In Our Perfect Sky

Published in Midwest Flyer – February/March 2019 issue

For many centuries, humankind has longed to touch the sky and fly free and fast like the birds do. It wasn’t until December 17, 1903 that we took that first small step toward that goal. Now we not only cruise throughout the sky, but we have also stepped into the heavens. As an allied group of astronauts continually circle the earth onboard the International Space Station, they learn more every day that will in time, help us to reach further and further into space.

A little closer to home, however, is our perfect sky. Everyday aviators by the thousands take to the air and travel all over the globe. Many fly only a few thousand feet above the ground, while others fly several miles high, with a few that reach the upper limits of our atmosphere. One might think that only those who go the highest can see and experience the myriad of natural phenomena that occurs in our sky.

Things like an azure blue sky, puffy white clouds, and brilliant red and pink sunsets are optical phenomena associated with small particles in the atmosphere. Rainbows, coronas and glories are types of optical phenomena associated with liquid water droplets in the air. Halos, parhelias (sun dogs), and sun pillars are optical phenomena caused by ice crystals in the air. Nearly all of these items listed can best be seen from the ground.

To understand these phenomena, we must first have a very basic understanding of light and its properties. Almost half of the solar energy has wavelengths within the visible spectrum. The human eye is sensitive to this specific portion of the spectrum. In addition, human eyes have nerves called rods and cones that allow us to see light and dark, and color. The rods sense the light and dark, and the cones sense color. We see white light when the visible light striking the cones is essentially equal in intensity. A majority of what we see is due to incident light reflection.

Incident light can be explained as the light emanating from the source, and perhaps illuminating an object that you are observing. For instance, the light coming from the Sun is incident light when it hits a tabletop. If a mirror is placed in that “beam of (incident) light,” the light bounces off the mirror. In other words, it is reflected. And for the purists in the audience, the angle of incidence equals the angle of reflection.

Refracted light is light that has been bent as it passes through a more dense material. For instance, light passing through hexagonal (six sided), pencil-like ice crystals will exit the crystals at an angle different from that when it entered the crystal. This action can create a “rainbow-like” effect called “parhelia,” or commonly referred to as “sundogs.”

Sundogs develop when transmitted light is refracted by hexagonal, plate-like ice crystals that are falling with their long axis horizontally situated. The ice crystals bend and disperse the Sun’s light. The result is a stunning prismatic effect. Because light travels in different wavelengths, we see it as different colors. Thus, when a sundog develops, the color red (long wavelength) will always be on the inside portion of the sundog, and violet (short wavelength) will be on the outer-most portion. This is because red light bends the least, while violet bends the most.

Air molecules are very small, yet highly selective in the light wavelengths they will scatter. They are very effective at scattering green, blue, and violet light. This is why we see the perfect sky as blue. The higher you climb, the deeper blue the sky appears because fewer and fewer air molecules are there to scatter the light. That also explains why astronauts see a black sky, because there are no air molecules in space.

Sometimes in the bright sky of day or the inky black sky of night, the Sun or Moon, under proper atmospheric conditions, can produce a halo or ring of light surrounding it. The halo is created when light from the Sun or Moon is actually refracted by ice crystals usually associated with high, thin, wispy cirrus clouds. But the result is most always visually spectacular.

So, this winter, take time every day to look up. Now that you have a little more knowledge of the natural phenomena of the sky*, when you fly, or when you are on the ground, you will be able to better observe and enjoy the magnificent natural events that occur in our perfect sky.

*For additional information about natural sky phenomena, go to a truly outstanding, information packed website produced by Dr. Les Cowley at:  https://www.atoptics.co.uk/

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This entry was posted in Columns, Columns, February/March 2019, MN Aeronautics Bulletin and tagged , , , , . Bookmark the permalink.

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