John Calvin Commentary 2 Peter 2:17

"These are springs without water, and mists driven by a storm; for whom the blackness of darkness hath been reserved." — 2 Peter 2:17 (ASV)

These are wells, or fountains, without water. He shows by these two metaphors that they had nothing within, though they made a great display. A fountain, by its appearance, draws people to itself because it promises them water to drink and for other purposes; as soon as clouds appear, they give hope of immediate rain to irrigate the earth. He then says that they were like fountains, because they excelled in boasting and displayed some acuteness in their thoughts and elegance in their words; but yet they were dry and barren within. Hence, the appearance of a fountain was fallacious.

He says that they were clouds carried by the wind, either without rain or that burst forth into a calamitous storm. He thereby indicates that they brought nothing useful and that they were often very hurtful. He afterwards pronounces upon them the dreadful judgment of God, so that fear might restrain the faithful. By naming the mist or the blackness of darkness, he alludes to the clouds that obscure the air. It is as though he had said that for the momentary darkness they now spread, a much thicker darkness is prepared for them, which is to continue forever.