What is what does it mean when there's a ring around the moon?

A ring around the moon, often called a <a href="https://www.wikiwhat.page/kavramlar/moon%20halo">moon halo</a>, is a luminous circle or arc of light that appears to surround the moon. It's a relatively common atmospheric optical phenomenon.

Here's a breakdown:

• Cause: Moon halos are caused by the <a href="https://www.wikiwhat.page/kavramlar/refraction">refraction</a> and reflection of moonlight by tiny ice crystals suspended in thin, wispy cirrus or cirrostratus clouds high in the atmosphere (typically 20,000 feet or higher). These ice crystals act like prisms, bending the light as it passes through them.

• Shape and Size: The most common type of moon halo is a 22° halo. This means the radius of the circle is about 22 degrees from the moon. The reason 22° is so common is due to the specific angle at which light is most efficiently refracted through the hexagonal ice crystals. Other less common halos can form at different angles.

• Appearance: The halo usually appears as a whitish ring, although sometimes faint colors (reddish on the inside, bluish on the outside) can be observed. This is because different wavelengths of light are refracted at slightly different angles.

• Weather Connection: Moon halos are often associated with approaching storm systems. The cirrus clouds that cause the halos frequently precede a warm front and its associated precipitation. So, seeing a moon halo can be an indication of changing weather conditions. They aren't a guarantee of bad weather, but it's a good prompt to check the forecast. It is because that <a href="https://www.wikiwhat.page/kavramlar/ice%20crystals">ice crystals</a> are generally associated with high-altitude clouds.

• Distinction from Corona: It's important to distinguish a moon halo from a corona. While both are rings of light around the moon, coronas are caused by diffraction of light by water droplets or small ice crystals in lower-altitude clouds (like altostratus or altocumulus). Coronas are smaller, often colorful, and appear much closer to the moon than halos.