Halo

A halo is a ring of white or colored light which appears around the sun, moon, or even a street lamp when conditions are right. This weather phenomenon is very common and can occur as often as once every three to four days, year-round. The formation of a halo requires ice crystals in the air rather than water droplets. Haloes around the moon and sun can occur at any time of year, since cirrus or cirrostratus clouds are high enough in the atmosphere to be made up of ice crystals.

Ground-level haloes, such as those around street lights or which briefly appear around headlights under some circumstances, can only happen in winter, generally speaking, when there is diamond dust in the air. Diamond dust is a mist of ice crystals which exists only at cold temperatures.

A halo around the sun or moon is usually at 22 degrees of arc from the light source, giving it the name of 22-degree halo. The halo is formed when light from the celestial body strikes hexagonal ice crystals in the upper atmosphere. These crystals refract the light like prisms, and send it to the observer's eye split into several different wavelengths. Because of the different angles that these wavelengths travel at, the ring of the halo appears red-tinted on the inside, white in the middle, and blue-tinted on the outside. This halo is produced by the sunlight entering and exiting through the edges of the ice crystal.

A rarer kind of halo is a 46-degree halo, which is caused by sunlight or moonlight which enters through the edge of ice crystals but exits through their flat underside. The resulting halo is a lot wider and dimmer, and you may not even see it if the cirrus layer that is refracting the light is fairly thin.

Interestingly, the sky between the sun or moon and the ring of the halo typically appears darker than the surrounding sky. This is because -- as is the case with Alexander's band in a rainbow -- the ice crystals here are unable to reflect any light to the observer because of the angles involved, and are thus tiny dark objects forming a sooty mist between the sun or moon and the halo.

If the conditions are right and most of the ice crystals are aligned with their flat surfaces towards the ground and sky, then the halo will also contain 'sun dogs,' bright patches flanking the sun and resembling two additional suns if they are large and brilliant enough. 'Moon dogs' can occur in the same way with the moon.

The presence of a halo is of interest to the weather observer not only because it is a distinctive visual phenomenon, but because it also indicates there are a lot of ice crystals in the upper atmosphere. If the ring is clear and complete, then the ice crystals are very numerous. Of course, this, in turn, means that there is a lot of moisture in the upper atmosphere, and the halo may be a sign that wetter weather is on its way -- a fact that has been known for centuries and used as a rough form of weather forecasting.