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July-August 2014

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

Often not much more than a brushstroke on the backdrop of daily life, the sky typically doesn't grab our attention beyond our glancing at a forecast to help with clothing or travel decisions. Even when the atmosphere rudely hurls cold rain, searing heat, or scouring wind our way, we simply ignore the vexation as we forge ahead with the task at hand. We're accustomed to seeing the powder and cobalt blue of a daytime sky, a hodgepodge of cumulus, stratus, and cirrus clouds coursing overhead, and fiery dawns and sunsets. Sometimes, however, the sky grants us a spectacle outside the ordinary and humdrum—one that captivates us like few other creations can. These phenomena inspire us to ponder their genesis, and then to wonder how the requisite conditions arise and then coalesce to create such exquisite and unusual forms. We might even be led to ask, of all atmospheric phenomena, which are the strangest and most extraordinary?

Is it even possible to compile such a list, and if so, how do we go about doing so?

In my efforts to compile a list of the most spectacular of the skies, I mined a pastiche of anecdotal reports, decades of direct experience, and as much scientific data and research as possible. A very simple principle guided the creation of the list: Each phenomenon must have been observed and identified—and it must stand out as exceptional, either as rarely occurring, notably different, or extreme in magnitude of energy release–anything but humdrum. We can remotely observe large phenomena with instrumentation such as terrestrial-based Doppler radar and satellite-based imagers (the data from which allow positive identification of the phenomenon), but we see and experience most of these phenomena directly. Do some atmospheric phenomena occur beyond human sensory perception, in the infrared, ultraviolet, or other bands of the electromagnetic spectrum? Definitely. We already know about many. Arguably, we don't observe most atmospheric phenomena—due to the vastness of the sky. And we likely see but do not mentally identify certain phenomena, such as some mirages and other refractive effects, because they occupy such a small field of view, or certain electrical effects due to their fleeting nature.

I employed no taxonomical structure during the drafting of this list, such as grouping phenomena in the low, middle, and high atmosphere, and I did not arrange it, such as by refractive phenomena, electrical phenomena, etc. I don't believe this list to be exhaustive, but I hope it provides a broad sampling of the strange and spectacular. Nor do I want to imply any ordered ranking, i.e., least extraordinary to most extraordinary. Despite this list's inherent limitations, I hope that readers—even those who are not frequently captivated by the sky—will find these spectacles mesmerizing and at least intriguing. Maybe even a few readers will have a desire to seek them with their own eyes.

The following is Part 1 of my list of 30 extraordinary, strange, and often spectacularly beautiful atmospheric phenomena, with Part 2 to follow in a subsequent issue of Weatherwise.

The Aurora

Both the aurora borealis (“northern lights” in the Northern Hemisphere) and aurora australis (“southern lights” in the Southern Hemisphere) occur when the earth's geomagnetic field “focuses” charged particles emitted by the sun toward the planet's north and south magnetic poles. This solar energy excites electrons of oxygen and nitrogen atoms of the earth's upper atmosphere, which then return to their pre-excited states, releasing photons (electromagnetic radiation) of various wavelengths. We see aurorae as “curtains” and amorphous streaks and blobs of various wavelengths of visible light, corresponding to white, green, yellow, and sometimes blue and red light.

Middle/Upper Atmospheric Transient Luminous Events (TLEs) Including “Blue Jets,” “Red Sprites,” “ELVES,” and Other Related Phenomena

Noticed and reported anecdotally for decades by pilots, yet formally detected and recorded only since 1989, electrical flux related to thunderstorm activity causes transient luminous events, or brief flashes of light in the middle and upper atmosphere upon which researchers have bestowed very eccentric, and notably unscientific, names. Triggered by positive cloud-to-ground lightning (+CG), a “sprite” or “red sprite,” which is a massive red- or orange-colored body of ionization, erupts far above the top of a thunderstorm for a few milliseconds in a region of the atmosphere between 30 and 55 miles above sea level. Researchers have divided sprites, the most common of the TLEs, into four categories, based largely on their appearance: jellyfish sprites, carrot sprites, column sprites, and angel sprites. Sprite halos sometimes precede sprites, lasting just a millisecond at an altitude that is at the top of the developing sprite. Neither as common nor as large as sprites, “blue jets” emanate from the tops of thunderstorms to altitudes of up to 30 miles. Blue starters are similar to blue jets, but extend to just roughly 12 miles above sea level. Most likely triggered by high-current cloud-to-ground lightning strokes, ELVES (emission of light and very low frequency perturbations due to electromagnetic pulse sources) can be as large as 300 miles in width, yet last less than a millisecond—which is too brief for human observance. Other TLEs include “gigantic jets,” “gnomes,” “pixies,” and “trolls,” as well as 1 millisecond bursts of gamma ray radiation and TIPPS (trans-ionospheric pulse pairs), which are very high-frequency, high-energy pulses that originate in the region of high-energy thunderstorms. Usually difficult to detect without specialized equipment, TLEs, in aggregate, occur millions of times per year.

High Altitude Upward Lightning

Lightning bolts can and do shoot straight up from cumulonimbus clouds. Reported as long ago as 1903, viewers can see these upward bolts from aircraft above or near the top of an active thunderstorm or on the ground from a distance. Researchers repeatedly observed upward lightning during high-altitude flights of the “Sprites94” campaign. Seeking to record TLEs, these researchers observed and identified upward lightning bolts that “were brief and very bright, and extended upward from the cloud tops for much shorter distances than the blue jets” and occasionally preceded them.

Supercell Thunderstorms

The supercell ranks as the most violent and dangerous of all types of thunderstorms, with its ability to unleash softball-sized hail, roof-ripping wind gusts, relentless lightning, and tornadoes. One of the most photogenic of the atmosphere's creations, a strong rotating updraft in the core of the storm (a mesocyclone) distinguishes supercells from other thunderstorms. Classified by meteorologists into three categories (high-precipitation, low-precipitation, and classic), supercells form mostly in mid-latitudes. They occur most frequently in the “Tornado Alley” region of the central United States. Atmospheric scientists believe that mesocyclones form when vertical wind shear creates strong rotating horizontal “tubes” of air. These powerful updrafts then “tilt” upward vertically. Depending on the amount of atmospheric instability and the strength of the vertical shear in the lowest roughly 10,000 feet of the storm's vertical profile, supercell thunderstorms can unleash incredibly destructive tornados with winds over 300 mph and damage paths more than a mile wide. NWS Doppler radar systems can detect mesocyclonic rotation in supercell thunderstorms on their radial velocity display, and dual polarization radars, which are a recently added capability, can detect debris balls lifted from the ground by tornadic winds, giving remote Doppler observers the ability to detect tornados as they tear across a landscape. Eyewitness observers sometimes can see mesocyclonic rotation when a wall cloud develops. A rotating wall cloud may descend from the rear aspect of the cumulonimbus and spawn a tornado, which descends from the wall cloud's base. Shelf clouds, which also form on supercell thunderstorms (and other thunderstorm types as well), look similar to wall clouds, but they form due to descending air at the front of the storm (shelf clouds are a type of arcus cloud formation, a low-level, horizontally oriented form), whereas a wall cloud forms due to rising air and at the rear of the storm.

Crepuscular Rays, Anti-Crepuscular Rays, and Cloud Shadows

Crepuscular rays, often called “god beams” (presumably for their “heavenly” appearance), “sunbeams,” “sunrays,” and other fanciful as well as not so imaginative names, form as very nearly parallel beams of sunlight stream through openings in clouds. (“Crepuscular” refers to twilight, near the time that this phenomenon most often occurs.) Crepuscular rays illuminate dust and other matter in the air, making them visible as dramatic shafts of light. Although parallel, the rays appear to diverge from the sun due to the effect of perspective. Anticrepuscular rays seem to converge at the “anti-solar point,” or the spot just above or below the horizon directly opposite the sun. The inverse of crepuscular rays, “beams” of cloud shadows seem to emanate radially from the sun, like crepuscular rays.

Mammatus Cloud Formations

Mammatus clouds, which are a kind of “accessory cloud” (defined by NOAA as “a cloud which is dependent on a larger cloud system for development and continuance”), captivate weather watchers due to their bulbous, homogenously patterned, almost “clonal” structure. Technically called “mammatocumulus” (“mammary cloud”—named due to their resemblance to breasts), these pouch-shaped features form most dramatically on the underside of thunderstorm anvils, but many different cloud types can spawn them. Often erroneously believed to be a sign of an impending tornado, mammatus actually form most frequently near the end of a thunderstorm's life cycle, usually after the threat of severe weather has passed. While atmospheric scientists continue to research the specific mechanisms for their formation, descending, saturated air that is colder than its surroundings seems to be the primary reason for their genesis.

Positive Cloud-to-Ground Lightning Discharges

The vast majority of cloud-to-ground lightning, called a negative cloud-to-ground stroke (-CG), connects a negative charge center in a cloud to a positive charge center on the ground and lowers negative charge to the ground. Much less common, a positive cloud-to-ground lightning discharge (+CG) connects a positive charge center in a cloud to a negative charge center on the ground and lowers positive charge to the ground. Striking the ground as far as 25 miles from a storm's core, a +CG stroke can explode as a “bolt from the blue” with no warning. Such strokes can traverse a much longer pathway than their negative siblings. But why?

As a typical thundercloud develops, positive charge migrates toward its top, and negative charge accumulates toward its base. A voltage difference (electrical potential) develops between the centers of positive and negative charges. Weak in-cloud lightning may begin when voltage differences grow to several hundred thousand volts per meter. The negative charge center near the bottom of the thundercloud causes mobile positive ions in the ground to collect beneath the cloud. The dry subcloud air is not as electrically conductive as cloud air, which is full of droplets and ice crystals, and so the voltage difference in dry air must grow to perhaps a million volts per meter or more before nature opens a brilliant, fleeting bridge to equilibrium along the path of least resistance–a +CG lightning stroke.

If a positive charge center is not located over a negative charge center lower in the cloud, it can induce a negative charge in the ground and initiate a spark, a +CG. Because positive charge typically builds high in the thundercloud, the sheer distance to earth means that some +CG strokes require tremendous charge and electrical potential, upward of 10 times that of a typical –CG stroke–possibly as much as 300 coulombs (a coulomb is a unit of electrical charge that denotes a discreet and very large number of electrons) and one billion volts. If this much charge were transferred to the ground in one millisecond (the duration of a fairly long +CG stroke), the current would be 300,000 amperes, though only for an instant.

According to NOAA, +CG lightning accounts for less than 5% of the 20 million or so CG strokes per year in the contiguous United States. (Other sources claim that +CG lightning accounts for 10% of worldwide CG strokes.) Positive CG lightning usually exhibits just one stroke, whereas − CG usually exhibits multiple strokes. Still, because a +CG stroke carries far more current and lasts much longer than a − CG stroke, this type of lightning can prove far more dangerous. NOAA identifies +CG lightning as a significant cause of forest fires and power line damage, despite its rarity. Under certain circumstances, such as during the latter stages of a thunderstorm, +CG lightning strokes can outnumber − CG strokes.

Bipolar Lightning

Successive lightning strokes with different polarities (changing from negative to positive or vice versa) comprise a bipolar lightning strike. Researchers have identified three distinct categories of bipolar lightning based on how and when polarity changes. They have determined that it strikes very prominent features, such as tall buildings and mountain tops, and is usually upward-branching (most lightning is downward branching). The genesis of this rare form of lightning is poorly understood.

Lenticular Clouds

Though they sometimes inspire fear of an imminent extraterrestrial encounter in some people (due to their “flying saucer” forms), lenticular clouds are some of the most beautiful of nature's creations due in great measure to their uncanny symmetry. Shaped like a lens or lentil (hence the name lenticular), these clouds often form on and above prominent mountain peaks. Lenticulars emerge when orographic lifting (lifting due to airflow meeting a physical barrier) cools water vapor in the air, causing it to condense. If the airflow is laminar, a lenticular cloud forms near the peak of its ascent. On the lee aspect, the air descends and warms through compression, evaporating the cloud droplets. “Anchored” to the terrain, the cloud is continually forming on its upwind side and dissipating on its downwind side as air flows through it. Lenticular clouds can also form downwind of a mountain ridge, when descending air rebounds due to buoyancy, forming a second line—or multiple lines—of wave clouds. Some of the most dramatic lenticulars are the “stacked plates” (multiple lenticular clouds atop one another) often seen over very prominent peaks, such as the volcanoes of the Cascade Mountains. One of the most beautiful spectacles in the natural world occurs when the atmospheric phenomenon called “alpenglow,” where particulates in the atmosphere at sunrise and sunset absorb shorter wavelengths of sunlight, leaving only the longer orange to red wavelengths to reach an object, “paints” these stacked lenticulars brilliant hues ranging from gold to crimson.

Lenticular clouds can also form, somewhat ironically, atop another cloud. Called a pileus, or cap cloud, these lenticulars often emerge over the tops of towering cumulus clouds, which, like mountains, can act as barriers to the prevailing wind flow. Rarely, pileus clouds serve as a canvas for dramatic cloud iridescence or irisation. Also rarely, pileus clouds may form atop rising columns of volcanic ash.

Banner Clouds

Not to be confused with a snow plume (which is a surficial effect of gusts blowing snow off of a mountain peak), banner clouds form when high winds produce low pressure on the lee side of a mountain peak, causing water vapor to condense and forming a cloud. Banner clouds are witnessed most commonly by mountaineers deep in high mountain ranges, such as the Alaska Range of North America and Asia's Himalaya.

Haboobs

Often inspiring fear and dread at their approach, haboobs, a type of dust storm, occur throughout the world's arid lands. Derived from the Arabic word haab, meaning “wind,” a haboob forms when quickly descending air from a thunderstorm or line of thunderstorms crashes onto the ground and turbulently lifts dust and other particulate matter into the air—sometimes as high as three vertical miles—and carries it across the landscape at speeds upwards of 60 mph. Often densely laden with desert debris, haboobs can reduce visibility to near zero within minutes and sweep the landscape for longer than an hour. Synoptic-scale dust and sand storms born out of frontal activity last longer and sweep larger areas, but typically don't produce such strong winds or total obscuration.

Minor Whirlwinds

Rising air due to strong convection will often—seemingly spontaneously—form a rotating column of air over land that can pull dust and other debris thousands of feet aloft along its spindly path into the sky—a phenomenon colloquially known as a dust devil, and in some places, a land devil. Minor whirlwinds can also form over water (water devils). During fires, intense updrafts can spin flames into rotating columns—a phenomenon called fire devils, firenados, or fire whirls.

Noctilucent Clouds and Nacreous Clouds

By far the highest of the earth's clouds, polar mesospheric clouds (PMCs), more commonly known as noctilucent clouds (NLCs), were first observed in 1885, two years after the Krakatoa eruption sent millions of tons of volcanic ash high into the upper atmosphere. Typically forming only in high latitudes during summer, noctilucent clouds roam the top of the mesosphere exclusively, which is the coldest and driest place anywhere in the atmosphere. These clouds form when miniscule amounts of water vapor diffuse from the troposphere to the upper mesosphere or form there from a photochemical reaction that destroys methane. This vapor then condenses into ice crystal clouds at temperatures below −205°F—temperatures occasionally found at over 50 miles above the earth's surface.

The German man who first noticed them in 1885, T. W. Backhouse, described ethereal, blue clouds high in the night sky long after sunset. Too thin to be seen during daylight hours, noctilucent clouds can only be viewed when the sun, the clouds, and an observer are ideally aligned—long after sunset.

Scientists continue to speculate exactly how they form, noting the necessity for cloud nuclei. Some believe that Krakatoa's fine ash provided the necessary nuclei, and the phenomenon's continued re-emergence has modern scientists speculating that meteoric dust may provide that critical component today. NLCs have been extensively photographed by astronauts on the Space Shuttle and International Space Station. On April 25, 2007, NASA launched the Aeronomy of Ice in the Mesosphere (AIM) satellite to study noctilucent clouds. That program has provided troves of data about these rare, high-altitude clouds as well as the atmosphere as a whole.

Related to noctilucent clouds, nacreous clouds, also called mother of pearl clouds, are a type of polar stratospheric cloud (PSC) that resides at altitudes from 10 to 15 miles above sea level. They form at high latitudes during the winter, when the lower stratosphere is exceptionally cold. Seen more frequently than noctilucent clouds, nacreous clouds consist of ice crystals and can be seen well after sunset.

Sometimes ground viewers confuse another phenomenon, cloud iridescence, with nacreous clouds. Nacreous clouds are often iridescent, but other clouds in the troposphere are, too. Not uncommon, iridescence occurs when cloud droplets or ice crystals are small enough that they diffract sunlight into amorphous, multicolored swaths throughout sections of a cloud they compose. Cloud iridescence occurs most often on thin sections of altostratus or cirrostratus clouds, with the main requirement being that the liquid droplets or ice crystals be of uniform size.

Fogbows

Similar to a rainbow, fogbows form when light rays from the sun (or in some very rare instances, sunlight reflected by the moon, at night) pass into individual water droplets composing a fog bank or mist, and are internally refracted and reflected. However, unlike raindrops participating in a rainbow, suspended droplets of fog are exceedingly small, on the order of 10 micrometers in diameter, just 15-25 times greater than the wavelengths of visible light. The result is diffraction of the refracted light emerging from the droplet and broadening of the rainbow colored rays so that they merge into each other to create white light—a white rainbow or fogbow.

Sometimes a fortunate observer will witness a related rare phenomenon simultaneously with a fogbow, depending on the geometry of the viewer, the sun, and fog: a Brocken specter (also called the specter of Brocken). Named after a high mountain in northern Germany, a Brocken specter forms when the sun casts a shadow of a person downward onto a bank of fog (the viewer needs to be on a peak or ridge, above the fog or mist). The specter moves only if the person moves, and witnesses report that it seems larger than expected. In addition to the shadow, the Brocken specter includes a glory, or a series of concentric colored rings encircling the head of the shadowed viewer. Diffraction plays a major role in the formation of the glory, which is centered on the antisolar point.

Waterspouts

NOAA classifies waterspouts into two categories: tornadic and fair weather. The less common of the two, a tornadic waterspout, forms just like a land tornado, emerging from the base of a thunderstorm and building downward. A tornadic waterspout can spend its entire lifespan over water, or begin as a land tornado and then move over water. A fair weather waterspout, with wind speeds less than that of an EF0 tornado (less than 67 mph), forms under rapidly building cumulus clouds, and first becomes visible as a swirl of spray at the surface, which then builds upward. A condensation funnel usually forms shortly thereafter in the lower pressure associated with maximum rotation. Fair weather waterspouts have a terrestrial counterpart in what meteorologists call a “land spout,” which is a tornado-like form with potentially dangerous winds, but which does not reach the intensity of an actual tornado.

When steam fog covers the surface of a body of water (commonly caused during outbreaks of very cold air over warm ocean water), vortices known as steam devils may form as the water's warmth convectively propels a rotating column of steam into the air.

Thundersnow, Thundersleet, and Thundergraupel

Lightning and its acoustic sibling, thunder, commonly accompany rain showers, but they can also, on rare occasion, accompany other forms of precipitation—snow, sleet, or graupel (lightweight, air-filled balls of ice, usually less than 0.5 inches in diameter).

In midlatitudes, convection is more vigorous during the summer than in other seasons because the temperature contrast between the planet's surface and the tropopause (the upper limit of the troposphere and the limiting altitude of thunderstorms) reaches its annual maximum, rendering the atmospheric column more unstable, provided that moisture is present. During cold months, atmospheric instability is limited and rather infrequent. Thus, most wintertime precipitation is stratiform (resulting from gentle, large-scale ascent) rather than convective, and hence thunderstorms with snow, sleet, or graupel are rare.

Snow-producing thunderstorms typically occur during late winter and early spring; they form just like “ordinary” thunderstorms in conditionally unstable air, but the atmospheric column, except very near the ground or within a frontal inversion aloft, must be below freezing. If air within a frontal inversion aloft is above freezing, snow falling into this layer melts, but droplets refreeze as they enter much colder air below the inversion to reach the ground as sleet. Convection occurs above the frontal inversion, if one is present.

Supercooled liquid water probably exists in all thunderstorms; it is partly responsible for separation of charges in the thundercloud and also for marked riming of snow crystals and the production of graupel. Even if graupel doesn't fall with thundersnow, one usually finds some rime on the snowflakes.

People who experience thundersnow typically don't see the bolt itself, just a diffuse flash, but they can hear thunderclaps.

Heat Bursts and Other Warm Winds

This rare atmospheric phenomenon descends from the sky onto the earth's surface with virtually no warning, raises the temperature 20 or more degrees, dramatically lowers humidity, and brings wind gusts that can top 80 mph. Heat bursts “drop out of the sky” almost always in the dead of night, and they normally last only a few minutes, but rarely more than an hour.

As noted on p. 43 in the September-October 2012 issue of Weatherwise, heat bursts occur only under very special conditions:

  • Thunderstorms recently in the vicinity, often in the dissipating stage

  • A region of stratiform rain toward the rear of the thunderstorm complex

  • Sloping back side of the anvil, extending upwind with respect to the air flow around the cloud

  • Dry air and steep lapse rate in the air immediately beneath the anvil

  • Moderate to strong air flow in the mid-troposphere along the sloping back side of the anvil

  • Precipitation evaporating into drier air beneath the anvil

  • A shallow, stable layer of air near the ground

Strong heat bursts rush earthward, suddenly raising temperatures, drying crops, uprooting trees, and flattening fences. A recent heat burst occurred near Wichita, Kansas, just after midnight on June 9, 2011, and caused the temperature to climb from 85°F to 102°F in 20 minutes.

Adiabatic heating in a descending air flow causes a number of regional warm winds, such as the chinook along the east slopes of the Rocky Mountains and the föhn of Europe, among others. These rather common winds form when a mass of moist air ascends the windward slopes of a mountain range. The rising air cools, vapor condenses, and precipitation falls, usually up to the crest of the range. Depleted of moisture, this air then descends the leeward slopes and compresses, thereby warming and lowering the humidity. Under mountain wave conditions, downslope windstorms can occur with gusts above 100 mph. The warm, dry air coupled with strong winds greatly increases the threat of wildfires.

Strong cross-mountain winds and steep surface pressure gradients across the mountains often cause such winds. In January 1982, a weather station in the foothills west of Fort Collins, Colorado, recorded a chinook wind of 140 miles per hour. At Wondervu, southwest of Fort Collins, a similar gust was recorded. Such winds can cause blankets of snow to sublimate (transitioning directly from solid to vapor, skipping the liquid phase), due to very low humidity.

Another regional hot wind, the Santa Ana, occurs when warm, dry air flows downward from the Mohave Desert toward the southern California coast, heating compressionally (adiabatically).

ED DARACK is an independent author and photographer; learn more at www.darack.com. 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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