A 22-degree lunar halo is a striking atmospheric optical phenomenon that appears as a large, faint ring of light encircling the Moon at an apparent radius of approximately 22 degrees. This halo forms when moonlight passes through millions of hexagonal ice crystals suspended in thin, high-altitude cirrostratus clouds in the upper troposphere. As the light is refracted—or bent—by the ice crystals, it is deflected by exactly 22 degrees, creating a circular halo. Unlike a rainbow, which displays vibrant colors, a lunar halo is usually white or pale with a subtle hint of color, since moonlight is not bright enough to produce strong spectral hues. These halos are best seen when the Moon is bright and full or nearly full, and the surrounding sky is clear enough to allow the high cirrus clouds to be visible. While commonly seen in colder climates, they can occur anywhere in the world and are often interpreted as harbingers of changing weather, since cirrostratus clouds often precede a warm front or approaching storm system.