How could Moshe "raise his hand upon the heavens"?
Rashi #1: The words "Al ha'Shamayim" mean (not 'upon the heavens' but) "towards the heavens."
Rashi #2 (citing the Midrash): Hashem raised Moshe above the heavens. 1
Sifsei Chachamim: Thereby empowering him to bring the next three Makos, which took place in the atmosphere. Refer to 7:14:8:2, 7:26:1:2**, and 9:14:1:2.
Why did Moshe need to stretch his hands towards the heaven?
R. Bachye: Because, to bring down the hail from the heaven, required Tefilah with the ten fingers ? corresponding to the ten Sefiros ? outstretched, as a result of which people died de the terrifying noise of the thunder-claps and Makas Barad, which was perfomed with the agreement of the Beis-Din shel Ma'alah. 1
And that explains why, to remove Makas Barad, Moshe had to stretch his hands heavenwards (Pasuk 29) to harness Dvine mercy. See R. Bachye.
