From page 2 of the article:

"Clouds indicate that the protocol or system component is working hard, while raindrops indicate that activity is really ramping up. A lightning bolt signifies that the protocol or component is running near capacity."

Talk about dumbing things down...

Looking at the interface the icons represent different load thresholds which can be customised, e.g. 100K ops per second on the http component gives a hurricane icon. So I guess it's a slightly whimsical at a glance status.