How does a halo form around the sun?

ALLISTER AALDERS @allistercanada Allister Aalders is the weather specialist for the SaltWire Network, providing forecasts and analysis for Atlantic Canada. #AskAllister



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Down Memory Lane

Many people were up early Tuesday morning to wish fishers well on setting day in P.E.I., including Tim Gallant. Tim took some photos, but it wasn't only fishing boats that he photographed – he also caught this sun halo over the harbour in North Rustico, P.E.I. Tim asked me how sundogs form, noting that it doesn't seem to happen often and that conditions must need to be just right. In this case, Tim's photo appears to be more of a sun halo. But the formation process for both is similar. The key ingredient is high cirrus clouds that are thin enough to see sunlight still. Cirrus clouds are found at heights of more than 20,000 feet in our atmosphere and are composed of tiny ice crystals. For a halo, the light from the sun or the moon refracts in the ice crystals, which slows down and bends the light, changing its direction. The light then reflects and is displayed as a white or coloured ring which forms about 22 degrees away from the sun or moon. The same process of refraction and reflection is needed for sundogs, also known as mock suns. However, sun dogs are spots to the left, right, or both, of the sun, 22 degrees away from it. Of course, it's not surprising that halos around the sun or moon can be a signal of future weather. According to folklore, “A ring around the sun or moon means rain or snow is coming soon." This is often true as high clouds arrive first in advance of unsettled weather systems. Thanks for your weather question, Tim.