In the spring, as the days get longer and warmer, we have more reasons to want to be outside (except for pollen sufferers), and so there are more opportunities to look at the sky. If you know where and when to look, on many days you can see colored arcs, bands or circles; these are all part of the halo family.

Halos are formed by refraction and reflection of sunlight or moonlight through suspended ice crystals. They are typically seen in veils of cirrus or cirrus-stratus clouds. Since they generally form at altitudes above about 15,000 feet, the ice crystals that make up these clouds do not melt, even in the tropics. This means we are able to see the halo phenomenon in the summer as well as the winter. The ice crystals that produce the most interesting halos are almost always six-sided plates or columns (pencil-shaped crystals).

Among the most common of the halos is the 22 degree halo and the parhelia. To get a complete halo (the full circle or ring around the Sun or moon), the ice crystals have to be randomly oriented, so that the crystals are pointed in all directions. When this happens, at least some crystals are properly positioned in each part of the halo to direct light toward your eye. However, ice crystals are not always alligned in different positions. If they all have a similar orientation, then the refracted light will only come from spots on either or both sides of the Sun. These spots, which can be quite bright, are called parhelia. You often hear them referred to as mock suns or sun dogs. In any case, the parhelia and the 22 degree halo, as the name implies, are always located 22 degrees from the Sun. This is because the ice crystals that form them bend light at an angle of a 22 degrees (minimum angle of deviation). For example, light coming from the Sun may be refracted or deflected as it encounters a side or face of an ice crystal. As the light ray passes through the crystal and exits through another side, it is refracted a second time, but in the same direction as the first deflection. This results in a total deviation from the path of the original sunlight of 22 degrees.

Not all halos are seen 22 degrees from the Sun. Depending on the orientation and type of crystal, light can be bent at an angle of 46 degrees, and on rare occasions, 9, 18, and 24 degree halos have been observed. The most interesting and stunning halos are the multiple halo displays. These unique halo complexes may occur if different types of crystals (plates, columns, and dendrites, for instance) are present in the same cloud, or if ice crystals are suspended overhead by the wind. This happens relatively frequently in polar locations. The South Pole is renowned for having magnificent halo displays.

Reds, oranges, and yellows are the colors most often observed in halos. One reason for this is that blues and greens tend to blend in more with the blue sky, against which most halos are seen. The reds are on the inside of a halo (closest to the Sun). In rainbows, the red band is on the outside of the bow. The colors in halos are generally not as vivid as the colors in rainbows. This is because with halos the different wavelengths band (colors) are not spread-out as much as with rainbows, and consequently the colors overlap more. Also, when looking at halos you're looking toward the Sun and the glare washes out some of the color. When you look at a rainbow, you're looking in a direction opposite of the Sun and usually the background sky is rather dark. This is why it's easier to photograph a rainbow. However, some halos can be very striking and show beautiful coloration. The circumzenith arc is sometimes confused with rainbows - it looks like a rainbow overhead. Although you've seen lots of pictures and paintings of rainbows, halos actually occur more frequently. You're lucky if you can see three or four rainbows a year, but halos can be seen every few days or so. If you recall from one of the questions last fall, rainbows cannot be seen during the middle of the day. The Sun must be within 42 degrees of the horizon in order to see a rainbow. Additionally, during the winter, in much of the U.S. the precipitation is frozen. Rainbows only put on a display in the presence of liquid water. As previously mentioned, the ice crystals that are responsible for halos are present in the highest clouds in summer as well as winter.

Another difference between rainbows and halos is that poems and songs have been written about rainbows - can you think of a song about halos (angel songs don't count). Maybe we can rectify this right now. Somewhere through the halo - well, that's a start anyway.

Sometimes you can see small concentric rings very near the Sun or moon that consists of rather faint colors. This is a corona. It results not from ice crystals, but rather from water droplets which all have a uniform size. These droplets diffract the light in such a way to produce an aureole. Coronae are easier to see around the moon - the Sun's brightness makes observing them difficult in the daytime.