Moonbow

True moonbows

By definition, true moonbows are associated with atmospheric rain events and arise at a time when conditions are optimal in terms of levels of reflected solar light being reflected from the Moon's surface. All true moonbows are therefore short-lived and transitory in nature. True moonbows are independent of all terrestrially fixed geographical features such as waterfalls for their existence.

Moonbows have been mentioned at least since Aristotle's Meteorology (circa 350 BC), and are associated with the ancient Greek Lunar goddess, Selene.

Viewing

Moonbows are most easily viewed when the Moon is at or nearest to its brightest phase full moon. For moonbows to have the greatest prospect of appearing, the Moon must be low in the sky (at an elevation of less than 42 degrees, preferably lower) and must not be obscured by cloud. In addition, the night sky must be very dark. Since the sky is not completely dark on a rising/setting full moon, this means they can only be observed two to three hours before sunrise (a time with few observers), or two to three hours after sunset. And, of course, there must be water droplets (e.g. from rain or spray) opposite the Moon. This combination of requirements makes moonbows much rarer than rainbows produced by the daytime sunlight. Moonbows may also be visible when rain falls during full moonrise at extreme latitudes during the winter months when the prevalence of the hours of darkness gives more opportunity for the phenomenon to be observed.

It is said that the definition of the colours depends upon the size of the moisture drops present in the air: the smaller they are the less vivid the colours. Below 1/500 inch diameter they usually refract more or less white light as the component colours are merged.

False moonbows

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

Spray, mist or fog related bows are classified as false moonbows. Such false bows (particularly waterfall spray bows) are dependent on geographical features. Moreover, since the same 'spray bow' is very often visible & reproducible in daylight hours, it means that such a bow cannot be linked exclusively to the effect of moonlight.

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Locations

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.

References

References

1. "Moonbow".
2. Miller, James. (2018-05-28). "Examining the Phenomenon of Lunar Rainbows".
3. Valerie Porter, “The Second Field Book of Country Queries” p. 89
4. "Goat Island and Luna Island How Did they get The…".
5. (15 July 2011). "'Moonbows,' Lunar Rainbows, Visible At Yosemite National Park (VIDEO)". Huffington Post—Green Blog.
6. Bailey, Bill. (1995). "Kentucky State Parks". Glovebox Guidebooks of America.
7. (1999). "The Historic Cumberland Plateau: An Explorer's Guide". Univ. of Tennessee Press.
8. "A Lunar Rainbow A Wonderful Sight Not To Be Missed...…". Victoria Falls Travel Guide.
9. (June 4, 2012). "Photographer captures moonbow over Waimea". Hawaii News Now.