Exodus 10:21

Darkness -This infliction was specially calculated to affect the spirits of the Egyptians, whose chief object of worship was the Sun-god; and its suddenness and severity in connection with the act of Moses mark it as a preternatural withdrawal of light. Yet, it has an analogy in physical phenomena. After the vernal equinox the southwest wind from the desert blows some 50 days, not however, continuously but at intervals, lasting generally some two or three days. It fills the atmosphere with dense masses of fine sand, bringing on a darkness far deeper than that of our worst fogs in winter.