The SCI 301 Puzzler

On occasion, usually when it is stormy, wind blows from the south side to the north side of St. Thomas and dark clouds move through the lower altitude saddles of the main ridge of Crown Mountain. As the leading edge of a cloud moves out over the sea, it becomes lighter and eventually invisible. The structure doesn't really change, it is more like a fade-to-clear video trick. It is fascinating to watch! Explain what is happening and why? (Hint: Consider how the cloud formed in the first place.)
An animation for those who have never seen it.



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