A ring or halo around the sun or moon is an optical phenomenon caused by the refraction of light through ice crystals suspended in the atmosphere. Halos can appear as bright white rings, rainbow colored rings, or even as partial rings. While beautiful to observe, halos are also useful to meteorologists as indicators of impending changes in the weather.
What Causes Rings Around the Sun or Moon?
Halos are caused by the refraction and dispersion of light passing through ice crystals suspended within cirrus, cirrostratus, and altostratus clouds in the upper troposphere. The ice crystals act as prisms, bending the light at a 22° angle to create a ring around the sun or moon.
The size of the ice crystals and their orientation determines the form of the halo. Larger hexagonal ice crystals tend to create 22° halos, whereas smaller ice plates and columns create 46° halo rings and parhelia, also known as sun dogs or mock suns. The ice crystals must be oriented horizontally to properly refract the light and create a halo.
As light passes through the ice crystals, shorter wavelengths like blues and greens are bent at a larger angle than longer wavelengths like oranges and reds. This leads to rainbow colored rings, with red on the inside and blue on the outside.
Common Types of Halos
There are many types of halos that can occur around the sun or moon. Some of the most common are:
The 22° halo is the most common type of halo. It appears as a large, bright white ring with a radius of about 22° around the sun or moon. The ring is caused by refraction through randomly oriented hexagonal ice crystals.
Smaller ice crystal plates and columns tend to create 46° halos. These are smaller, fainter rings with a radius of 46° around the sun or moon.
Circumzenithal and Circumhorizontal Arcs
These vibrant rainbow colored arcs occur when plate and column ice crystals are aligned horizontally. Circumzenithal arcs rise vertically from the sun and circumhorizontal arcs run parallel to the horizon.
Sun dogs are bright spots of light that form on either side of the sun at the same elevation. They are also called mock suns or parhelia. Sun dogs form from the refraction of light through horizontally aligned plate ice crystals.
These partial rings are sections of a 46° halo which rise vertically from the sun. Tangent arcs are caused by tall, vertically oriented column ice crystals.
Moondogs are the lunar version of sun dogs – bright spots that form on either side of the moon due to ice crystals in the atmosphere.
When are Halos Most Likely to Form?
Halos tend to form when cirrus, cirrostratus, or altostratus clouds containing ice crystals are present at altitudes of 5,000 to 30,000 feet in the upper troposphere. The ice crystals in these high clouds are required for halos to occur.
Halos frequently form ahead of a warm front, as increasing high altitude clouds signal changing weather patterns. Behind cold fronts halos also regularly form in the stream of humid air. Halos can also occur with storms, tornadoes, hurricanes, or unstable and turbulent weather conditions.
The best time to observe halos is when the sun or moon is low on the horizon. Halos are brighter and more prominent when the light source is lower in the sky.
Halo Formation by Season
The frequency of halo formation depends on the season and latitude:
|Very frequent, especially at higher latitudes
|Frequent, high altitude cirrus clouds are common
|Rare, due to fewer high clouds
|Frequent, with increasing frontal systems
Halos are more common closer to the poles where very cold temperatures lead to frequent high altitude ice crystal clouds. In tropical regions, halos are very rare due to a lack of extensive cold air masses.
Halos as Weather Predictors
While beautiful to observe, halos can also serve as useful indicators of impending weather changes. Some associations between halos and weather patterns include:
Halos often precede warm fronts as high cirrus clouds blow in above a mass of warm, humid air advancing from the south. Expect changing conditions and precipitation within a day or two after halo observations.
Behind a cold front, cold dense air can undercut warmer air and cause a stable layer of cirrostratus clouds ideal for halos. Halos following cold fronts indicate fair weather but another front may be approaching.
Halos may signal the approach of a severe storm system several days in advance. Unsettled weather typically follows within 48 hours of halo observations.
If halos form on otherwise clear days, a weather front and precipitation may be moving within the next day or so. Sudden halo formation means changes are imminent.
While not an infallible tool on their own, halos can help astute weather watchers discern trends in atmospheric patterns and instability to anticipate coming changes.
Observing halos requires having the sun or moon in view along with high altitude cirrus clouds to create the refraction. Some tips for halo viewing include:
- Watch the sky near dawn or dusk when the sun or moon is low.
- Orient yourself with the sun or moon at your back for the best view.
- Block the sun partially with a building or tree to cut glare.
- Use sunglasses or filters when viewing solar halos to protect your eyes.
- Photograph the halo if possible to better study the structure.
- Note the position of parhelia, arcs, and spots relative to the halo.
With attentiveness and optimal conditions, anyone can observe these luminous displays high in the sky. Appreciating the beauty while considering the meaning can make time spent halo watching richly rewarding.
Rings and halos around the sun or moon occur when light is refracted by ice crystals in high altitude cirrus clouds. These atmospheric optical phenomena can take many forms, including 22° halos, sun dogs, tangent arcs, and circumzenithal arcs. Halos are indicators of unstable weather and changing conditions, often forecasting the arrival of a warm or cold front by a day or more. These dazzling displays provide insight into coming weather and are a visual delight if seen under the right conditions.