"As the rain": Just as rain is life for the world, so, words of Torah. —
But then (why not say:) Just as with rain, part of the world is happy (with it) and part, sad (e.g., One whose pit and vat is full of wine, and his vat and threshing floor is exposed to the rain, is grieved by it) so, words of Torah! It is, therefore, written "My word shall flow as the dew": Just as with dew, all the world is happy with it, so, words of Torah.
"as winds upon the herbage": Just as winds raise the grass and make it flourish, so, words of Torah raise their disciples and make them flourish, viz. (Proverbs 4:8) "Caress it (Torah) and it will uplift you."
"Like showers upon the grass": Just as showers descend upon the grass and beautify and preen it, so, words of Torah beautify and preen their disciples. And thus is it written (Proverbs 1:9): "For they (words of Torah) are a chaplet of grace to your head," and (Ibid. 4:9) "It (Torah) will set a chaplet of grace upon your head."