Lunar halo sparks curiosity, moon gazers Monday night - WTOC-TV: Savannah, Beaufort, SC, News, Weather & Sports

What was that ring around the moon?

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Lunar halo on Monday night. (Source: Pat Prokop) Lunar halo on Monday night. (Source: Pat Prokop)

SAVANNAH, GA (WTOC/WAFB) - WTOC was flooded with phone calls Monday night from people curious about the ring that appeared around the moon.

The phenomenon is called a lunar halo. Senior Meteorologist Pat Prokop says the ring is caused by ice crystals in cirrus clouds high above around 30,000 feet. The crystals refract (bend) the light in all directions by an angle of 42 degrees forming the circle. Even if the moon was a rectangle, the refracted light would still be round.

Lunar haloes are somewhat rare because they require a combination of events:

  • The moon must be located overhead, or nearly so
  • There can be few, if any, clouds in the low and middle levels of the atmosphere
  • A thin deck of cirrus clouds, so thin that you can see the stars through them
  • By the way, did you see the "star" sitting right next to the moon? That's the planet Jupiter, making Monday night's halo even extra special.
  • "Solar haloes" also occur occasionally, but are generally less common (harder to see) than the lunar halo, because of the advantage of the night sky with the moon.

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