The reference to waterless clouds was probably a common metaphor at the time, an image which is also found in Proverbs 25:11. For people living in a climate where rain was precious and always welcome, the implication of this phrase is that these heretics bring nothing of value. They make promises that sound good but they cannot fulfill them. When Jude goes on to talk of them being swept along by the winds, he might also have their false doctrine in mind (see Ephesians 4:14). Their faith is not rooted in Scripture but in the ideas of their day and age.1
12 These are hidden reefs at your love feasts, as they feast with you without fear, shepherds feeding themselves; waterless clouds, swept along by winds; fruitless trees in late autumn, twice dead, uprooted;