John Calvin Commentary Exodus 9:22

"And Jehovah said unto Moses, Stretch forth thy hand toward heaven, that there may be hail in all the land of Egypt, upon man, and upon beast, and upon every herb of the field, throughout the land of Egypt." — Exodus 9:22 (ASV)

And the Lord said. The rod of Moses is again employed to bring on the storm, not so much for Pharaoh’s sake, as that Moses may be more encouraged for the remaining contests, when he sees the proof of his vocation renewed.

Meanwhile, we may observe the trial of his faith, since before he had received the command to stretch out his rod toward heaven, he had not hesitated to predict to Pharaoh the grievous and miraculous hail.

But if anyone thinks that this is an ὕστερον πρότερον, and that what was first in order of time is related last, I will not debate it. However, this seems more probable to me, and also to be rightly gathered from the text: that when the day had elapsed, Moses was commanded to execute that for which the means was previously unknown to him.

Hence, Moses himself learned, and we also ought now to learn, that all the elements, although without sense, are still ready to render any kind of obedience to their Maker. For, at the stretching out of the rod, the air was troubled in an incredible manner, so that it hurled down an abundance of hail for the destruction of beasts and men.