Exodus 10:22

22. And there was a thick darkness in all the land of Egypt three days As all the plagues seem in some way connected with natural causes, this preternatural darkness is by many assigned to a sand-storm, such as accompanies the simoom or the chamaseen, miraculously increased in intensity. Neither of these winds are ever known to produce so deep a darkness as is here described, though they obscure the sun and cause a twilight gloom. The simoom, or samoom, is a hot parching wind, raising clouds of dust and sand, which give the whole air a reddish-yellow tinge, and make the sun at first look like a globe of blood, and then blot it wholly from view. It is painfully suffocating both to man and beast while it lasts, but it is of short duration, especially if extremely hot and violent, generally passing within half an hour. The chamaseen, or khamsin, is less hot and violent, but lasts two or three days at a time, occurring at frequent intervals during a period of fifty days before and after the vernal equinox, that is, during March and the first of April, which corresponds well with the time of the plague of darkness. It darkens the sun with clouds of sand, which fill the air like a yellow fog, or like a heavy storm of snow or hail; men and beasts hide themselves while it rages, the inhabitants taking refuge in the innermost apartments or in subterranean vaults. The streets are deserted, as in the night, and all business ceases. But all these characteristics must have been supernaturally intensified to produce the effects described in the text. Both the simoom and the chamaseen are local, and very limited in range, so that such a wind might have blows up the Nile valley and left Goshen, in the eastern part of the Delta, untouched.