In the plague of hail, there was a melding of two contradictory forces, water and fire, in order to punish the Egyptians.

To what can the plague of hail be compared? It can be compared to two forceful legions that were waging war against each other. Eventually, the time for the king’s war arrived [i.e., the king waged a war], and the king made peace between the two legions and they executed the king’s mission together. So too, fire and hail are antagonistic to each other. Once the time for war against Egypt arrived, the Holy One, blessed be He, made peace between fire and hail and they struck Egypt. This is the meaning of: “There was hail and fire igniting amid the hail” (Exodus 9:24). What is the meaning of the term “igniting”?… After an Egyptian was struck by the hail, the fire would burn him. (Shemot Rabba 12)