מדרש על ברכות 9:2

The clouds draw water from the depths, as it is said, "He causeth the vapours to ascend from the ends of the earth" (Ps. 135:7), and in every place where the King commands them, there they cause rain (to fall), and forthwith the earth becomes fruitful and yields produce like a widow who becomes pregnant through debauchery. But when the Holy One, blessed be He, desires to bless the produce of the earth, and to give provision to the creatures, He opens the good treasuries in heaven and sends rain upon the earth, namely, the fructifying rain, and forthwith the earth becomes fruitful like a bride who conceives from her first husband and produces offspring of blessing, as it is said, "The Lord shall open unto thee his good treasury the heaven" (Deut. 28:12).