What was that ring around the black part of the moon on last sunday?
Why is there sometimes a ring around the moon? Is it useful for weather forecasting? Contrary to what some people think, the ring around the moon is not due to failure of the moon to use the proper laundry detergent. Rather, the ring is caused by the reflection of moonlight off of ice crystals in high, thin clouds. The ring has a radius of 22 degrees, which is set by the properties of water. Most often, the ring around the moon (and also around the sun) is caused by high, thin cirrus clouds — sometimes too thin to be seen by any other means. Cirrus clouds tend to precede weather fronts, meaning that bad weather is likely ahead. So, it is often said that a ring around the moon or sun indicates rain. To the best of my knowledge, professional meteorologists do not use the appearance of a ring to forecast the weather.
