What are "Aliyosav"?
Radak: They are the clouds.
Malbim: They are the vapors that rise.
Does rain water only mountains?!
Radak: It mentions mountains, because they need rain more than valleys and plains. The latter are moister than mountains. Also, one can irrigate valleys and plains from river water; he cannot do so for mountains.
Why does it mention rain now?
Radak: After mentioning the gathering of water, which was on day three, and the benefit of water on the dry land, it mentions the vegetation, which was also created on day three 1 - "Tadshei ha'Aretz Deshe Esev" (Bereishis 1:11). Before the earth sprouted vegetation, "v'Ed Ya'aleh Min ha'Aretz v'Hishkah Es Kol Pnei ha'Adamah", and then "va'Yatzmach Hashem Elokim Min ha'Adamah" (ibid., 2:6, 9).
Malbim (14): Earlier creations are preparations for later creations.
What does "mi'Peri Ma'asecha Tisba ha'Aretz" teach?
Radak: This repeats the previous matter in different words. The 'Pri' of the clouds is rain. Rain is an action of Elokim's Chachmah and a great wonder. Elihu mentioned it among the Creator's wonders "ba'Asoso la'Matar Chok" (Iyov 28:26), and Hashem told Iyov "ha'Yesh la'Matar Av Oh Mi Holid Eglei Tal" (ibid., 38:28). Elokim's deed (clouds) is the father, and rain is the fruit.
Malbim: The land is satiated from the rain, and from the rivers that result from it.
