Sun Halo - WAFB 9 News Baton Rouge, Louisiana News, Weather, Sports

Sun Halo

Question:

I saw a video on a halo around the sun, and I thought it was very interesting! Can you "shed some light" on this phenomenon? What exactly is it? What causes it?


Answer:

Where do the "halos" around the sun and moon come from?

As we've mentioned over the years at WAFB -- and occasionally shown examples -- the "halo" around the moon and/or sun is produced by high, thin cirrus clouds.  Cirrus clouds are ice clouds rather than liquid water droplets that make up most other cloud types.  During the day, these cirrus clouds are sometimes difficult to see, but often result in "filtered" sunshine and may give the sky a slightly pale appearance as opposed to a clear-blue cloud-free sky.  At night, these thin cirrus clouds are often virtually invisible: stars shine through and the sky appears totally clear.

The ice crystals that make up the cirrus cloud deck act like tiny reflectors at a precise angle from the sun/moon.  The angle, or size, of the halo (usually about a 22 degree angle from the centerpoint of the moon/sun) is a function of the "shape" of the ice crystals.  I've even seen double halos ... two circles indicating two different shapes of ice crystals in the cirrus clouds.

So how do the ice crystals take on different shapes?  The shape of the ice crystal is a function of the temperature at which the liquid (or water vapor) freezes.  Yep, they don't all look like tiny ice cubes -- instead, at very high altitudes and usually well below the freezing point (32F), they tend to freeze into either tiny "platelets" or "needles."

Hope this helps!

Jay Grymes
Chief Meteorologist
WAFB Storm Team

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