Impressive Sun Halo Spotted Over Las Vegas; How Do These Form?

Brian Donegan
Published: November 14, 2017

An impressive sun halo appeared in the sky over Las Vegas on Monday morning thanks to the presence of a thin layer of high-altitude cirrus clouds.

Halos are rings of light that can encircle the sun or the moon, and they usually occur when cirrus clouds are covering the sky.

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A sun halo was spotted over Las Vegas on Monday morning, Nov. 13, 2017.
(National Weather Service-Las Vegas)

A mix of chemistry, physics and geometry are the main components for sun halos.

The atmosphere consists of a mix of gases, including oxygen, nitrogen and water vapor. At high altitudes in the sky, water vapor condenses and then freezes into ice crystals. As sunlight passes through those ice crystals, the geometry of the crystals causes sunlight to refract, or bend, similar to what happens when light passes through a prism.

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Randomly-oriented hexagonal ice crystals with diameters less than 20.5 micrometers are responsible for the halo observed in the sky, according to the University of Illinois at Urbana-Champaign.

This geometric size and shape causes light to undergo two refractions as the light passes through the ice crystal. Once the second bend is made, the light appears as a halo in the sky.

The process works for any celestial light source, which means moon halos form under the same physical and geometrical properties. Additionally, the process is similar to how rainbows are formed, which is why colors can sometimes be seen in the halo.

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Scientifically, halos are called "22-degree halos" because the two refractions bend the light by 22 degrees from its original direction. This means the halo can be seen when your eye makes a 22-degree angle with the sun or the moon.

Weather Forecasting: What Do Sun Halos Mean?

The old weather saying, "Ring around the moon means rain soon," contains a little bit of truth. Halos need ice crystals in order to form, and ice crystals are usually present within high-altitude cirrus clouds. These clouds arrive days before an advancing cold front or warm front, which typically brings rain.

However, not all cirrus clouds are associated with storm systems, and some halos can merely signal an increase in water vapor in the upper atmosphere. Therefore, rain may not necessarily occur after the presence of a halo, resulting in a forecast bust for this weather folklore.

Brian Donegan is a digital meteorologist at Follow him on FacebookTwitter and Instagram.

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