When he made a decree for the rain - A statute or law (חק chôq) by which the rain is regulated. It is not sent by chance or hap-hazard. It is under the operation of regular and settled laws. We cannot suppose that those laws were understood in the time of Job, but the fact might be understood that the rain was regulated by laws, and that fact would show that God was qualified to impart wisdom. His kingdom was a kingdom of settled law and not of chance or caprice, and if the rain was regulated by statute, it was fair to presume that he did not deal with his people by chance, and that afflictions were not sent without rule; compare the notes at Job 5:6.
And a way - A path through which the rapid lightning should pass - referring, perhaps, to the apparent "opening" in the clouds in which the lightning seems to move along.
The lightning of the thunder - The word "lightning" here (חזיז chăzı̂yz) properly means "an arrow," from הזז hāzaz, obsolete, to pierce through, to transfix, to performate; and hence, the lightning - from the rapidity with which it passes - like an arrow. The word "thunder" (קולות qôlôt) means voices, and hence, "thunder," as being by way of eminence the voice of God; compare Psalm 29:3-5. The whole expression here means "the thunder-flash." Coverdale renders this, "when he gave the mighty floods a law;" but it undoubtedly refers to the thunderstorm, and the idea is, that he who controls the rapid lightning, regulating its laws and directing its path through the heavens, is qualified to communicate truth to people, and can explain the great principles on which his government is administered.

When he made a decree for the rain - When he determined how that should be generated, viz., By the heat of the sun evaporation is produced: the particles of vapor being lighter than the air on the surface, ascend into the atmosphere, till they come to a region where the air is of their own density; there they are formed into thin clouds, and become suspended. When, by the sudden passages of lightning, or by winds strongly agitating these clouds, the particles are driven together and condensed so as to be weightier than the air in which they float, then they fall down in the form of rain; the drops being greater or less according to the force or momentum, or suddenness, of the agitation by which they are driven together as well as to the degree of rarity in the lower regions of the atmosphere through which they fall.
A way for the lightning of the thunder - ודרך לחזיז קולות vederech lachaziz koloth. קול kol signifies voice of any kind; and koloth is the plural and is taken for the frequent claps or rattlings of thunder. חז chaz signifies to notch, indentate, or serrate, as in the edges of the leaves of trees; חזיז chaziz must refer to the zigzag form which lightning assumes in passing from one cloud into another. We are informed that "this is a frequent occurrence in hot countries." Undoubtedly it is; for it is frequent in cold countries also. I have seen this phenomenon in England in the most distinct manner for hours together, with a few seconds of interval between each flash. Nothing can better express this appearance than the original word.

When he made a decree for the rain, and a way for the lightning of the thunder. Decreed within himself that he would give it; for rain is his gift alone, and which none of the vanities of the Gentiles can give, and a wonderful blessing to the earth it is; and which God bestows on all sorts of men, both good and bad, and causes it to fall sometimes on one place and sometimes on another, sometimes in greater, sometimes in lesser showers; and according to his sovereign pleasure he gives or withholds it; the effects of which are quickly seen. Mr. Broughton renders the clause, "he made a bound for the rain, and a way for the lightning of thunder", or "the lightning and the thunder", as Ben Gersom, who thinks the copulative "and", is wanting. Thunder is from God, it is his voice, and the word here used is in the plural number, "voices" (m), signifying various claps of thunder; and lightning generally accompanies it, which, though first perceived, they are both at once the eye doing its office quicker than the ear; and a cloud also is usual; and so some render the word for lightning, as in Zac 10:1; it may signify the way of the lightning out of the thunder cloud, and attending claps of thunder; the thunder breaks the cloud and makes a path for the lightning: the Targum is,
"a path for the lightnings, which run with the voices or thunders;''
but, though the course or path the lightning steers is very quick and very extensive from east to west, and cannot be traced by us. God that made it knows it, and he knows the path and place of wisdom. Sephorno interprets this of the thunder and lightnings at the giving of the law, which he understands by wisdom, as do other Jewish writers: Pliny (n) speaks of thunder and lightning as chance matters; but Seneca (o) more truly ascribes them to divine power and Providence, as here.
(m) "vocum", Piscator, Mercerus, Drusius. (n) Nat. Hist. l. 2. c. 43. (o) Nat. Quaest. l. 2. c. 13. 31.

The decree regulating at what time and place, and in what quantity, the rain should fall.
a way--through the parted clouds (Job 38:25; Zac 10:1).

When - At the first creation, when he settled that course and order which should he continued. A decree - An appointment and as it were a statute law, that it should fall upon the earth, in such times, and places, and proportions.