''There was hail and fire flaming amid the hail'' [Va'eira 9:24]
The Midrash states that the Plague of Hail was a miracle within a miracle: the hail did not extinguish the fire and the fire did not melt the hail. Rather, both elements joined forces in smiting the Egyptians.
R' Acha compared this to a king who had two very powerful legions of soldiers. To find greater favor in the king's eyes, each legion tried out-doing the other when they went out to war. The competition between the two legions escalated to the point that they hated one another.
This hatred continued for some time until a major war threatened to break out in the king's land. The king summoned both legions to appear before him, and he told them the following:
''I know'' said the king, ''that you are both very powerful and dedicated to my service. I usually send only one of you at a time to the battlefront. But now a major war looms on the horizon, and I need assistance from both of you. But what shall I do about your mutual hatred? You must make peace between yourselves and go out to war united. Then we will be victorious!''
So it was with the Plague of Hail. Hail and fire cannot co-exist because the nature of fire is to melt hail and the nature of hail is to extinguish fire. But in this instance, Hashem made peace between them and together they struck at the Egyptians.
Source: Rabbi Yisrael Bronstein