Exodus 10:21

This plague, like the third and sixth, was sent without warning. It is not said how the darkness was produced, but in all probability it had a natural basis, like the other plagues. It resembles the darkness caused by the khamsin, a S. or SW. wind, excessively hot and charged with fine dust, which blows about the time of the vernal equinox. The darkness is often local, covering a belt or strip of the country. The unusually dense gloom would excite the superstitious fears of the Egyptians, who worshipped the sun-god Ra. For a vivid description of the terrors of this plague, see book of Wisdom, Exodus 17.