One of my guide books to clouds lists 53 different cloud classifications. On each page, a careful paragraph describes the subtle distinctions between types, explaining how to identify the exact moment when one cloud evolves into another. At some point the entire body of water droplets crosses an imaginary line to be called something else. To say that clouds are ephemeral is simply a temporally broader observation.
If the whole idea of classifying transient bodies seems nearly pointless to you, you're not the first to think so. The sorites paradox sums it up nicely. Darwin recognized this problem with species concepts, of which there are now at least a couple dozen floating around. Systematics is simply a recognition that there is some salient cluster of characteristics that allows us to make a little more sense of what we see, from clouds to barnacles.
"I look at the term species", Darwin wrote, "as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other." Which is just about all we can agree upon. But at least we agree on that.