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.

(MORE: A Cargo Carrier on Columbia River and 18 Other Weird Things Captured on Radar)

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.

(MORE: The Science Behind a Double Rainbow)

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.

(MORE: The Science Behind Moonbows, or Rainbows Seen at Night)

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 weather.com. Follow him on FacebookTwitter and Instagram.


The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

Featured Blogs

Meteorology of Saturday's Colombian Flood Disaster That Killed 254

By Dr. Jeff Masters
April 3, 2017

At least 254 people were killed in the in the city of Mocoa (population 40,000) in southwest Colombia near the border of Ecuador early Saturday, when torrential rains triggered a debris flow on a nearby mountain that surged into the town as a huge wall of water carrying tons of mud and debris. The disaster is the fourth deadliest weather-related disaster in Colombia’s recorded history.

Iconic American Destination Virtually Isolated for Rest of Year

By Christopher C. Burt
March 24, 2017

Half of the village of Big Sur, on the coast of central California, has lost its only access to the north following the demolition of the flood-damaged Pfeiffer Canyon Bridge along State Route 1 (also Rt. 1 or SR 1) on March 19. Although Rt. 1 to the south of Big Sur has reopened to traffic (after mud and rock slides were cleared) it is a long 70-mile journey along the windy but spectacular highway to Cambria, the next town of any significance where supplies can be had. CalTrans (California Department of Transportation) estimates it will take 6-9 months to rebuild a new bridge over the canyon.

An extraordinary meteorological event; was one of its results a 1000-year flood?

By Stu Ostro
October 5, 2015

The confluence of meteorological ingredients the first weekend in October 2015 resulted in an extraordinary weather event with severe impacts. Was one of them a 1000-year flood?

Why the Arrest of a Science-Loving 14-year-old Matters

By Shaun Tanner
September 16, 2015

By now, many of you have heard or read about the arrest of Ahmed Mohamed, a 14-year-old high school student from Irving, Texas. Ahmed was arrested because school officials called the police after he showed one of his teachers his homemade clock. Mistaken for a bomb, Ahmed was taken into custody, interrogated, shamed, suspended (still on suspension today, Wednesday), and reprimanded. All of this after it has been found that the "device" he brought to school was indeed, a homemade clock.