Jamieson, Fausset and Brown on Exodus 10:21-23

Ex 10:21-29. Plague of Darkness.

21-23. Stretch out thine hand toward heaven, that there may be darkness--Whatever secondary means were employed in producing it, whether thick clammy fogs and vapors, according to some; a sandstorm, or the chamsin, according to others; it was such that it could be almost perceived by the organs of touch, and so protracted as to continue for three days, which the chamsin does [Hengstenberg]. The appalling character of this calamity consisted in this, that the sun was an object of Egyptian idolatry; that the pure and serene sky of that country was never marred by the appearance of a cloud. And here, too, the Lord made a marked difference between Goshen and the rest of Egypt.