The evaporative cooling effect is how sweat works to cool the body. A hot object warms up moisture that surrounds it. That moisture absorbs the heat and evaporates. It moves the heat away from the original body, cooling it.

Similar is true of online communities.

“The Evaporative Cooling Effect describes the phenomenon that high-value contributors leave a community because they cannot gain something from it, which leads to the decrease of the quality of the community.” - Cornell blog post (link below)

Evaporative cooling explains how once-hot communities die and why online communities end up sucking.

Inspiration: Evaporative Cooling Effect in Social Networks, Cornell University