Monsoons lit up Utah sky with a rare type of rainbow

SALT LAKE CITY — Monsoonal storms provided Utah with much-needed moisture and spectacular lightning displays over the weekend, and also helped provide some residents with a rare type of rainbow.

A large rainbow filled the sky west of Garden City about an hour after sunset Sunday night, said Corey Wiley, who submitted a photo of it to KSL through iWitness. KSL viewers provided similar photos over the weekend after noticing similar experiences over Park City, Rockport Reservoir and other parts of the state.

These were all a specific type of rainbow called a moonbow, or "lunar rainbow," which is extremely rare because it requires specific conditions to be visible.

Rainbows, of course, form when sunlight collides with water droplets at the right angle to produce colors in the sky. But when there's enough moonlight in the sky, located in the right position away from storms, it can also create the same visual, although not as strong, the National Oceanic and Atmospheric Administration explains.

"Very rarely, a full moon will produce enough light for our eyes to see a faint 'moonbow' across the sky," the agency wrote in 2018.

The moon has to be less than 42 degrees high for it to form, KSL Weather meteorologists wrote in 2006, responding to a question about a previous event that produced one near Lake Powell.

Although the next full moon isn't until Friday, the moon — at 74% to 82% illumination over the weekend — still produced enough light for moonbows to be visible.

More could form this week over Utah as monsoonal moisture continues to push through the state, producing scattered evening storms on Monday and possibly more on Tuesday, while the moon inches closer to fullness.

It's something to keep an eye on if the moon is visible and storms form in the right location away from it.

The probability lowers by the middle of the week, as drier air moves into the state.

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