Jasmine and Chrissy from NIU STEAM recently shared their insights on the Sound of Science on WNIJ. During their latest presentation, they received questions about halos that can be seen around the Moon at night and across the horizon when the Sun is low in the sky during a typical Northern Illinois Winter. Both situations occur due to ice crystals in the atmosphere, which form depending on conditions like temperature, humidity, and the presence of clouds.
When water in the atmosphere exists in a super-cooled liquid state below freezing point, it can freeze into ice crystals when it comes into contact with tiny particles in the air. Cirrus clouds, found in the highest parts of the atmosphere, are an excellent source for ice crystal formation due to their high altitude and low temperature.
When light from either the Moon or Sun passes through these ice crystals, it can be reflected, refracted or dispersed based on their shape. The most commonly seen patterns are 22-degree halos and 46-degree halos formed when light passes through hexagon-shaped ice crystals.
During sunrise or sunset, a phenomenon called Parhelia occurs where one or two bright patches of light appear within a solar halo on either side of the Sun known as “sun dogs.” These occur because sunlight passes through a greater thickness of Earth’s atmosphere during these times, increasing interactions with ice crystals. Jasmine and Chrissy explained that sun dogs are more likely to occur during sunrise or sunset due to this reason.
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