Atmospheric optical phenomena can be caused by reflection, refraction, scattering, diffraction and interference. Some of these processes are caused by the difference in density between hydrometeors (liquid and solid water particles) and air.
Common ice-crystal optical phenomena include the sun pillar, parhelic circle, subsun, 22° halo, 46° halo, circumzenithal arc, sun dogs, subsun dogs and tangent arcs. They are usually found by looking toward the sun. The hexagonal column shape of many ice crystals creates 60° and 90° prismatic effects. The somewhat random orientation of these crystals causes sunlight to be returned in different directions from different crystals. The phenomena we see are the superposition from many crystals. Many other rarer halos can occur, some of which are associated with hexagonal pyramid crystals.
Raindrop phenomena are a bit easier to describe, because the approximate spherical shape eliminates orientational dependence. Rainbows are seen by looking away from the sun. Cloud droplets produce corona, iridescence, glory, and aureole. Scattering by air molecules cause blue sky and red sunsets. Refraction in air causes amazing mirages.
This brings you to the end of this book. Along the way, you examined the physics of atmospheric pressure, wind, temperature, and humidity. You saw how these concepts weave together to make the cyclones, precipitation, thunderstorms, fronts, and hurricanes that we experience as weather. You have the quantitative wherewithal to critically evaluate weather-related issues, and to consider sound decisions that affect society and the future of our planet.
If this is your last course on meteorology, be confident that you have the background to delve into the meteorology journals to learn the latest details that might help you in your engineering or scientific work. If you plan to continue your study of meteorology, the overview that you have gained here will help you put into context the advanced material that you will soon be learning.
As a farewell, I invoke an Irish blessing: “May the wind be always at your back, may the sun shine warm upon your face, and may the rains fall soft upon your fields.” Sincerely, Roland Stull
A SCIENTIFIC PERSPECTIVE • Great Scientists Can Make Big Mistakes Too
Isaac Newton explained many atmospheric optical phenomena in his books on optics. Not all his theories were correct. One of his failures was his explanation of the blue sky.
He thought that the sky was blue for the same reason that soap bubbles or oil slicks have colors. For blue sky, he thought that there is interference between light reflecting from the backs of small water droplets and the light from the front of the drops.
Although Newton’s theory was accepted for about 175 years, eventually observations were made of the polarization of sky light that were inconsistent with the theory. Lord Rayleigh proposed the presently accepted theory in 1871.
Like Newton, many great scientists are not afraid to propose radical theories. Although most radical theories prove to be wrong, the few correct theories are often so significant as to eventually create paradigm shifts (radical changes) in scientific thought. Unfortunately, “publish or perish” demands on modern scientists discourage such “high risk, high gain” science.