Weather Science

Those ghostly moon halos and how they can help predict the weather.

Growing up, I’m sure Rochesterians are familiar with a number of mythical sayings for fascinating weather occurrences. For colorful, brilliant skies, there’s the well-known “Red sky at night, sailor’s delight. Red sky in morning, sailor’s warning”. There is also the less known but equally peculiar cricket theory, which states in order to calculate Fahrenheit temperature, count a cricket’s chirps over fourteen seconds and add fourteen. Many have actually seen success when predicting weather changes through this concept. However, for those of you who have difficulty keeping up with a cricket’s rapid chirp rate, you may want to look to the moon to foresee a change of weather.

Photo: Dan Bush (via

Large rings around the moon known as halos are without a doubt one of the most captivating weather phenomena, but did you ever realize you could predict the weather by looking at one? Before we jump into how it’s possible to forecast by merely peering at the sky, we need to understand how halos form around the moon. A halo is a whitish ring that encircles but does not touch the moon. They are merely an optical sensation, formed by the splitting of light (refraction) caused by atmospheric ice crystals. Since light must shine through this thin layer of ice crystals, halos are thus generally associated with cirrostratus (wispy, sheet like) clouds that drift about 20,000 feet above the earth’s surface.

As with any optical illusion, halos are based entirely on the perspective of the individual who sees them. That’s why, like rainbows, these rings around the moon are personal. Everyone sees their own particular halo, made by their own particular ice crystals, which are different from the ice crystals portraying the halo to the person standing next to you.

People often wonder if these halos are consistent or if they vary at all. Different crystal habits, orientations, and zenith angles can produce different halos, the most common being the 22° halo. This halo gets its name by forming a circle 22° away from the moon. Because ice crystals are randomly oriented in space, there are a variety of directions light rays can enter and exit the crystals. However, light rays that form the 22° halos enter one side and exit out the opposite, the most common occurrence, making this the most common halo.

The other known halo is the 46°, located 46° away from the moon. This halo is much less common because ice crystals do not enter one side and exit the other; they exit out of the bottom or top of the ice crystal.

Photo: Colin Chatfield (via

Regardless of the type of halo, many have claimed this optical phenomenon can be a telltale sign of precipitation in the near future. People have stuck to the old saying of “ring around the moon means rain soon”. Although this may be seen as a tad far-fetched, there is truth to this saying, since high cirrostratus clouds often come before a storm. So in addition to listening to the weatherman for tomorrows forecast, check the moon before bed, you never know what you may find.