WX BRIEF

Atmospheric light shows

Beyond clouds and storms, the sky can serve up some curious and fascinating displays for pilots.


Light and shadow

St Elmo's Fire tracks along an aircraft windscreen. The phenomenon is caused when an exposed object acts as an electrode to transform surrounding atmospheric gases into plasma that glows as energy is emitted.

Objects in the way of sunlight can create light and shadow effects that can be just as impressive as the more colorful displays created by water droplets. Like the shaft of light streaking through a window pane to play across a dark and dusty room, the visibility of light and shadow in the atmosphere is due to particulates, such as dust, aerosols or minute water droplets suspended in the air.

In sufficient quantity shafts of light or shadows may be greatly enhanced, and may cause light diffraction into colorful hues.
Crepuscular rays, as their name implies, are most commonly seen at low Sun angles such as sunrise or sunset. Because of this, they may already have a yellow or reddish hue from the longer path the light must take through the atmosphere.

Crepuscular rays produce some of the more impressive shadowing effects. These shafts of sunlight (or moonlight) are simply sunlit sections of sky interspersed with the shadows created by irregular objects in their way. Most often these form as light passes around or through breaks in the clouds, but crepuscular rays may be seen as light streams through trees, over a mountain range or any other obstacle.

The interesting thing about crepuscular rays is that they appear to radiate out from the object blocking the light even though they are actually parallel to each other. This radiance is simply due to perspective. The effect, which makes the rays appear to spread out from the Sun, will also work if you turn around. Anticrepuscular rays disappearing away from the Sun will appear to converge toward the horizon.

Bending light

Air itself may play optical tricks on one's eyes. When we think of a mirage, we conjure an image of a thirsty explorer crawling through the desert toward an oasis that is not actually there. Indeed, mirages are commonly associated with deserts, but may appear in just about any place where there is a discontinuity between the surface air temperature (below the line of sight) and the temperature of the air above it.

This sort of air temperature difference also frequently occurs above a runway and where cold air flows over a warmer surface, such as a warm ocean. The result is that light passing between the warmer and colder air is refracted, just like light passing between air and water.

The classic desert mirage is an inferior mirage. That means that the image one sees is beneath the actual object being seen. In most cases, what the viewer is seeing is the blue sky. The light from the sky, instead of being intercepted by the ground ahead, is bent toward the viewer, resulting in a shimmery phantom image that may appear as an oasis in the desert or perhaps a wet or slick runway.

The opposite mirage effect is a superior mirage. In a superior mirage, the surface air is colder than the air above it, causing light reflected from a distant object to be refracted downward toward the viewer. The result is that a distant object will appear higher on the horizon than it really is. Because frequent superior mirages over the Strait of Messina caused the appearance of a city suspended above the water, this sort of mirage is sometimes called a Fata Morgana after the Arthurian legend of the sorceress Morgana creating castles in the air.

Auroras

Among the most spectacular of all atmospheric light shows, however, are the Northern and Southern Lights. These curtains of quietly undulating reds, greens and blues are common visitors to the high latitudes, but if conditions are right and the lights intense enough, they have been seen almost into the tropics.

Auroras are the result of charged particles given off by the Sun entering the top of Earth's atmosphere and colliding with the oxygen and nitrogen molecules in the atmosphere's upper layers—usually the thermosphere—about 50 miles (80 km) up.

The colors are due to light being emitted by the molecules as they release energy. Oxygen tends to give off greenish or reddish-brown light, while nitrogen commonly produces red or blue. Green is the most common auroral color, and the colors are altitude dependent due to what molecules are primary at different heights.

Because they occur highest in the atmosphere, red auroras are the ones that can be seen at the lowest latitudes.

From our vantage point aloft, and free from most surface light interference, auroras become much easier to see from an aircraft than from the ground. What might just be a faint reddish glow to the citizens of New York might be a spectacular display of waving reds and greens for someone flying 30,000 ft above the city's lights.

Lightning, jets and sprites

Of course, not all light shows are of the benign variety. Thunderstorms often produce incredible light shows in the form of lightning. At their height, strong thunderstorms may be producing hundreds of strokes every minute. These discharges of raw electricity can travel miles across the sky and branch out into infinite and intricate patterns. In-cloud lightning illuminates the cloud from within, enhancing the ominous presence of the storm.

But while a lightning show viewed from a safe distance can be quite impressive, even more so are the occasional viewings of jets and sprites. Very rarely, when viewing a distant line of thunderstorms from our cockpit at night, you might notice a bright reddish flash appear well above the storms. Or, even more infrequently, a geyser of bluish light might shoot out of the top of a storm, well into the stratosphere. These unusual effects are known as upper-atmospheric lightning.

Reddish discharges that appear high above the storms are known as sprites. They are often quite large, occupying the space between about 30 and 56 miles (50–90 km) above the ground.

They usually last less than a second, and their existence wasn't even confirmed until one was captured on video in 1989. Scientists believe sprites are triggered by lightning activity. Strong, hail-producing storms may also occasionally produce a blue jet—a momentary streak of intense blue light, thought to be ionized nitrogen which extends 25–30 miles (40–50 km) above the surface. Unlike sprites, blue jets don't appear to be triggered by lightning, and have been recorded only a few hundred times, as opposed to the several thousand observations of sprites since 1989.

Also associated with thunderstorms is a more eerie light show. St Elmo's Fire has been seen by sailors for thousands of years as their rigging glowed while they battled their way through a gale. In fact, it is so called because St Elmo is the Roman Catholic patron saint of sailors. St Elmo's Fire is no stranger to pilots either.

From the earliest dope-and-canvas bi­planes, pilots have seen exposed surfaces start to glow when in or near thunderstorms. In fact, when flying through clouds that may contain an ionizing charge, pilots fairly often report seeing St Elmo's Fire on their windscreens, nacelles or wing­tips. St Elmo's Fire is actually plasma, or gas that becomes ionized and glows when the electrical field surrounding it becomes strong enough. A protruding object such as a propeller or nacelle acts as the electrode, facilitating the current.

Finally, one of the most beautiful atmospheric light shows can be the simple occurrence of sunrise or sunset. When the Sun is near the horizon, the shorter wavelengths of light are scattered away by the longer path through the atmosphere, and the sky fills with glorious shades of red, orange and yellow.

Clouds are illuminated from beneath, and ethereal crepuscular rays may dance across the sky. No matter what ad­verse weather conditions you may have flown through earlier, watching a sunrise or sunset while flying through relatively clear skies can set everything just right.

Karsten Shein is a climatologist with the National Climatic Data Center in Asheville NC. He formerly served as an assistant professor at Shippensburg University. Shein holds a commercial license with instrument rating.

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