Commentaries:
This is an example of the connection between idolatry and rain, saying that idols are powerless to control the heavens, yet it is an easy thing for God (see I Kings 18:20-46). The signs that we see (rain being given in due season or no rain being sent when it is necessary) can indeed be blessings or warnings from God. Only God is able to manipulate these things to these ends. We should not take these things lightly. Jeremiah's words here are very pointed.
Richard T. Ritenbaugh
The Two Witnesses (Part Six)