Charles Ellicott Commentary Exodus 9:29

"And Moses said unto him, As soon as I am gone out of the city, I will spread abroad my hands unto Jehovah; the thunders shall cease, neither shall there be any more hail; that thou mayest know that the earth is Jehovah`s." — Exodus 9:29 (ASV)

That thou mayest know how that the earth is the Lord’s. —Compare to Exodus 9:15. It was the general belief of the Egyptians, as of most ancient nations, that each country had its own god or gods. Pharaoh had already admitted Jehovah’s power (Exodus 8:8), and now regarded Him as the God of the Hebrews (Exodus 8:28). God desired to have it generally acknowledged that He was the God of the whole earth.