An age-old adage, 'Ring around the moon, rain soon', may hold more truth than initially believed, as the moon offers several methods for predicting weather patterns. A frequently observed 'moon halo', or lunar halo, could signal approaching snowfall, especially on chilly winter evenings. These halos are caused by light refracting through ice crystals within a thin cloud layer.
Jonathan Erdman, senior meteorologist at weather.com, explained: "One thing to look for is whether these thin, wispy cirrus clouds eventually lower and thicken. If so, then snow or rain is probably on the way." The next time you notice an unusual ring encircling the moon, it may be your cue to keep your winter jacket nearby or be ready to defrost your vehicle, or perhaps carry an umbrella for the day. However, it's always recommended you monitor the weather forecast to be certain.
While the halo isn't actually snow, it serves as an indicator of atmospheric conditions that could lead to snowfall and makes for a useful winter weather trick. The rings can form due to several key factors, primarily ice crystals present when moonlight passes through crystals within high-altitude clouds in the atmosphere. Another factor is simply atmospheric conditions themselves, such as the presence of high-altitude clouds which form when moisture rises and freezes in the upper atmosphere. That natural occurrence, however, is often an indicator that milder weather is on its way.
Adding credence to this folklore theory, it's also thought that the number of stars visible within a moon halo can indicate how many days until poor weather arrives. One source indicates that smaller, more sharply defined rings may suggest snow within eight to 12 hours, whilst larger, less distinct rings could mean 48 to 72 hours. According to a Quora discussion thread about lunar rings, one user, Vincent Emery, drew comparisons to rainbows, noting that it is essentially the same phenomenon but with the moon and cloud instead of the sun and rain.
