Exodus 9:8

8.And the Lord said unto Moses. God does not now postpone the time of the punishment, but redoubles the plagues in a continuous series; nor does he threaten Pharaoh, but, leaving him, executes the judgment which He decreed; both because it was now more than sufficiently manifested that admonitions were of no avail with him, and also that his desperate wickedness might be reproved in every way. For although I have lately said that all which happened is not fully related, still the narrative of Moses rather leads us to infer, that nothing about the boils was previously told to Pharaoh, but that the ashes (105) were sprinkled, when he had no suspicion of anything of the kind. But it did not happen naturally that the heaven was darkened by the dust, and that the disease arose from thence; for how could a few ashes cover the whole air? But by this visible sign the tyrant was taught that the calamity which ensued was inflicted by Moses and Aaron. Moreover, God invested His servants with high and power, when He gave them command over the air, so that they should envelop it in darkness, and poison it with contagion. Hence we gather, that the devil’s are called the princes of the air, not because they govern it according to their will, but only so far as the permission (106) to wander in it is accorded to them.