Malbim: Until now, he saw Olam ha'Mal'achim and Olam ha'Ofanim and the influence and connection between the angels that move the Ofanim and the Ofanim. Now he saw how the Chayos receive influence from above them - Olam ha'Kisei, which stands on Olam ha'Chayos. He depicts a Raki'a on the heads of the Chayos. The Raki'a is the division between above and below. Just like our world has a Raki'a over it, which is where vapors ascend - until there pertains to the land. From the Raki'a and above pertains to Shamayim. No living being can survive there ¹ - "va'Yikra Elokim la'Raki'a Shamayim" (Bereishis 1:8). Every world has Reki'os between it and the worlds above and below. Rain and influence come from the world above, via the Raki'a. It says "u'Demus? Raki'a" because it is not in actuality, only his mind perceived it. It is stretched over their heads; the Chayos' heads receive influence via the Raki'a, and influence those below them.

Rashi citing Targum Yonasan: It is hard ice.

Radak: [The ice] is Nora (awesome) in its whiteness and brightness.

Malbim: In the lowest Raki'a above the land, vapors gather - according to physicists, from below, and according to Chazal, from above. They form clouds; sometimes they freeze and harden, and form ice. So it depicts that the Divine influence that descends from Olam ha'Kisei, which is very spiritual, will thicken and congeal and become like ice, to the point that the Chayos can receive it. It is elevated above the physical ice that we know. Their fear, which they get from dew of light influenced from above, is depicted as ice. Fear freezes blood and natural heat, and casts frost like crumbs. This is a Mashal to help us understand the freezing of influence when it descends, compared to its fineness when it was in its place.