What causes a halo around the moon?
Have you ever looked up at the night sky and saw a clear ring around the moon? You're not dreaming—it actually happens.
ST. LOUIS - Have you ever looked up at the night sky and saw a clear ring around the moon? You're not dreaming—it actually happens.
As a burst of cold air is expected to make its way into the area this week, those who catch a glimpse of the sky could possibly see a halo around the moon.
In short, ice crystals in the upper atmosphere are refracted in the moonlight, forming a circular shape, according to BBC Sky at Night Magazine. The National Weather Service states that these lunar halos are typically white, although there can be some hints of red inside the halos if color is visible.
As this effect is dependent on ice crystals in cirrus clouds—around 30 miles above ground—it explains why this natural phenomenon mainly exists in the winter.
These tiny ice crystals are hexagon-shaped, which translate into a 22-degree ring diameter. This ring is always the same size, according to Live Science. However, there may sometimes be a second ring that is 44 degrees in diameter.
The reason it is always the same size is due to the hexagonal ice crystal bending light between its sides, creating this consistent halo. You can test this by putting your hand up to the sky with your fingers stretched out, with your thumb over the moon, and your pinky should be near the halo, the World Meteorological Association said.
A halo around the moon is also folklore for rain on the way, with some saying the number of stars inside the halo is the number of days it will take for the weather to arrive.
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