Stretch forth thine hand toward heaven. The action was appropriate, as the plague was to come from the heaven. Similarly, in the first and second plagues, Aaron's hand had been stretched out upon the waters (Exodus 7:19, Exodus 7:20; Exodus 8:6); and in the third upon "the dust of the ground" (Exodus 8:17).
And upon every herb of the field—i.e; upon all forms of vegetable life. (Compare Genesis 1:30; Genesis 9:3.