Ring Around The Sun Or Moon, Rain Will Come Soon

| December 6, 2018 @ 9:30 am
An icy sunrise peaks over the Manzano Mountains in Albuquerque, NM on February 21, 2017.  (Photo Credit: Jannette Posis)

Early last year, many Albuquerqueans were surprised to see a rainbow encircling the sun as it rose over the Manzano Mountains. Many forms of atmospheric optics, such as coronae, sun dogs, glories, and sun halos, are produced when light passes through tiny cloud droplets or ice crystals in upper-level cirrus clouds. Solar coronae (not to be mistaken with the sun’s outer atmosphere but rather an atmospheric optic) may consist of several colorful concentric rings around the celestial body and a bright central circle called the aureole.

When light travels through thin clouds made up of nearly uniform-sized, individual water droplets, aerosols, or even pollen, diffraction or scattering of light may occur by the outer “skins” of the droplets.  Since light has different colors of different wavelengths, each color diffracts differently. This scattered light radiates outward from all points on the droplet’s surface, resulting in a circular diffraction pattern.

The angular size of the corona is also dependent upon the diameter of the cloud droplets (between 0.001-0.1 mm), hence smaller droplets result in larger coronae. (This can be properly described by Mie’s theory, in which the intensity of scattered light is proportional to the geometrical cross-section of the particle squared and inversely proportional to the fourth power of its wavelength). Sun halos and rainbows differ from coronae in the sense that they are formed by refraction from larger ice crystals and water droplets, respectively.

Coronae, sun dogs, glories, and sun halos are not an uncommon occurrence, but they aren’t something that you would expect to see every day.

To learn more about space weather and other atmospheric phenomena, please click here!

© 2018 Sharon Sullivan

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