The fifth plague was an epidemic that struck the Egyptians’ livestock. The sixth plague was a skin inflammation that erupted into blisters on the skin of the Egyptians and their cattle. The seventh plague was a rain of hail formed miraculously of ice together with fire.
Combining Mercy and Severity
וַיְהִי בָרָד וְאֵשׁ מִתְלַקַּחַת בְּתוֹךְ הַבָּרָד כָּבֵד מְאֹד וגו': (שמות ט:כד)
The hail was very heavy, with flashing lightning in the midst of the hail. Exodus 9:24

Water and fire derive from and express the Divine attributes of mercy and severity, respectively. Thus, the uniqueness of the plague of hail was its blend of ice and fire, Divine mercy and severity. Similarly, although this was a particularly severe plague, as indicated by the harsh warning preceding it, this very warning included merciful instructions how to avert it.

Only G‑d can override nature and combine fire and ice. In the same way, it is only by rising above our natural limitations and connecting ourselves to G‑d that we can be both strict and merciful at the same time – both for our own benefit and for the benefit of others.1