There are some patterns that you experience and recognize, but that you fail to name or define. Then someone comes up with a picturesque name precisely capturing its essence. This is one of those:

[Evaporative Cooling] occurs when the most high value contributors to a community realize that the community is no longer serving their needs any more and so therefore, leave. When that happens, it drops the general quality of the community down such that the next most high value contributors now find the community underwhelming. Each layer of disappearances slowly reduces the average quality of the group until such a point that you reach the people who are so unskilled-and-unaware of it that they’re unable to tell that they’re part of a mediocre group.

(Via Another Way to View the “Decline” of HN)