Sun Halo. Source: https://sites.google.com/site/thebrockeninglory/
Sun halos are some of the most interesting and frequent atmospheric optical effects, and along with sun dogs and glories, are the results of physical properties that occur in the atmosphere.
Sun Halo. Source: Wunderground
In order to understand how these optical effects occur in the sky, it’s important to recognize that light is radiation that travels across the universe at certain wavelengths. Our eyes are able to intersect all the wavelengths that fall within what’s referred to as the visible spectrum, and they range from around 400nm (nanometers) to 700nm. Blue light exists around 400nm and wavelengths below that it exist in the ultraviolet spectrum while red light exists around 700nm and any and all wavelengths beyond it exist in the infrared. The rest of the colors we say in day to day life all exist somewhere within that range!
The EM spectrum. Source: newgradoptometry.com
All of that light travels from the sun and enters our atmosphere, where it interacts with everything from the molecular level all the way up to the synoptic (large-scale) level. It’s during this stage that light will experience some degree of reflection or refraction. While reflection is a commonly used term in our day-to-day lives, refraction isn’t so much.
Refraction from a prism. Source: annisaidris.wordpress.com
Refraction refers to when light changes its speed and the direction. Another way to think about it is to visualize a prism, which forces light to slow down and bend towards a new direction, resulting in light “breaking” and appearing as a rainbow once it makes it to our eyes. And much like the prism’s effect on incoming light, water droplets and ice crystals also bend light as it travels through the atmosphere. In fact, it’s when these particles refract light at certain angles that we end up getting all of our atmospheric optical effects!
For sun halos, we need light to be refracted off ice crystals in high cirrus clouds:
For sun dogs, we need for there to be refraction off ice crystals at an angle of around 22-24 degrees from the sun in any direction. Depending on where you’re standing and if the conditions are right, you may be able to see the entire sun dog from your field of vision!
For glories, you’ll likely have to be either on a hill or up in the air looking towards a thin layer of clouds or fog and facing away from the sun, given that this phenomenon occurs via the scattering of light from behind the observer. Other descriptions for this phenomenon include fog bows and Brecken specters.
And there you have it. The next time you’re in an environment that’s ripe with these conditions you might just get lucky enough to see some of these atmospheric optical effects first-hand. As always, feel free to share any and all of your pictures of any of the effects we discussed here or any that we might have missed!
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©2020 Meteorologist Gerardo Diaz Jr.