Exodus 9:19
Send therefore now, and gather thy cattle, and all that thou hast in the field; for upon every man and beast which shall be found in the field, and shall not be brought home, the hail shall come down upon them, and they shall die. Send therefore now, and gather thy cattle, and all that thou hast in the field; for upon every man and beast which shall be found in the field, and shall not be brought home, the hail shall come down upon them, and they shall die.
Before the hail fell, advance command was given to gather cattle, servants, and everything in the field into houses and barns. Every man and beast left in the field when the hail came down would die, be killed and destroyed. By providing this warning and a way of escape for all who would choose to act upon it, God showed mercy in the midst of judgment.
The warning gave opportunity to act, so that safety depended on heeding God's instruction and those who heeded could avoid the disaster. Those who feared, regarded, and believed the word brought their servants and cattle inside into houses, while those whose hearts were hardened like Pharaoh's, who disregarded, despised, and disbelieved the warning, left them exposed in the field. The warning tested the exalted pride of the Egyptians and revealed how many hearts were really affected by God's wonderful dealings for Israel, whom the Egyptians had despised and cruelly treated.
The warning tested whether people took the word seriously and showed who had learned to fear God. The hail plague to which the verse belongs came with fire, lightning, and thunder. The time of the plague was announced beforehand so it could not be dismissed as chance.
The hail showed that the whole earth, including thunder, hail, and storm, was under the command of the God of the Hebrews. In response Pharaoh humbled himself, confessed that he had sinned and that the Lord was righteous, and asked Moses to intercede so the storm would cease. The hail illustrates the disasters that will come upon those who disregard God's law.