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SWA Meteorology: Halos

rmiller
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It’s a photo that’s been circulating around the interwebs: a spectacular display of halos, arcs and lines all around the sun, taken by Joshua Thomas in Red River, New Mexico on January 9. We’ve all seen halos around the sun and moon, but I bet you’ve never seen anything like this! One of the subjects meteorologists study in college is optical physics in order to explain things like halos, but it took an expert physicist in atmospheric optics to identify the many halos here. Dr. Les Cowley (it’s misspelled on the picture) from Atmospheric Optics (www.aoptics.co.uk) has diagrammed the picture to tell us what’s what. Halos are formed when millions of ice crystals are present 3 to 5 miles above the ground. All ice crystals are near perfect hexagons. When the sun’s rays pass through them, the rays are reflected and refracted, resulting in halos. The smallest halo around the sun is the most common: it’s a 22 degree halo, meaning the angle from the center of the sun to the halo is 22 degrees. It’s produced when the ice crystals are tilted 60 degrees to one another. An even larger halo, a 46 degree halo, is rare and produced when the ice crystals are tilted 90 degrees to each other. (Geometry DOES come in handy sometimes!) So what caused all these halos and arcs in the sky? We see ice crystals in cirrus clouds all the time, but rarely halos like this. Sometimes in areas where ski resorts make snow using “snow cannons”, ice crystals from these cannons drift through the air, producing halos. It’s sort of like when you spray water from a hose and see a rainbow. Ice crystals are sprayed into the air, creating halos. There’s a ski resort in Red River, NM that does, indeed, make snow. The sun is also low in the sky in the photo, indicating maybe some ice crystals didn’t need to be high in the atmosphere to catch the sun’s rays. Perhaps they were making snow in Red River on January 9. But unusual halos and arcs can be seen even when snow making machines aren’t nearby. Just last Friday in Dallas, a sun dog and parhelic circle were seen in the cirrus clouds above!

Photo courtesy of Terry Pace

Folklore says when a halo is seen around the sun or the moon that rain is on the way. What halos indicate are the presence of high, thin clouds made of ice crystals. Sometimes it rains afterward, sometimes it doesn’t. But if you’re near a ski resort that will be making snow, keep the camera handy for any unusual halos, and maybe your picture will go viral, too!