As summer is drawing to a close, it means not only the start of a new school year, but also the approach of the next full moon. Scheduled to occur on September 4, the full moon will once again bring about the opportunity to catch sight of a rare and beautiful phenomenon called a moonbow.
Also referred to as a lunar rainbow, this phenomenon can only be seen regularly at two locations -- Victoria Falls in Zimbabwe, Africa; and right here in the United States at Cumberland County State Falls Park in Corbin, Kentucky.
The Cumberland Falls Web site predicts that moonbows will be visible for several days before the full moon, beginning on September 2, and can possibly still be seen as late as September 6. Every year, the moonbows draw thousands of visitors with cameras to the falls in the middle of the night to catch a glimpse of the unusual occurrence.
Moonbows take their light source from the moon, rather than the sun, and can be seen best on clear nights when the moon is full. This means that they are best seen in the fall and winter, when skies are clearer. They also require the presence of precipitation or moisture, such as the mist produced by waterfalls. The moon's light reflects off the water droplets in the air to create the rays of light. The location of the Cumberland Falls enables the moon to be at just the right angle and to shine the proper amount of light on the mist to create the phenomenon.
These lunar rainbows appear as a white beam to the naked eye since they occur at night when color perception is lower. However, when photographs are taken with a long exposure, they can result in rainbow-like images. Here's a cool photograph from Environmental Graffiti Web site, shot with a long exposure so you can see the full colors. You can also look at this picture from Courier-Journal.com
While moonbows have also appeared at other locations, including the Waimea Canyon in Hawaii and Yosemite Falls, they cannot be seen routinely. Humidity also affects the visibility of moonbows, resulting in dimmer colors. Cloud cover also blocks light from passing through to the mist, thus obscuring the moonbows.
Sightings of moonbows date back to as early as Aristotle. More recently, Texas State University Professor Don Olson and a team of astronomers began recording observations of moonbows over Yosemite Falls, beginning in 2005. By 2007, he and his team had designed a computer model that could accurately predict when moonbows would occur, and for how long they would be visible.
____
Brittany Stoner
Fox Weather Correspondent
Penn State University, Print Journalism and Spanish
