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Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

February 1998

A moon ring's pearly glow is one of several optical phenomena occuring in the troposphere. Scott Nielsen, ©1998A moonring.

Moon rings and rainbows

Sunlight, moonlight and water vapor work the angles of the atmosphere to produce brilliant, evanescent light shows.

Anita Carpenter


A moon ring's pearly glow is one of several optical phenomena occuring in the troposphere.

© Scott Nielsen

The color in white light || Sundogs and sun pillars
Coronas || Rainbows

A pearly-white ring encircles the moon or a dazzling rainbow streaks across the sky for just a moment as an afternoon storm is swept aside by sunny weather. The unexpected pleasure stops us in our tracks. We gaze skyward. In a minute, maybe less, the fleeting image vanishes as quickly as it came, and life goes on. We take our watery atmosphere for granted, except for those rare times when sundogs, coronas or brilliant rainbows enliven the sky watcher's world. Physics can explain what we see in those transient seconds.

Our atmosphere – the gaseous envelope surrounding the Earth – is composed of several gases, primarily nitrogen, oxygen, argon and carbon dioxide. Water vapor and minute bits of dust, salt and clay-silicate particles swirl about in the mixture.

The atmosphere has four vertical layers that radiate from Earth and vary by temperature and composition. Proceeding from the ground outward, they are the troposphere, stratosphere, mesosphere and thermosphere.

All of the "optical phenomena" – halos, sun dogs, sun pillars, coronas and rainbows – are produced at specific levels in the troposphere, which extends from the surface up to about 12 kilometers, eight miles or 42,000 feet depending on which measuring stick you pick. Here, the temperature decreases rapidly with altitude – averaging 59°F at the surface and a right chilly -69°F at 11 km. Commercial jets fly in the upper reaches of the troposphere.

High-level cirrus clouds form in extremely cold air where water vapor condenses on tiny dust and salt particles to form ice crystals. These crystals can grow into different shapes – plates; columns; sharp, stellar dendrites or needles. The crystals can randomly float in a cloud or can fall in layered groups as the shapes interact with air currents and fall through the troposphere. The hexagonal plates fall with their flat sides parallel to he earth's surface; the long columns lay on their sides.

The colors trapped in white light

Halos, sundogs and sun pillars are produced when light passes through or reflects off hexagonal plates or columns. Each hexagonal ice crystal acts like a tiny prism. When white light from the sun or moon passes through the crystals, it bends or refracts. Since each color wavelength bends at a slightly different angle, white light separates into many colors and the color spectrum from violet to red becomes visible.

Here comes a bit more physics. If the incoming light penetrates the sides of the hexagonal plates or columns at a 60° angle, the light is bent to 22°, which we see as a circle of light or halo at 22° from the sun or moon. If the light just happens to pass through the ends of the ice crystals where a 90° prism angle exists, the light refracts at 46° and we can see that too. Since it's more likely that light passes through the sides of a crystal than its ends, we see the 22° halo more frequently.

Halo colors can vary. The refractive angles are slightly less for the longer wavelength of red light and shorter for the violet hues, so colored halos progress from the inside to the outside band from red followed by yellow, green and violet on the outer edge. Due to the random orientation of ice crystals and the random chance if crystals hitting the light just right, the colors in most halos are not very intense. Since moonlight is weaker than sunlight, the halos around the moon typically appear as a pearly-white ring.

What do weather watchers deduce when seeing a ring around the sun or moon? Rain or snow. Since the halo is produced when light refracts off ice crystals in cirrus clouds and cirrus clouds portend worsening weather, a ring around the sun or moon is a fairly accurate indicator of precipitation within 24-48 hours.

Sundogs and sun pillars

Occasionally brilliantly-colored spots of light can be seen on either side of the sun or level with it. The spots are called "sundogs," "mock suns," or "parhelia." Sundogs are produced when light passes through a thin layer of cirrus clouds whose hexagonal plates are oriented horizontally.

Sundogs refract light when ice crystals are aligned horizontally in thin clouds.

© Scott Nielsen
Sundogs refract light when ice crystals are aligned horizontally in thin clouds.

The resulting images create a streak of sunlight exactly 22 to the left and right of the real sun. So, when we see sundogs, we know the exact position of the ice crystals in those high-flying clouds.

Sun pillars occur near sunrise and sunset when the sunlight reflects – not refracts – off the top or bottom of the approximately horizontal hexagonal plates. They appear as vertical columns of light that are 5-15 above or below the rising or setting sun. The sun pillar's position and intensity depend on the sun's angle and the tilt angle on the ice crystal. That's why these pillars are so infrequently observed.

Coronas

Time to move lower in the troposphere where light meets water droplets instead of ice.

A corona, meaning "crown," is a disc of light which appears to be surrounding and touching the sun or moon, giving it a hazy appearance. Because looking too intently at the sun can cause blindness, it's easier and safer to look for a corona around the moon.

Coronas are produced when light shines through a thin layer of middle-height clouds (altostratus), which are formed of uniform-sized spherical water droplets. As the light shines between and around the droplets, it diffracts and separates into colors producing alternate bands of bluish and faint reddish light. The bluish tinge appears closest to the moon and gradates into a yellowish-white band that merges into a redder outer ring. The more uniform the droplet size, the purer the corona colors appear. The smaller the droplet size, the larger the diameter of the corona ring. Since coronas are produced by lower clouds composed of water droplets, they indicate that rain- or snow-producing clouds are much nearer and precipitation is imminent.

Rainbows

Arguably the most beautiful of these optical sights is the rainbow. The single rainbow we see most often is called the primary rainbow. It forms when sunlight refracts as it enters a spherical water drop, reflects once off the back of the raindrop and refracts again as it leaves the drop. Since each color wavelength refracts and reflects at a slightly different angle, the colors separate and become visible as wide, beautiful, visible bands at an average of 42 above the earth's surface – usually red on the outside, grading to orange, yellow, green, blue, indigo and violet. Remember the mnemonic ROY G BIV to recall the color sequence.

Since light in the raindrop is being reflected back toward the sun, we must be standing between the sun and the rain to see a rainbow. By the by, we each see our own, personal rainbow and it is strictly dependent upon our position relative to the sun and the storm. If the sun is higher than 42 in the sky, we can't see a rainbow. That's why you never see a rainbow at midday. We most often see rainbows in late afternoon facing east after a rain shower has passed. Morning rainbows in the west are possible if you are standing between the rising sun and an approaching storm.

Sometimes a secondary rainbow is positioned 50 above the main rainbow. It's created when sunlight refracts upon entering a raindrop and reflects twice in the drop before refracting as it exits. Because of the double reflection, the light rays cross between their entrance and exit points. Consequently, the color sequence in the secondary rainbow is reversed – violet on the outside and red on the inside. Since a small amount of energy is lost in the second reflection, the secondary rainbow is never as brilliant as the primary one.

Seeing any rainbow is special, but for me, catching a double rainbow is an extra treat. It amazes me that some rainbows fade very quickly and others are so stunning, so bright and magnificent. Some appear as partial arcs while others are so complete they arch from ground to ground.

Still, I don't find it any more beautiful than the stark bold ring around the moon on a cold, still winter night. Each is a unique gift that graces the sky. Just knowing that it's possible is incentive enough for me to take long walks and scan the sky day and night to relish these fleeting visions.

Anita Carpenter keeps an eye to the sky observing nature near her Oshkosh home.