This is a good article about the Hebrew concept of the firmament:
Of course rain clouds have water, but where do the water in the clouds come from? The ancient West Semitic belief was that water pours through windows in heaven to the clouds (just as snow and hail come from heavenly storehouses). From ancient Cananite literature:
KTU 1.4 vii 25-39: "Let a window ('urbt) be opened (yptch) in the house, a casement in the midst of the palace! Baal opened a rift in the clouds; his holy voice (i.e. thunder) Baal gave forth; Baal repeated the issue of his lips".
The same words used in the Baal Cycle to refer to the "opening" of a "window" in Baal's palace occur in the OT (cf. Hebrew 'rbwt "window" and ptch "open") to refer to Yahweh opening up the windows of heaven to bring the waters to the earth (Genesis 7:11, 8:2, 2 Kings 7:2, 19, Isaiah 24:18, Malachi 3:10). The mabbul "flood" that Yahweh is enthroned upon in Psalm 29 is also the same word that occurs in Genesis 7-8 to refer to the floodwaters cast down to the earth from heaven: "Yahweh sits enthroned over the flood, Yahweh is enthroned as king forever" (Psalm 29:10). Yahweh's heavenly palace or temple is described as built on top of the firmament (a solid structure, cf. heaven being stretched out like a metal sheet in Job 37:18, and cf. the references to the firmament or the tent of heaven containing the sun, moon, and stars in Genesis 1:14 and Psalm 19:1-6) on top of the waters of heaven"
Psalm 104:2-3: "He wraps himself in light as with a garment; he stretches out the heavens like a tent and lays the beams of his upper chambers (h-mqrh `lywtyw) on their waters (b-mym). He makes the clouds his chariot and rides on the wings of the wind".Ezekiel 1:22, 26: "Spread out above the heads of the living creatures was what looked like a firmament (rqy`), sparkling like ice (k-`yn h-qrch) and awesome... Above the firmament (mm`l l-rqy`) over their heads was what looked like a throne of sapphire, and high above on the throne was a figure like that of a man".
4Q403 1:41-44: "Sing to God, awesome in power, all you wise spirits of light, to exalt together the utterly brilliant firmament of heaven (rqy` zw twhr thwrym) that girds his holy temple. Praise him, divine spirits, praising forever and ever the firmament of the uttermost heaven (rqy` rwsh mrwmym), all its beams (qyrwtw) and walls, the work of its construction".