Seeing a rainbow usually evokes a feeling of hope and peace. Its colors add to the beauty of nature as if the sky is smiling down on Earth. On rare occasions, some rainbows appear upside-down like NASA's recent photo.
It was a stunning image of an inverted rainbow, also known as "circumzenithal arc," that appeared over a palm tree in Ragusa, Sicily, on February 24. According to NASA, the particular rainbow is an example of an "ice halo" that forms due to refraction and reflection in flat, hexagonal ice crystals.
Astronomy Picture of the Day: A circumzenithal arc forms above a palm tree Ragusa, Sicily on February 24.
Astronomy Photo of the Day
The inverted rainbow was photographed when the sun was low in Sicily and was spotted by 47-year-old Marcella Giulia, an Italian astrophotographer and a primary school teacher, MailOnline reported.
It is a rare event to photograph the circumzenithal arc as they are usually obscured by big clouds that is why not many people see them. As Giulia wrote in her caption, rainbows are typically ignored because people nowadays are more likely to bow down when using their phones even though they appear over the sky many times.
Many people call the circumzenithal arc the smiling rainbow because its shape makes the colors appear inverted compared to the common rainbows observed in the sky. Giulia noted that the upside-down rainbow has the most vivid and bright shades out of all the halo phenomena seen in the photo and is even more striking than the typical rainbow.
Due to its beauty and uniqueness, NASA has chosen Giulia's photo of the circumzenithal rainbow as its "Astronomy Picture of the Day" on Friday, March 11.
How Do Circumzenithal Arc Forms?
As NASA explains, the zenith of the circumzenithal arcs is at the center and the color red is on the outside, unlike the typical rainbow after a downpour whose arcs are pointed toward the horizon. The American space agency said that to see the circumzenithal arc, one has to look straight up when the sun is low in the sky.
Typical rainbows are formed due to the refraction of sunlight in water droplets. In contrast, circumzenithal arcs result from refraction and reflection in flat hexagonal ice crystals formed in high, thin clouds. As sunlight refracts through the horizontal ice crystals at a specific angle, the light enters a flat surface and exits through a side prism that causes its unique inverted rainbow effect.
The UK's Meteorological Office added that circumzenithal arcs are commonly associated with Cirrus clouds where ice crystals readily form. This type of rainbow is quite common as they occur throughout the year, although they are mostly hidden in clouds.
For a circumzenithal arc to appear, a combination of atmospheric conditions must be present. The Met emphasized the importance of height, depth, and position of ice clouds for the light to enter the specific angle convex to the sun. More so, the observer's position is also important because the arc's visibility can vary over short distances.
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