What is a “snow blink”?

Dear Tom,

I have come across the term “snow blink” used in a technical manual but I can find no meaning for the term. What is a “snow blink”?

Jeff Maliss,
Champaign, Ill.

Dear Jeff,

“Snow blink” is a bright white glare on the underside of clouds, produced by the reflection of light from a snow-covered surface. The term is used in polar regions.

A different, but similar, kind of “blink” can often be observed here in the Chicago area when metropolitan lights illuminate the undersides of clouds at night. In situations when the city is covered by a high overcast in the range of 10,000 to 30,000 feet above the ground, the sodium vapor lights from the metropolitan area cast an orange glow on the underside of the overcast — a glow that can sometimes be seen as far as 75 miles away when the air is clear.

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