By the breath of God frost is given - Not by the violent north wind, or by the whirlwind of the south, but God seems to "breathe" in a gentle manner, and the earth is covered with hoary frost. It appears in a still night, when there is no storm or tempest, and descends upon the earth as silently as if it were produced by mere breathing. Frost is congealed or frozen dew. On the formation and cause of dew, see the notes at Job 38:28. The figure is poetical and beautiful. The slight motion of the air, even when the frost appears, seems to be caused by the breathing of God.
And the breadth of the waters is straitened - That is, is contracted by the cold; or is frozen over. The waters are "compressed" into a solid mass (במוצק bemûtsaq), or are in a state of "pressure" or "compression" - or so the word used here means. What were before expanded rivers or arms of the sea, are now compressed into solid masses of ice. This, also, is proof of the greatness and power of God, for though the cause was not understood by Elihu, yet there was no doubt that it was produced by his agency. Though the laws by which this occurs are now better understood than they were then, it is no less clearly seen that it is by his agency; and all the light which we obtain in regard to the laws by which these things occur, only serve to exalt our conceptions of the wisdom and greatness of God.

By the breath of God frost is given - The freezing of water, though it is generally allowed to be the effect of cold, and has been carefully examined by the most eminent philosophers, is still involved in much mystery; and is a very proper subject to be produced among the great things which God doeth, and which we cannot comprehend, Job 37:5. Water, when frozen, becomes solid, and increases considerably in bulk. The expansive power in freezing is so great, that, if water be confined in a gun-barrel, it will split the solid metal throughout its whole length. Bombshells have been filled with water, and plugged tight, and exposed to cold air, when they have been rent, though the shell has been nearly two inches thick! Attempts have been made to account for this; but they have not, as yet, been generally successful. The breath of God freezes the waters; and that breath thaws them. It is the work of Omnipotence, and there, for the present, we must leave it.
The breadth of the waters is straitened - This has been variously translated; מוצק mutsak, which we here render straitened, we translate Job 37:18 melted. Mr. Good thinks that the idea of a mirror is implied, or something molten; and on this ground it may be descriptive of the state of water formed into ice. He therefore translates: -
By the blast of God the frost congealeth,
And the expanse of the waters into a mirror.
I have only to observe, that in the act of freezing wind or air is necessary; for it has been observed that water which lay low in ponds did not freeze till some slight current of air fell on and ruffled the surface, when it instantly shot into ice.

By the breath of God frost is given: and the breadth of the waters (g) is straitened.
(g) That is, frozen up and dried.

By the breath of God frost is given,.... By the word of God, as the Targum; at his command it is, at his word it comes, and at his word it goes, Psalm 147:15; or by his will, as Ben Gersom interprets it, when it is his pleasure it should be, it appears; it may be understood of a freezing wind from the Lord, for a wind is sometimes expressed by the breath of his nostrils, Psalm 18:15; and as the word "God" added to things increases the signification of them, as mountains of God are strong mountains; so the breath of God may signify a strong wind, as Sephorno notes, the north wind (q);
and the breadth of the waters is straitened; by the frost they are reduced and brought into a narrower compass; or made hard, as Mr. Broughton renders it; so hard as to walk upon, to draw carriages on, and lay weights and burdens very great upon; or become compact or bound together, like metal melted, poured out, and consolidated; though some think it refers to the thawing of ice by the south winds (r), when the waters return to their former breadth; which is done by the breath or commandment of God, as appears from the place before quoted from the psalmist, Psalm 18:15; for it may be rendered, "and the breadth of the waters is pouring out", so the Targum, when thawed; or through the pouring down of rain, so the Syriac and Arabic versions, "he sends forth plenty of water".
(q) "Induroque nives", &c. Ovid. (r) "----cum vere reverso Bistoniae tepuere nives", &c. Statii Theb. l. 2.

the breath of God--poetically, for the ice-producing north wind.
frost--rather, "ice."
straitened--physically accurate; frost compresses or contracts the expanded liquid into a congealed mass (Job 38:29-30; Psalm 147:17-18).

The waters - The waters which had freely spread themselves before, are congealed and bound up in crystal fetters.