Atmospheric light shows
Beyond clouds and storms, the sky can serve up some curious and fascinating displays for pilots.
By Karsten Shein
Comm-Inst, Climate Scientist
Rays from the setting sun illuminate the underside of a stormy cloud deck as seen from a US Airways A319. Scattering of blues and greens provides the reddish hues of twilight, and the clouds create an impressive canvas.
You've been dodging embedded cells as you weave your way through the cold front. Looking at the scope, you know it won't be much longer before you are in the clear air behind it, but meanwhile, back in the cabin, your company's executive board is being bounced around like pinballs. You'll definitely need something to soothe their ruffled feathers.
Fortunately, as the cumuli fall away below, Mother Nature provides just what you need. After apologizing for the rough air, you draw your passengers' attention out the windows. Just off the wing, your charges catch a glimpse of the aircraft's shadow.
It is surrounded by a brilliant halo all the colors of the rainbow. On the other side of the aircraft, they see the thinner clouds above take on various pastel hues. Glories and iridescent clouds are just a few of the airborne displays that can make a routine or even rough flight a bit more enjoyable.
While water in the atmosphere is the cause of clouds, and the energy transported by that water is the fuel for most of our severe weather, it is also the factor behind many of the interesting displays of light we see in the sky. Water droplets and ice crystals both have the ability to reflect, refract and/or diffract light.
Reflection is pretty well understood as a shift of the path of light to a new incident direction after striking a surface, but refraction and diffraction need a bit more explanation.
Refraction also refers to a change in direction of the path of light—but, unlike reflection, refraction takes place when the light passes from one medium, such as air, into another, such as water. Usually, with reflection, all wavelengths are shifted equally, so the colors don't appear to separate. With refraction, each color wavelength of light changes direction to a different degree, resulting in color separation and a rainbow effect.
Although it sounds similar to refraction, diffraction is a slightly different process. Diffraction refers to the bending of the light path around the edge of an obstacle. Like refraction, the amount of bending is dependent on the color's wavelength, so the colors separate, with the longest (red) wavelengths bending least, and the shortest (blues) the most.
One of the most commonly recognized optical phenomena is the rainbow. It is formed by a combination of refraction and reflection and can be seen when you have the Sun at your back and are looking toward the rain.
First the sunlight refracts as it enters the raindrop, then the separated light reflects off of the back of the droplet. Further refraction as the light leaves the droplet separates the colors further.
Since you are in a fixed location, only the colors directed right at your eye will be visible. The other colors bounced out of a particular raindrop will miss your field of view. The result is that from droplets at a certain angle above or below you, you will see only red light. The droplets just above or below that red will produce orange, and on to yellow, green, blue, and violet as the angle between your eye and the droplet increases.
One of the neat things about rainbows is that they always appear to be 42° above the horizon—the result of the angle of refraction/ reflection for the outermost red light. If you see them from above, or anywhere where the ground doesn't interfere with your viewing angle below your line of sight, you might even see a completely circular rainbow.
22-degree halo surrounds the Sun. Halos are the result of sunlight being refracted by randomly oriented ice crystals in high, thin clouds. Often, sun dogs appear in the halo on either side of the Sun.
When the Sun is particularly bright or the rain is backdropped by sufficiently dark skies, you may also see a secondary rainbow. Secondary rainbows occur when the sunlight is reflected twice from the inside of the droplet.
This double reflection causes the output to appear weaker and in reversed color order. Sunlight may reflect internally several times, resulting in tertiary, quaternary or even more multiple arcs.
Cloud droplets also create a variety of optical effects. One of the most common for pilots to see is known as a glory. The glory was named after the effect it produces—a heavenly halo. It is similar to a rainbow in that the light from behind you is refracted by the cloud droplets and reflected back to your eye. However, glories have one added property—diffraction—since the light is directly behind you and bends around you to reach the cloud droplets.
Because the light reflects almost directly back to your eye, the diameter of a glory is very small, and appears to closely encircle the shadow of the aircraft.
When cloud droplets are very small and the clouds are high and thin, as is often the case with alto or cirrocumulus, light rays may diffract around individual drops, creating pastel hues that can become quite vivid. Iridescence is not, however, associated with cirrus, because these clouds are more likely filled with ice crystals that will more commonly produce halos.
Sunlight passing through a high thin cloud may also produce a corona around the Sun as light is diffracted by the cloud droplets. Coronae are about as large as glories and form when the Sun is directly behind the cloud, and surrounding a bright center, the colors will transition from blues to reds, but without the clear differentiation of a rainbow.
When the cold temperatures high above the Earth transform cloud droplets into ice crystals, refraction and reflection become dominant processes, and create a number of light shows. One of the most common of these is the halo. Halos may form around the Sun or Moon as light is refracted through the crystals.
Because of the shapes of these crystals and the way they orient themselves as they fall, halos may form in concentric bands of light and dark around the Sun or Moon, or they may contain some color separation from light refraction and appear as a prismatic ring.
The key to having a complete halo is to have a random orientation of the plate-shaped crystals making up the cloud. This allows light from crystals above and below the Sun's elevation to be oriented toward the viewer's eyes, letting them see the enhanced, prismatic light. The most common halo is a 22° halo—so called because the refraction results in the deflection of light about 22° from its original path.
When it forms around the Sun—or a very bright Moon—it may also have brighter spots on either side and above. On the sides, these spots are known as sundogs. They form when the orientation of the falling crystals aligns with the horizontal plane of the Sun.
The more of the cloud crystals are in that orientation, the brighter the sun dogs become. If the cloud also has longer hexagonal crystals in it, the cloud may produce arcs around the top, sides or bottom of the halo ring. At sunrise or sunset, a bright pillar of light may appear above the Sun. These pillars are caused by light reflecting off the flat surface of the crystal as it falls.