THE AIRLINE PILOTS FORUM & RESOURCE
Corona |
| Source: Excerpt from The Book " Weather " |
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A Corona (meaning crown) describes one or more luminous disks of light that occasionally appear around the Sun or Moon. They occur when these objects are viewed through a thin layer of cloud consisting of water droplets.
A corona is caused by light diffraction --- the slight bending of light as it passes by the edge of an object. This process causes the colors that make up white light to separate, because each color has a different wavelength and bends at a different angle. A corona forms when sunlight passing through a cloud is diffracted by
water droplets in the cloud. The short wavelength
of blue light diffracts the most and forms the inner
ring, while red forms the outer ring. While
orange, yellow, and green may be visible in bright
coronas, blue and red are the predominant colors.
Sometimes, several rings will be visible, becoming
fainter the farther they are from the center.
A corona is best seen when the Moon, rather than the Sun, is providing the light (right picture) , as the brightness of the Sun tends to blind the observer to the subtle effects of diffracted light. Small water droplets produce the largest coronas, and those of a uniform size produce the brightest colors. If the cloud contains a variety of differently sized droplets, the rings become deformed and irregular and the colors diffuse. Newly formed clouds tend to contain uniformly small droplets, so the best conditions for a colorful and well-defined corona are a bright Moon shining through a thin layer of newly formed, uniform cloud, such as altostratus. |
Acknowledgement: John W. Zillman, William J. Burroughs,
Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot and Richard Whitaker.
