Nature’s illusions: Don’t believe your eyes

April 11, 2017

 

Jordan Smith
Zenith News

Probably the best-known weather-related optical illusion is the rainbow, which is simply white sunlight refracting as it passes through countless raindrops, breaking into all the colors in the visible spectrum. (No pot o’ gold—sorry!)


It was Isaac Newton who first inferred that white light is a combination of every color in the visible spectrum, when he demonstrated how a beam of sunlight transforms into a rainbow as it passes through a prism. No two people ever see the exact same rainbow. Its appearance depends on several factors, including the position of the sun, the observer, and the movement of raindrops.


Moonbows occur when sunlight reflecting off the moon refracts into visible colors through rain. The colors tend to be much less vibrant than those of a rainbow.


Fogbows occur when sunlight passes through the water droplets in fog. These droplets don’t disperse light as well as rain, so fogbows rarely show any color.


Coronae (Latin for “crown”) appear as luminous rings around the sun or moon (or sometimes another a planet or even a bright star). As nearby water or fog diffracts the light. “Refraction” is when light passes through an object; “diffraction” happens when light is bent, causing coronae to often appear blue or red.


(The sun also has a corona, which is not an illusion. It’s the sun’s outermost layer of plasma, extending millions of miles outward. You can’t see it except during a solar eclipse, and don’t even try. It will hurt your eyes.)


Clouds near the sun or moon can result in iridescence, which are irregular patches of color in the sky. They also occur by diffraction, but iridescence tends to appear as asymmetrical patches of color, often near the edges of clouds. Moonlight iridescence is the most easily viewable, but the colors are much less vivid.


Like coronae, halos form as rings around the sun or moon, but halos require ice crystals rather than water droplets. Nearly all halos are white because ice reflects, rather than refracts or diffracts, nearly all the light.


Sundogs appear as two bright spots on either side of the sun, as though viewing the sun through a lens (which you should never actually do). Like halos, sundogs are produced by ice—but only by crystals of a particular shape, making sundogs (and, more rarely, moondogs) more common in very cold climates.  


Sometimes, though rarely, just as the sun is rising or setting, a brief green flash appears on the horizon. The angle at which sunlight passes through our atmosphere determines the color of the sky, resulting in a red sunrise or a purple sunset, and a blue sky during most of the day. As it rises or sets, the atmosphere refracts the light into a vertical spectrum, which vanishes or appears one by one, starting with red, then orange, then yellow.


Due to dust in the air, blue and violet are scattered sufficiently to prevent us from seeing them, leaving green—for one brief moment—as the only visible color. You’re most likely to catch a green flash over open water, where there are no obstructions to the horizon.


Mirages are not limited to the stereotypical hallucinations in the desert—although that kind does exist. When the surface of the earth is sufficiently heated, it inverts air density. Light can be refracted back upward, producing mirror images, usually of an object on or near the horizon, just above the surface. A void between the ground and the mirage can look like a lake or a puddle.


During hot summer days, look down a long, unobstructed roadway and you will most likely see this sort of mirage. A mirage can also occur where air near the surface is colder and denser than the air just above it, resulting in inverted images floating in the sky above a real object, such as a ship.


Smog is, unfortunately, not an illusion, but it does produce optical effects. The burning of fossil fuels produces smoke, which increases condensation nuclei in the air. Smog has to either be blown away by wind or dispersed by rain. As a result, warm cities with little rainfall, such as Los Angeles and Mexico City, have become well known for their smog and its human health effects.


Wildfires, and even volcanic eruptions, can produce similar effects, as smoke increases the number of condensation nuclei in the troposphere and enhances the natural scattering of sunlight, producing paler daytime skies and intense oranges and reds at sunrise and sunset.


This far north, we can sometimes see auroras, shifting curtains of luminous color in the night sky. Incoming electrons from solar emissions meet with gas molecules in our upper atmosphere, producing an electronic burst of light that usually appears green. Auroras are most common at the equinoxes, but we don’t yet know why.


We’ll likely see our last snowfall around mid-April, followed by sunny, warm conditions, which may cool off with some showers as we head towards May flowers.

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