【英语时差8,16】彩虹的鬼表哥

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Rainbows are one of nature's most ephemeral beauties, but there's a related phenomenon that's even more ghostly.   

These are the pale white, rainbow-like arches that are sometimes seen on foggy days or in dense¹ fog banks. What's responsible for these so-called "fogbows"?

 

Fogbows are created in much the same way as normal rainbows are. Round droplets³ of water in the atmosphere can separate sunlight into different colors, just like a prism or a piece of crystal. This is called "refraction."

 

You'll see a rainbow when the sun is behind you, but it's shining on raindrops in the sky in front of you. Pure sunlight is white--it contains all the colors of the spectrum--but each individual color is refracted out of the droplets at a slightly different angle. A rainbow is the refracted light from millions of tiny raindrops.

 

A rainbow only appears when the water droplets are small enough to be more or less round, but large enough to focus the colors clearly. The best droplet² size for rainbows is about one or two fiftieths of an inch across. Larger drops tend to wobble as they fall, refracting the sunlight haphazardly⁴. Smaller drops can refract the light well enough, but they are too small to focus the colors clearly. The diameter of a fog droplet is only about one five hundredth of an inch, much too small for this sharp focus. The light is refracted just like in a rainbow, but the colors all bleed into one another, and the fogbow appears a ghostly white.  

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