Spray Bow at the Lower Yosemite Fall

Numerous places in the world feature spray-, fog- or mist-induced bows. In the United States such bows may be seen in relation to various waterfalls including Niagara Falls, New York, Yosemite National Park, California and Cumberland Falls, near Corbin, Kentucky. Victoria Falls, on the border between Zambia and Zimbabwe is also widely known for spray moonbows.

Spray moonbows are also seen with some regularity in the cloud forests of Costa Rica, in mountain towns like Monteverde and Santa Elena. These occur when clouds of mist are blown in from the Caribbean by the Christmas Winds. The Christmas Winds happen from the end of December through late January or early February.

These clouds of mists create a streaming pattern of stripes giving rise to their popular name in Spanish, pelo de gato ("cat's hair). Moonbows happen in this part of Costa Rica almost every full moon in the months of December through February. The bows that are caused by Pelo de Gato are not limited to just before dawn but can happen after sunset too, but it does need a full or nearly full moon.

Moonbows are also found in wet regions of Hawaii, such as Kauai (with the moon rising in the east during light rain) and the Big Island of Hawaii.




Takakkaw Falls, Yoho National Park, British Columbia, Canada




Spray Bow

A true moonbow is lit by the Moon itself, not directly by the Sun. In contrast, a colored rainbow (or a white fogbow) seen when the sun is rising or setting, or in twilight, is not a moonbow because it is still produced by sunlight. Moonbows have been mentioned at least since Aristotle's Meteorology (circa 350 BC), and also in an 1847 publication.




A colored circle around the moon is not a moonbow it is usually a 22° halo produced by refraction through hexagonal ice crystals in cirrus cloud. Colored rings close to the moon are a corona, a diffraction phenomenon produced by very small water droplets or ice crystals in clouds.

Moonbows are most easily viewed when the moon is near to full (when it is brightest). For true moonbows, other than those produced by waterfalls or sprays, the moon must be low in the sky (less than 42 degrees and preferably lower) and the sky must be dark. And, of course, there must be rain falling opposite the moon. This combination of requirements makes moonbows much more rare than rainbows produced by the sun.





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