(a) The MESHECH CHOCHMAH (Parshas Re'eh, 16:5) addresses this question. He
cites a Yerushalmi (Avodah Zarah 3:1) which says that while those who
worship Avodah Zarah believe that their idols have power over the world,
they admit that their idols have no power over the seas. The Meshech
Chochmah says that in the same vein, they admit that their idols have no
power over water in general. That is why we find (Sanhedrin 67b) that water
nullifies the powers of Keshafim (sorcery). [Similarly, the Mishnah in Kelim
(17:13) says that anything which comes from the sea cannot become Tamei.]
The same applies to rain; as the verse in Yirmeyah (14:22) implies, no
idolater believes that an idol can bring rain. For this reason, the Mishnah
refers to rains as "Gevuros Geshamim," for rain is something that shows the
unique and unparalleled power of Hashem which everyone acknowledges and
which no one attempts to attribute to any other perceived power.
We find that all of the shipmates of Yonah prayed to their various gods to
stop the storm at sea. According to the assertion of the Meshech Chochmah,
why did they do that if they know that their gods have no power over the
sea? Also, the Gemara (Avodah Zarah 55a) talks about a certain Avodah Zarah
that visited people in their dreams and told them that if they offer it a
sacrifice it will bring rain to them. (They offered a sacrifice and it
indeed rained.) We see, then, that the idolaters *do believe* that their
idols have control over the sea and rain!
It must be that they believe that sometimes other powers can persuade the
true G-d to manipulate the rains or the seas, but they know that their gods
do not have any permanent or independent power over water. The keys to the
seas are in the hand of the G-d of G-ds.
The Meshech Chocmah's contention might explain why the Torah, in the Parshah
of "v'Hayah Im Shamo'a" that we recite each day in Keri'as Shema, states
that when the Jewish people follow the will of Hashem, then the rains will
come in ample supply and at appropriate times (Devarim 11:14), and
conversely, when the Jewish people do not follow the will of Hashem, He will
withhold the rains (ibid. 11:17). Since the rain is a clear indication of
the power of Hashem, it is a fitting way of demonstrating the reward (or
punishment) for following (or defying) the will of Hashem.
The Gemara says that the keys to rain, childbirth, and Techiyas ha'Mesim are
in the hands of no one but Hashem. The RA'AVAN writes that like rain,
childbirth and Techiyas ha'Mesim demonstrate the power of Hashem. For this
reason, they are also mentioned in the blessing of Hashem's Gevurah in the
Shemoneh Esreh ("Atah Gibor..."). Following the Meshech Chochmah's approach,
we may suggest that these are also things that no idolater would say that
his idol can do -- no one else makes claim to the power of childbirth or
Techiyas ha'Mesim, and therefore these things are also referred to as
Gevuros of Hashem.
(b) The VILNA GAON (Aderes Eliyahu, end of the first verse in v'Zos
ha'Berachah) suggests a somewhat different approach. He explains that "the
four keys" refer to acts that defy nature. According to the laws of physics,
it is absolutely impossible for the dead to be resurrected. Similarly, the
fall of rain (and weather in general) follows no natural physical,
predictable law. It is not like sunrise and sunset, or the motions of the
planets which can be predicted thousands of years in advance. In this sense,
rainfall is "supernatural," and demonstrates the presence of the Creator
more than other events. This is why it is referred as "Gevuros Geshamim,"
and why it constitutes a demonstration of Hashem's might.
The BEN YEHOYADA here (DH Sheloshah) follows a similar approach, adding that
the "Shali'ach" to whom all keys but these three are given is none other
than the forces of nature.
(c) In BENAYAHU (beginning of Daf 7a) the author of the Ben Yehoyada
suggests another approach. As the Gemara (ibid.) says, rain is greater than
the resurrection of the dead since it is beneficial not only to the
righteous, but even to the wicked.
The Mishnah tells us, "Who is mighty ("Gibor")? One who conquers his will."
The only acts that can be described as "hard," as it were, for the Creator,
are acts in which he conquers His will and shows generosity even when it is
not deserved. This is what the Gemara means in Pesachim (118a) when it
asserts that providing sustenance for every person is as hard before Hashem
as the Splitting of the Red Sea. Hashem split the sea even for the sinners
of Israel: "The idol of Michah passed through the sea!" Similarly, Hashem
provides sustenance -- through rain -- for all beings, be they deserving of
it or not. In this sense, it is fitting to refer to rain as the "might of
Hashem."
(d) The Torah tells us in the verses of v'Hayah Im Shamo'a that in Eretz
Yisrael we should not feel free to act as we do out of Israel. Israel is
lacking in natural self-replenishing water sources, unlike Egypt, and if we
sin Hashem will immediately make us aware of the fact by taking away our
water sources (as happened in the times of Eliyahu and Achav).
In this manner, rain, or the lack of it, causes us to always lift our eyes
towards heaven in prayer, and to trust in and pray to Hashem. It also
reminds us constantly of Hashem's presence, by letting us witness Divine
retribution in a direct cause and effect manner. Since it reminds us of
Hashem's presence and His omnipotence, it is appropriate to refer to it as
the "might of Hashem." (M. Kornfeld)
(Although at first glance these four approaches may appear to differ from
each other considerably, upon further analysis it should become evident that
they are complementary. They reflect four perspectives of the same
approach.)