On the night before the now-infamous "Presidents' Day Blizzard of 2003, the phone rang incessantly at the weather center at News 12 Westchester in New York. But the inquiries posed to the meteorologist on duty that night were not about the impending big snow.

In advance of the clouds, a Full Moon was shining brightly, and right next to the Moon was a very bright, silvery "star," the planet Jupiter. But that's not what all the phone calls were about, either.

Rather, people were asking about a large and unusually bright ring, or halo, that surrounded the Moon.

Halos are produced when light from the Sun or the Moon strikes ice crystals suspended high in the atmosphere at altitudes above 25,000 feet (7.6 kilometers). The ice crystals reside within a rather thin, uniform veil of cirrostratus cloudiness.

The practical value of halos for weather forecasting follows from the fact that cobwebby cirrostratus clouds usually precede a warm front and its accompanying precipitation. Actually, a halo in winter doesn't have absolute significance; but during the warmer months, it usually foretells the coming of a thickening cirroform ceiling of warm air overhead; in most cases, a long, slow rain should eventually arrive in about 12 to 18 hours.

But your chances of accurately predicting precipitation depend chiefly on your location; the closer you are to normal storm tracks, the more likely a halo predicts rain. Precipitation tends to develop more quickly with an associated halo that appears in winter as opposed to summer.

The precise size of the halo is due to the shape of the ice crystal. Light enters one side of the crystal and is refracted, or bent, because light travels more slowly through ice than through air. This light leaves the crystal through another side and refracted again. This light is usually bent at an angle of 22 degrees, creating the most commonly observed halo.

Halos seldom have much color; they are commonly soft white circlets in the sky.

But sometimes, especially with solar halos, they can appear like vivid rainbows with a dull red appearing on the inside and bluish white on the outside. (Important note: If you attempt observation of a ring or halo around the Sun, make sure you block the Sun out with your hand so that you don't end up looking directly at it and risking blinding yourself.)

There's another halo sometimes called the great halo, which is not seen very often. It appears at an angle of 46 degrees from the Sun or Moon and forms on the same principle as the normal 22-degree halo. In the great halo, light enters the top of the crystal and then emerges from one of the sides, or it enters one side and then passes out the bottom.