Why is the sky dull when a storm is coming on?

The light of day is almost all due to the Sun. The stars are shining, of course, as they do all the time, but they are so far away that the light of all of them put together counts for nothing compared to the light of the Sun; nor does the light of the Moon count for anything when it happens to be up during the day. Thus we may say that the light of the day is due to direct sunlight and to skylight, which is sunlight reflected from the sky – that is to say, from the air.

When a storm is coming on, clouds gather, and these clouds are thick and dense, so that they cut off the light of the sky, and so we say that the sky is dull. If we went up in a balloon above the clouds, we should find ourselves in brilliant sunshine, even when it was almost as dark as night to the people of the Earth below.

More reading:
Thunderstorm (Wikipedia)

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