Is it natural for hailstones to fall on people and animals, and kill them?
Malbim: Normally, it is natural 1 . However, here they fell only on the enemy, and not on Yisrael; this was a miracle.
Even if such giant hailstones are sometimes found, it is astounding that they be in Eretz Yisrael, which nowadays normally gets hail or snow only about twice a year, in winter. The verses imply that there was not so long from crossing the Yarden in Nisan until now, i.e. it was still summer! (PF)