Question

what do you mean by the overcast sky

Answer 1

overspread or covered with clouds; cloudy: an overcast day. Meteorology. (of the sky) more than 95 percent covered by clouds.

Answer 2

ANSWER
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Overcast sky is one which is fully covered with clouds. No sun is visible. Such a Sky can be observed before rain or storm.

Fighter Planes cannot not fly in such a Sky because of very low visibility.
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