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Iridescent Clouds
| Article
# : |
12261 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
2 / 1987 |
714 Words |
| Author
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Pekka Parviainen
Pekka Parviainen is a free-lance writer living in Turku,
Finland. |
The rain has ceased and now the sky is filled with fragmentary clouds scudding through blue sky. While jogging, you are dodging puddles and for an instant the reflected sun strikes your eyes. "Just a moment - did I see colors or - ?" You take the next puddle and the thin arm of a cloud appears pastel red and green. You turn your keen eye to the sky, but find it too bright for direct observing. Back to the puddle for another look, but now the cloud is gone and you wonder whether you really saw anything at all.
A similar observation may take place during cycling. A pinkish solar disc speeds through the trees and leaves, but when you stop to get a confirmatory look, you see nothing but bright clouds.
Iridescent clouds are a new and surprising phenomenon to many people, but are actually detached extensions of corona that are observed around the sun or moon. They appear frequently, but because of their short angular distance from the sun they are hard to observe directly. Though hard to see, they show some of the purest colors seen in the sky -comparable to bright rainbows. The most characteristic colors of iridescent clouds are pastel red and pastel green. In the brightest displays, the whole spectrum of color may be visible, and then the dominating colors are rather blue and red.
A good way to get acquainted with iridescent clouds is to monitor their passing reflections as they drift through the mirrors of puddles or swimming pools. The best colors are found at two to ten degrees from the sun, although faint traces of color can be seen from thirty to forty degrees away. (Two degrees are approximately the width of your index finger when your hand is fully stretched out in front of you.) The angular distance of two degrees is critical to the iridescence and a cloud will appear ordinary until it reaches this angle from the sun.
At first glance, the colored parts of the clouds may appear irregular, but with further observation they often follow the visible contours of the clouds. A typical form of iridescence is seen as ribbons of color along the edges of the clouds. Another common form of iridescence is the corona, where the colors form concentric rings around the sun.
Not all clouds can produce iridescence. Clouds that produce the best shows are those that contain water droplets of uniform size, the same conditions that produce corona closer to the sun. If the diameter of the droplets falls within the range of 0.005-0.02 mm
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