E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A cloudy speck which indicates the approach of a storm. When Elijah heard that a speck no bigger than a mans hand might be seen in the sky, he told Ahab that a torrent of rain would overtake him before he could reach home (1 Kings xvii. 44, 45). Thomson allude&smacr; to this storm signal in his Summer.