With 60GHz it would take a storm with a rate of rain approaching 55mm (2") per hour to exceed a 20dB fade margin. That is quite a severe rain. A slight rain shower should not significantly impact a 1.5km link at 60GHz.
12"/hour is 0.2"/minute, so that for a couple of minutes doesn't make for ridiculous overall rainfall. Signal quality is affected by the current momentary rate, and having the signal drop for seconds or minutes is problematic.
In real world use if you want five nines availability of a 60 GHz link in Portland, OR, you need to keep the distance to about 700 meters or less. And it doesn't see really heavy rain showers (as Miami or Houston do), just constant drizzle. 300-400 meters max in Miami.
It's not so much the rain fade as the oxygen absorption that really gets you with 60Ghz[1]. You can get better distance and reliability with 70/80Ghz. They're expensive though.
