The video: Officials at Yosemite National Park have released a "stunning" video of something that few people have ever heard of, let alone seen: a "moonbow." Similar to a rainbow, a moonbow is a colorful optical effect caused by moonlight passing through mist or water vapor. For lovers of this rare natural phenomenon, 2011 is proving to be a banner year. A combination of heavy snowmelt in California's Sierra Nevada mountains has combined with clear, moonlit skies to produce several of the colorful arcs in Yosemite. (Watch a video below.) Moonbows are best viewed when the moon is full or nearly full, and low in the sky. They can occur when moonbeams hit water droplets in the atmosphere, but the most reliable viewing is near large waterfalls. Yosemite's cascades aren't the only ones that produce moonbows; they are sometimes seen at Cumberland Falls in Kentucky, Victoria Falls in Africa, and in Hawaii's rainy Waimea Canyon. 

The reaction: Wow, says Rob Lovitt at This is simply "an ethereal experience." What's exceptional about the Yosemite moonbows are their vivid colors, says Les Cowley, as quoted by Often, these natural phenomena appear white to the naked eye, because they're not bright enough to activate our eyes' cone color receptors. But if the moon is especially bright, as it has been in Yosemite, look out. See for yourself: