This incredible photo of a moonbow near Upper Yosemite Fall was taken by photographer Brian Hawkins.

Lucky visitors heading to Yosemite National Park this weekend will get a special treat if they stay up late enough. Conditions will be perfect for moonbows to make an appearance around the park’s waterfalls.

Moonbows are just like rainbows, only they happen at night when moonlight shines on water droplets. They are also known as lunar rainbows.

“Moonbows are rare because moonlight is not very bright,” said Les Cowley, an expert in atmospheric optics. “A bright moon near to full is needed, it must be raining opposite the moon, the sky must be dark and the moon must be less than 42° high.”

“Put all these together and you do not get to see a moonbow very often!” Cowley added.

Waterfalls can aid with the “raining opposite the moon” requirement, and Yosemite has plenty of waterfalls. The park also doesn’t have the problem of light pollution, making it a prime location to look for moonbows.

July 15 is the full moon. According to the U.S. Naval Observatory, the moon will be at an altitude less than 42° through the weekend, so viewing conditions should be perfect. There may be a few patchy clouds around tonight, but not enough to interfere. The rest of the weekend should be clear.

Cowley explained that to the unaided eye, moonbows usually appear without color because their light is not bright enough to activate cone color receptors in our eyes. However, he said that colors have been reported and might be seen when the moon is bright.

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