God's mercy sent milder penalties at first to turn the Egyptians away from disobedience and to save them from the final and awful calamity.
..........................But when that didn't work, He had to get stronger in His rebukes to convince them He was serious.
In the seventh plague, [read Exodus 9:16-35], "23 And Moses stretched forth his rod toward heaven: and the LORD sent thunder and hail, and the fire ran along upon the ground; and the LORD rained hail upon the land of Egypt."