What time - In the time; or after a time.
They wax warm - Gesenius renders this word (יזרבו yezorebû) when they became narrow, and this version has been adopted by Noyes. The word occurs nowhere else. Taylor (Concord.) renders it, "to be dissolved by the heat of the sun." Jerome, fuerint dissipati - "in the time in which they are scattered." The Septuagint, τακεῖσα Θέρμης γενομένης takeisa thermēs genomenēs - "melting at the approach of heat." The Chaldee, "In the time in which the generation of the deluge sinned, they were scattered." Castell says that the word זרב zârab in the Piel, as the word in Chaldee (זרב zerab) means "to flow"; and also that it has the same signification as צרב tsârab, to become warm. In Syriac the word means to be straitened, bound, confined. On the whole, however, the connection seems to require us to understand it as it is rendered in our common translation, as meaning, that when they are exposed to the rays of a burning sun, they evaporate. They pour down from the mountains in torrents, but when they flow into burning sands, or become exposed to the intense action of the sun, they are dried up, and disappear.
They vanish - Margin, "are cut off." That is, they wander off into the sands of the desert until they are finally lost.
When it is hot - Margin, "in the heat thereof." When the summer comes, or when the rays of the sun are poured down upon them.
They are consumed - Margin, "extinguished." They are dried up, and furnish no water for the caravan.

What time they wax warm they vanish,.... The ice and the snow, which, when the weather becomes warm, they melt away and disappear; and in like manner, he suggests his friends ceased to be friends to him in a time of adversity; the sun of affliction having looked upon him, they deserted him, at least did not administer comfort to him:
when it is hot they are consumed out of their place; when it is hot weather, and the sun has great strength then the waters, which swelled through the floods and fall of rain and snow, and which when frozen, looked black and big as if they had great depth in them, were quickly dried up, and no more to be seen in the place where they were; which still expresses the short duration of friendship among men, which Job had a sorrowful experience of.

wax warm--rather, "At the time when." ("But they soon wax") [UMBREIT]. "they become narrower (flow in a narrower bed), they are silent (cease to flow noisily); in the heat (of the sun) they are consumed or vanish out of their place. First the stream flows more narrowly--then it becomes silent and still; at length every trace of water disappears by evaporation under the hot sun" [UMBREIT].

Warm - When the weather grows milder. Hot - In the hot season, when waters are most refreshing and necessary.