rest of the world. In Part II of “The Extraordinary Sky: Seeking the Atmosphere's Strangest and Most Spectacular Phenomena,” we examine colored rain, quaternary rainbows, ball lightning, Saint Elmo's Fire, mirages, and numerous other phenomena that keep our eyes glued to the sky.

Clouds Forming Waves, Eddies, and Streets

When we look to the sky, we typically expect to see chaos and asymmetry, which, for many, represents the essence of nature's dynamism and disordered beauty. Patterned regularity in clouds often catches our attention—it's unexpected, but is as much a part of nature as the amorphous wisps of cirrus or disorganized cumulus we often see overhead.

A type of clouds forms at a variety of scales when two air masses of distinctly different density share a common interface, for example, at an inversion (where temperature increases with altitude), where lower density air overlies higher density air. The inversion supports wind shear across itself, and this gives rise to waves propagating along the inversion. Billow clouds form when the humidity in such a region reaches a sufficient level, allowing cloud material to make these waves visible, where they can be seen breaking over and curling just like water waves. Billow clouds, specifically caused by a process called Kelvin-Helmholtz instability, often develop in multiples, one after the other, with little separation between them.

Gravity wave clouds occur within a stably stratified layer of air, that is, a layer resistant to convection. A rising cumulus tower, impinging on the underside of a stable layer of air, can set gravity waves in motion. Another common source of gravity waves is cross-mountain flow. In either case, air forced upward into a stable layer quickly becomes colder than its surroundings (it becomes negatively buoyant). Gravity acts to pull this air back to a level where its temperature matches that of its surroundings, but the air overshoots its mark, sinking lower and becoming warmer than its surroundings. Positive buoyancy then brings the air back up, continuing an oscillation that only slowly damps out. These oscillations occur while the air has a horizontal component of motion. Often the crests of the gravity waves are marked by clouds and the troughs by cloud-free air. Sometimes the wave crests form long, parallel bands of clouds perpendicular to the wind direction, and separated by cloudless bands of roughly equal width.

Orographic lee wave clouds present a perfect example of this process. Given adequate moisture, a cloud will form near the crest of a mountain range as the prevailing wind, blowing perpendicular to the range, is forced up and over the barrier. Up- and down-oscillations in the flow downstream of the range often result in multiple, elongated lines of clouds at the crest of each wave. These cloud lines may extend tens of miles downstream, and chinook and related winds can cause such formations. One of the more famous regional lee wave clouds is the Sierra Wave, which forms over the Owens Valley, just east of the high Sierra. Related to this type of cloud is a rotor, which spins on a horizontal axis between the mountain range and the lee cloud formation. A rotor can spell disaster for aircraft of all types and sizes due to the associated extreme wind shear.

Another form of gravity wave cloud, the undulatus, typically forms near inversion layers. The most commonly observed form of undulatus, altocumulus undulatus, appears as a sea of closely and regularly spaced cloud elements in wave-like configuration. The undulatus asperatus cloud, a rare form of this type, has recently been nominated to have its own official classification—which would be a first since 1951. Atmospheric scientists have not yet determined how the undulatus asperatus forms, but the cloud often appears above the interface between two distinct air masses, which leads to patterned (undulating) turbulence.

Kármán vortex street cloud formations occur when a (generally low) cloud mass, typically over water, blows into a prominent island. The clouds form a “street” downwind of the island of interconnected, regularly spaced clockwise and counterclockwise vortices (eddies) that can stretch for hundreds of miles.

Another form of the cloud street is the horizontal convective roll. Sometimes extending for 100 miles, this form arises above typically flat portions of the globe, including plains and oceans. The horizontal convective roll features long, straight lines of cumulus clouds separated by equally wide sections of cloud-free air that form nearly parallel to the prevailing wind in a well-mixed layer above the surface. Average winds in this layer must be about 15 mph or greater, at least minimal vertical shear must be present in the layer, and heat flux from the ground must be occurring, normally due to solar heating.

Low-Sun Refractive Phenometeor (the Green Flash and Other Colored Flashes)

Not an optical illusion, not a retinal/physiological/psychological effect, and not a figment of the imagination, many know of this phenomenon and possibly have sought the most common iteration of it, the fabled green flash. Few, however, have actually identified the “flash” firsthand (although many have observed it without looking closely enough to actually identify it). Just what is it? As the earth rotates, light from the sun strikes a stationary viewer's position at a continuously changing angle, passing through a continuously varying thickness of atmosphere, from widest at the horizons to thinnest at the sun's zenith for the day. Acting not as a single, fixed lens, but as a complex “dynamic polyfocal” lens (as air constantly mixes and churns along any given atmospheric pathway), the atmosphere bends these light rays as they pass through its varying densities by a mechanism called refraction, specifically, atmospheric refraction.

Close to the horizon, refraction can affect a view of the sun in a number of ways. So can non-refractive atmospheric phenomena occurring simultaneously. When all these effects combine, the overall mechanism can be incredibly complex. Astrophysicist Dr. Andrew T. Young of San Diego State University, a leading expert on atmospheric optics, provides perhaps the best description of the formation of these types of phenomena. Young explains that the flash “photometeor,” which is actually very, very small—more like a dot or a tiny streak at the top of the disk of the sun than a brilliantly conspicuous flash, originates from a refractive process called dispersion, which causes an often green fringing at the top of the sun's disk and a red fringing at the bottom of it when at or near the horizon. This fringing, which is invisible without a high magnification telescope or telephoto lens, then becomes magnified by any number and combination of mirages, each formed through atmospheric refraction. The tiny “flash,” which can persist from less than a second to a few seconds, can arrive at a viewer's eyes as green, blue, violet, yellow, and even cyan, and, despite its name, isn't any more luminous than other parts of the sun at the time of its formation and short lifespan. The small dab of light can be seen either attached to or apparently “floating” above the sun's disk, depending on mirage effect(s) and strength(s), and can appear at or slightly above the horizon just after the top of the sun has “fallen” below the horizon. Due to the genesis of this type of phenomenon, its relative luminosity, relative size, and multiple iterations, “low-sun refractive photometeor” (a term coined by the author with input from Dr. Young) better describes it than simply “green flash,” which is just one of many of this phenomenon's forms.

Tertiary and Quaternary Rainbows

We're all familiar with the primary rainbow and the fainter secondary rainbow, but an ideal combination of sunlight and rain can create tertiary (third-order) and even quaternary (fourth-order) rainbows, when the sun's rays undergo three or four internal reflections in raindrops. Unlike primary and secondary rainbows, which are centered on the antisolar point and are seen when looking away from the sun, the tertiary rainbow encircles the sun at an angular distance of about 42°. Only five tertiary rainbows have been observed in over 250 years, and just one quaternary rainbow has ever been photographed, in Germany in 2011. Because multiple rainbows form through internal reflections, and some light passes out of the raindrop at each point of reflection to form a bow, each succeeding bow is fainter than the previous one, making tertiary and quaternary rainbows incredibly faint and very difficult to see.

Roll Clouds

A rare type of arcus (low-level, horizontally oriented) cloud, roll clouds are long, cylindrical formations that resemble giant white or gray flexible pipes rolling along the lower sky. They form in humid air, most often at the leading edge of a thunderstorm outflow and less often along cold fronts or strong sea breeze fronts. Sometimes stretching more than 500 miles, the “Morning Glory” roll cloud forms most commonly (and, somewhat predictably, every October) in the skies around the Gulf of Carpentaria in northern Australia, and can reportedly travel at speeds in excess of 60 mph.

Surface-Atmosphere Interactive Phenomena (Raining Fish and Colored Rain)

While we've omitted surficial effects of the atmosphere (such as the moving rocks of Racetrack Valley), these bizarre phenomena represent an admixture of sky and surface. In 2010, according to the United Kingdom's Telegraph newspaper, spangled perch (a white fish) rained down on the isolated desert outpost of Lajamanu, in Australia's Northern Territory, over the course of two days. Incredibly, this wasn't the first time, nor the second time, but the third time in less than 30 years that fish fell from the sky there. How do fish get into the sky, and then fall—in this case over 300 from their habitat? Nobody knows, as we have no confirmed eyewitness accounts of the transfer of fish (and other surface creatures such as frogs) into the sky, but certainly powerful thunderstorm updrafts can pluck fish and other creatures from the planet's surface, lofting them high into the atmosphere where internal currents can hold them for more than an hour before depositing them far from where they bade sea level goodbye. Over centuries, other types of animals have been reported to “rain” from the sky, including birds and bats.

A little less odd, colored rain—typically brown, red, and yellow—forms when updrafts, primarily over arid land, pull dust into a cloud, where it mixes with falling precipitation, giving the rain an “earthy” color.

Saint Elmo's Fire

A phenomenon associated with electrical potential is the seldom-observed Saint Elmo's Fire, named for Saint Erasmus (also known as Saint Elmo), bishop of Formiae, Campagna, Italy. It is not fire, and while it is related to lightning, it isn't lightning either. A faint bluish glow appearing typically on pointed objects such as masts or lightning rods, Saint Elmo's fire results from very high voltage gradients, which most commonly occur during thunderstorms. The voltage gradient ionizes nitrogen and oxygen atoms, creating plasma (ionized air), which fluoresces, or emits light. The phenomenon can last for several minutes, and witnesses have reported hearing faint hissing or crackling noises emanating from the glow. Sailors who encountered thunderstorms at sea would sometimes see this glow on the masts of their ships and attribute it to Saint Elmo, the patron saint of sailors.

Halos and Other Ice Crystal Phenomena

Elegant circles in the cold sky, or halos (also called “ice halos,” “icebows,” and “glorioles”), form due to refraction of sunlight by ice crystals in the atmosphere, primarily in clouds. The atmosphere produces many different types of halos. The form of each is based on ice crystal type, orientation, and other factors, including uniformity of crystal type and number density, and they span the “rarity spectrum,” from common to virtually unseen. The most frequently seen form, according to the German Halo Research Group, is the 22° radius halo, which is a circular arc around the sun (and sometimes the moon) with a radius of 22 degrees of arc.

Many kinds of arcs and lines appear in the sky, some due to refraction of sunlight, others due to reflection of sunlight, when planar ice crystals such as hexagonal plates act as tiny mirrors. Some of the more common ones, according to the researchers, include sun dogs (parhelia), or bright blotches of rainbow-colored light appearing to the right and left of a low sun near where the 22° halo would be, if present; sun pillars, or vertical shafts of white or orange light appearing above and/or below a low sun, and caused by reflections of countless planar ice crystals floating nearly horizontally; and the circumzenithal arc, or a short arc of multicolored light high above the sun, about where a 46° halo would be, if present.

Some of these arcs rank as the rarest of all atmospheric phenomena, including the Parry arcs (named after the English explorer Sir William Edward Parry, who first recorded them during his quest for the Northwest Passage) and the Kern Arc, which has only been photographed once.

Often confused with halos, coronal forms (coronae), which also encircle the sun (and often the moon), arise through diffraction of light by water droplets and ice crystals in high, thin clouds.

Complex Superior Mirages, Including Fata Morgana and Fata Brumosa Mirages

Virtually all of us have experienced an inferior mirage, often described as a “water-on-the-highway mirage,” where light rays from the sky bend upward toward a viewer due to atmospheric refraction, presenting an image of the sky that appears to be water “on the highway.” Identified far less frequently, the superior mirage forms in strong inversion conditions, where warm air overlies cold. Through refraction, the superior mirage presents an image to a viewer of an object above its physical location, and has a number of extremely rare manifestations. Severely distorting distant objects, and often presenting a “stacked” image of them, the fata morgana mirage is one of the atmosphere's liveliest phenomena. Named after the Italian name for Morgan le Fay, an Arthurian fairy who could change her shape, the fata morgana mirage occurs when a viewer sees a distant object through a refractive “duct,” where light rays from an object crisscross each other a number of times, forming a wildly distorted view of an object. Such ducts follow the curve of the earth, allowing a view, albeit very distorted, and sometimes incredibly so, of an object beyond the horizon. Reportedly so rare that it's only been photographed once, the fata brumosa, or “fairy fog,” also results from refractive ducting. The man who coined the term, François-Alphonse Forel, noted this phenomenon while studying Lake Geneva in Switzerland, and while calling the effect “fog,” he intended for the term to reference a “wave-like” visual effect of the lake flooding the distant shore.

Violent Tornadoes

Atmospheric scientists in North America classify tornadoes using the Enhanced Fujita Scale for Tornado Damage. Initially developed by Tetsuya Fujita, one of the pioneers of modern severe storm research, the Enhanced Fujita Scale classifies tornadoes into six categories, EF0 (wind speeds of 65-85 miles per hour) through EF5 (wind speeds over 200 miles per hour), based on damage assessment. Historically accounting for the vast majority of tornadoes (roughly 75%), EF0 and EF1 tornadoes are considered “weak” and typically inflict minor damage. “Strong” tornadoes, EF2 and EF3, which account for roughly one-fifth of this type of storm, can uproot trees, topple railroad cars, and tear walls from houses. “Violent” tornadoes, EF4 and EF5, historically have accounted for just 1% of all tornadoes, yet have caused about two-thirds of all tornado-related fatalities. Supercells, the most powerful type of thunderstorm, spawn EF4 or EF5 tornadoes, some of the most ferocious of the sky's feral beasts. While hurricanes and even thunderstorms unleash orders of magnitude more energy over their lifespans, violent tornadoes tear across landscapes with incredibly focused, virtually explosive energy release.

According to NOAA, of the 1,253 tornadoes that occur each year on average in the United States (ranging from 0 in Alaska to 155 in Texas), less than 0.1% reach EF5 intensity—an average of 1.2 per year in the United States, where roughly 75% of the world's tornadoes occur. One of the most powerful and devastating of these storms emerged on May 27, 1997, from a supercell over central Texas. Initially forming as a skinny, thread-like tornado at roughly 3:45 p.m., the storm quickly matured to an EF5 as it approached the outskirts of the Double Creek Estates neighborhood of the small town of Jarrell. Slowly moving, wide, and spinning with cataclysmic winds, the multi-vortex storm would completely obliterate the entire neighborhood of Double Creek Estates, scouring 30 permanent homes, eight mobile homes, and three businesses off their foundations as the storm gouged a path 7.6 miles long and roughly 1,320 yards wide. The storm killed 27 people, including two entire families, injured dozens, killed hundreds of cattle, removed soil to a depth of roughly 18 inches in some places, debarked and uprooted trees, tossed and trundled vehicles hundreds of yards, and ripped asphalt off of roads. For years, atmospheric scientists have studied this tornado, one of the most powerful ever recorded, including the genesis of the spawning supercell and, in particular, the role a line of gravity waves played in its formation.

Strong Tropical Cyclones

Called hurricanes in the Atlantic and eastern Pacific Oceans, typhoons in the western north Pacific Ocean and Philippines, and cyclones in the Indian and south Pacific Oceans, strong tropical cyclones are the only phenomenon on this list for which we have a precise count, as meteorologists fastidiously track them from genesis to dissipation using instruments located on the earth's surface, in the air, and in orbit. Humanity knows of strong tropical cyclones perhaps more than any other atmospheric phenomenon due to their potential for, and history of, immense death and destruction. Sometimes lasting longer than a week, the strongest tropical cyclones unleash tremendous amounts of energy, eclipsing that of all other types of atmospheric phenomena. Scientists have probed virtually all aspects of this type of storm, and articles and books have presented vast troves of information about them to the general public. Despite their global infamy, however, just 86 occur per year across the globe on average, according to NOAA.

Earthquake Lights

One of the most bizarre and puzzling of all of nature's phenomena, earthquake lights (EQL) occur prior to and during earthquakes in the lowest levels of the atmosphere. Over centuries, witnesses have reported EQL in many luminous forms, from skyward-shooting lightning that emanates from the ground, to ball-like forms hovering from ankle height to a few feet in the air, to purplish globes of radiance, to “flames” shooting from the ground. Scientists widely dismissed these sightings, thinking they were unrelated to earthquakes, until photographs surfaced in the mid-1960s in the wake of a string of earthquakes in Nagano, Japan. This spurred a number of theories, but few with merit. With small, yet mounting, anecdotal, photographic, and video evidence that seismic mechanisms created these lights, however, a team of researchers, including NASA senior researcher Friedemann T. Freund, set out to proffer an explanation. After poring through sighting descriptions, photographs, and video footage from over 65 cases spanning more than four centuries (including major historical events such as the 1906 San Francisco quake), and carefully analyzing geologic structures underlying the reported locations of these lights, the team developed a hypothesis. In their article “Prevalence of Earthquake Lights Associated with Rift Environments” published in the journal Seismological Research Letters, Freund and his colleagues explained that these lights likely originate from ionization created when tectonic forces in rifts (substantially vertically oriented faults where blocks of earth move away from one another) place extreme stress on certain types of rocks. These electrical charges can combine with others and escape into the atmosphere through fissures, creating luminous electrical discharges of a variety of forms in the lower atmosphere. Because the lights appear due to tectonic stresses that ultimately lead to an earthquake, and because these lights have appeared at times weeks before a seismic event, further research into the mechanisms that cause the ionization leading to these luminous displays may one day allow researchers to develop and deploy a reliable earthquake prediction system. Just as aurorae—a luminous phenomenon at the upper edge of the atmosphere caused by energy from outside the planet—grants us insight into the extraterrestrial, earthquake lights, which emerge in the lowest levels of the atmosphere and are generated by forces within the planet, have given us a flicker of further understanding of the intraterrestrial.

Ball Lightning

One of atmospheric science's greatest enigmas, this elusive electrical phenomenon has confounded those seeking to observe it and has defied explanation for centuries. Reportedly occurring during intense thunderstorms, ball lightning appears as an amorphous, glowing, sphere-like shape, frequently with dendritic, lightning-like tendrils emanating from it. The central “ball,” most commonly red, orange, or yellow, ranges in size from a few inches to more than 10 feet across. Witnesses have claimed that it hovers, slowly traverses the air, and can even pass through walls before vanishing after a few seconds—either by dissipation or through an explosion. Dismissed by scientists for decades as a physically impossible artifact of folklore, ball lightning has very little photographic documentation, yet it has inspired a hodgepodge of explanations over the years—some plausible, but many not. Its rarity and strangeness simply didn't inspire much serious research.

In February 2000, however, researchers John Abrahamson and James Dinniss of the Chemical and Process Engineering Department of New Zealand's University of Canterbury published an article in the science journal Nature entitled “Ball Lightning Caused by Oxidation of Nanoparticle Networks from Normal Lightning Strikes on Soil,” in which the authors detail their hypothesis. Based on simulated lightning strikes on soil, they hypothesized that cloud-to-ground lightning strikes initiate a process whereby a “ball” of semi-cohesive networks of miniscule (nano-scale) particles, laden with chemical energy, rise into the air and then release that energy as light and heat—ball lightning—through a rather complex process culminating in oxidation. In 2006, Israeli scientists at Tel Aviv University, who were interested in ball lightning and intrigued by Abrahamson's and Dinniss's theory, attempted to simulate the phenomenon by discharging a lightning-like arc onto sheets of silicon oxide. As predicted by the New Zealanders' theory, the strike did indeed create ball lightning. Brazilian and American researchers then successfully repeated the experiment in successive years. But while it was able to be created in a carefully controlled environment, the phenomenon still hadn't been scientifically observed in nature, despite hundreds of years of anecdotal accounts.

A few years later, however, in 2012, Chinese researchers who were observing lightning strikes with a video camera and a spectrometer noticed something unusual during their research. After a powerful cloud-to-ground lightning strike, a glowing ball, roughly five meters in diameter, rose from the strike point and traveled 15 meters before disappearing 1.6 seconds after emerging. Retrospectively, this was a “eureka” moment for the group of ad hoc ball lightning hunters. The spectrometer data would later reveal elements within the ball: calcium, iron, and silicon, identical to those found in the ground where the strike entered.

Ball lightning does exist, and science continues to unlock its secrets, but we still cannot explain it with complete confidence. For example, an article published in the Journal of Geophysical Research in 2102 by J. J. Lowke and coauthors argued that the Abrahamson−Dinniss theory doesn't explain ball lightning occurring inside of houses or airplane cockpits, as has been reported over decades. They proposed a second possible cause: atmospheric ions impinging and collecting on the inside surface of insulating glass windows. Much about ball lightning remains a mystery, and the phenomenon's secrets may remain hidden from science for decades or even centuries to come.

The Green Ray

The Green Ray, which has never been photographed, ranks among the very rarest of all of nature's phenomena. Only a handful of reliable accounts of the green ray exist, and all but one date back to the early 1900s. What is the “green ray”? While the green flash appears as just a tiny dot—a miniscule fraction of the diameter of the sun—the green ray, by some accounts, can stretch to many times the diameter of the sun, and shoot upward into the sky from the horizon at either sunrise or sunset as a beam of brilliant green light, much like a searchlight shining upward, and always appear over water. Atmospheric optics expert Andrew Young describes the green ray on his Web site, and gives relevant passages of all known published accounts, as well as one possible recent (2002) account. Its formation, however, remains a mystery, although Young proffers a possible explanation, whereby surface layer haze, caused by spray from whitecaps, scatters the light from a strong green flash. He believes that this phenomenon is essentially a crepuscular ray that appears green because a green flash is the light source. The haze, Young explains, must be perfect in density and form to scatter the light from the green flash but not cloak it. Young also notes another very rare type of atmospheric optical phenomenon: the sub-duct flash, which a viewer can see when positioned below a refractive duct. Complex in formation, the sub-duct flash culminates in a triangular-shaped flash, like an inverted pyramid atop an oblong sun at the horizon. Photographs of the sub-duct flash may exist; however, Young has simulated it with a computer program. An idealized sequence is available on his Web site.

Strange and Spectacular Indeed

From the greenish-yellow curtain of the aurora silently drifting across a dark sky, to a dark-gray tornado roaring across a prairie, to the fleeting and virtually unseen ball lightning, to the miniscule green flash, the sky's strange, spectacular, and phenomenal captivate our imaginations and inspire wonder—about the sky, as well as all of nature. These phenomena can be extraordinary in their forms, effects, and energy, and scientific study of them has and continues to provide lucid windows of insight into the most fundamental mechanisms that drive the atmosphere, at all scales. After pondering such a list, we might ask, how do some of these phenomena relate to others, directly and indirectly? Are there phenomena at such scales and of such rarity that they might remain completely undiscovered by humanity, forever? What secrets could these hypothetically undiscovered phenomena teach us about the atmosphere? And finally, how much of the atmosphere do we really understand? We continue to produce ever more data about the skies, which reveals much on an ongoing basis. And yet the more we study the atmosphere—and all its myriad phenomena, both exquisite and seemingly banal—ever more alluring horizons emerge for further exploration.

ED DARACK is an independent author and photographer; learn more at He would like to thank Weatherwise Contributing Editor Tom Schlatter for his tremendous contribution to this article through his thorough review and his subsequent suggestions for changes. This was a large, very research-intensive project that, due to Tom's breadth and depth of knowledge and his willingness to contribute to it, evolved into a far superior work than one without his participation.

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