Radak: Do any follies (idolatries) of the Goyim give rain?! We will scream for rain only to You!

Malbim: We know that rain does not come via angelic powers via serving idols like foolish Nochrim believe, and not via nature itself. Rather, it is from Your Hashgachah!

Rashi: Does Shamayim make fine rain by itself, not according to Your mouth?! When there is a double astonishment, the first begins with Hei ¹ , and the second with 'Im', e.g. "hab'Machanim Im b'Mivtzarim", "ha'Tov Hi Im Ra" (Bamidbar 13:19), "ha'Ma'os Ma'asta... Im b'Tziyon Ga'alah Nafshecha" (verse 19). Also here, "ha'Yesh b'Havlei ha'Goyim... v'Im ha'Shamayim."

Radak: If Shamayim gives rain - this cannot be, for when You want, You stop it from giving rain. This shows that giving rain is not in its jurisdiction. You open [Shamayim] when You want to. Therefore, we hope to you.

Radak: You withheld rain, and withheld the land from giving its yield. Also Zecharyah (8:12) said "Ki Zera ha'Shalom ha'Gafen Titen Piryah veha'Aretz Titen Es Yevulah veha'Shamayim Yitenu Tal'am... Es Kol Eleh."