Charles Ellicott Commentary Exodus 9:34

"And when Pharaoh saw that the rain and the hail and the thunders were ceased, he sinned yet more, and hardened his heart, he and his servants." — Exodus 9:34 (ASV)

Pharaoh ... sinned yet more, and hardened his heart. —As Pharaoh had never been so much moved before, so it now required a greater effort of his will to harden his heart than ever before; and thus he now sinned yet more than he had sinned up to that point. It seems strange that God's mercy should still have allowed him another chance (Exodus 10:3–6).