Ex 9:13 Then the LORD said to Moses, "Rise early in the morning and stand before Pharaoh, and say to him, 'Thus says the LORD God of the Hebrews: "Let My people go, that they may serve Me,

14 "for at this time I will send all My plagues to your very heart, and on your servants and on your people, that you may know that there is none like Me in all the earth.

15 "Now if I had stretched out My hand and struck you and your people with pestilence, then you would have been cut off from the earth.

16 "But indeed for this purpose I have raised you up, that I may show My power in you, and that My name may be declared in all the earth.

17 "As yet you exalt yourself against My people in that you will not let them go.

18 "Behold, tomorrow about this time I will cause very heavy hail to rain down, such as has not been in Egypt since its founding until now.

19 "Therefore send now and gather your livestock and all that you have in the field, for the hail shall come down on every man and every animal which is found in the field and is not brought home; and they shall die."'"

20 He who feared the word of the LORD among the servants of Pharaoh made his servants and his livestock flee to the houses.

21 But he who did not regard the word of the LORD left his servants and his livestock in the field.

22 Then the LORD said to Moses, "Stretch out your hand toward heaven, that there may be hail in all the land of Egypt-on man, on beast, and on every herb of the field, throughout the land of Egypt."

23 And Moses stretched out his rod toward heaven; and the LORD sent thunder and hail, and fire darted to the ground. And the LORD rained hail on the land of Egypt.

24 So there was hail, and fire mingled with the hail, so very heavy that there was none like it in all the land of Egypt since it became a nation.

25 And the hail struck throughout the whole land of Egypt, all that was in the field, both man and beast; and the hail struck every herb of the field and broke every tree of the field.

26 Only in the land of Goshen, where the children of Israel were, there was no hail.

27 And Pharaoh sent and called for Moses and Aaron, and said to them, "I have sinned this time. The LORD is righteous, and my people and I are wicked.

28 "Entreat the LORD, that there may be no more mighty thundering and hail, for it is enough. I will let you go, and you shall stay no longer."

29 So Moses said to him, "As soon as I have gone out of the city, I will spread out my hands to the LORD; the thunder will cease, and there will be no more hail, that you may know that the earth is the LORD'S.

30 "But as for you and your servants, I know that you will not yet fear the LORD God."

31 Now the flax and the barley were struck, for the barley was in the head and the flax was in bud.

32 But the wheat and the spelt were not struck, for they are late crops.

33 So Moses went out of the city from Pharaoh and spread out his hands to the LORD; then the thunder and the hail ceased, and the rain was not poured on the earth.

34 And when Pharaoh saw that the rain, the hail, and the thunder had ceased, he sinned yet more; and he hardened his heart, he and his servants.

35 So the heart of Pharaoh was hard; neither would he let the children of Israel go, as the LORD had spoken by Moses.

Again, the Egyptians gods were discredited here. Nut, the sky goddess could not do anything. Osiris, the god of crop fertility was shown to be powerless. Set, the storm god, could not help the Egyptians. 

Pharaoh thought of himself as sovereign and yet he was put in his place when God through Moses told him: "I have raised you up."

The first six plagues were distressing but this plague is now deadly. Things are getting worse for Egypt. 

The purposes of God as to why He dealt with Pharaoh and Egypt this way are the following:

a) that you may know that there is none like Me in all the earth verse 14

b) that I may show My power in you, and that My name may be declared in all the earth verse 16

c) that you may know that the earth is the LORD'S. verse 29