What did Hashem mean when He said that, this time, He would send all His plagues against the hearts of Pharaoh ... ?
Rashi: He was referring to Makas Bechoros, 1 which was equal to all the plagues.
Seforno: The third group of Makos (that struck the air, refer to 7:26:1:2**) that was about to begin, would strike him in the heart (they would hit harder than the first two groups) inasmuch as the Egyptians suffered the consequences long after the termination of the plagues - with regard to the devastation of all plants and food, and the physical illnesses in which they resulted. 2
Rashbam: He said it with reference to the plague of hail, which incorporated fire, hail, gigantic hailstones, sulfur, snow and a pillar of smoke.
Vilna Gaon: Hashem punishes evildoers via three legions - fire, wind and water. 3 And the Makah of Barad comprised all three 4 - water (the hail itself); fire (See Pasuk 24) and wind (which produced the thunder mentioned in Pasuk 23).
Some commentaries that Rashi is referring, not to the slaying of the firstborn (which was not about to take place yet), but to the Makah of hail ("Bechoros" is a Lashon of Bikurim, because it was confined to those crops that had already ripened (See Pasuk 31 [See Sifsei Chachamim, who elaborates]).
Seforno: And this was true even of the plague of darkness, which most certainly affected the atmosphere, leaving many sick people in its wake.
Hence He punished Sedom with fire, the Dor ha'Mabul with water, and the Dor Haflagah (the Tower of Bavel) He scattered with wind (Vilna Gaon).
Vilna Gaon: In any event,, Dam and Tzefarde'a were connected with water, Arbeh, with wind (See 10:13) and Shechin, with fire (See 9:10).
What is the significance of the phrase "Ba'avur Teida ki Ein Kamoni be'Chol ha'Aretz!"?
Ramban: Refer to 8:18:1:1**.
Seforno: When Pharaoh would see Hashem's mastery over the atmosphere that surrounds the earth, he would be forced to acknowledge His superiority. 1
Targum Yonasan: The plague of Barad left no room for doubt that the plagues were sent by Hashem and were not the work of any form of sorcery.
And as for His Mastery over the Heavens, that He reserved for K'ri'as Yam-Suf, when He would engage the Angel of G-d, the Pillar of Cloud and the Pillar of Fire (Seforno).