First of all, the PWS sweeps at 0° no matter what the radar tilt indication says. The indication appears only to remind the pilots, that the aircraft radar is emitting. Which it does as soon as PWS is switched on (even on ground).
PWS needs moisture of some kind to operade. Not sure though if it has to be droplets or if haze of some sort would suffice.
Through the return data of this moisture it can determine wind direcion and velocity for a short range of airspace in front of the aircraft. Sudden changes in either will result in a warning.
I've only ever had once since it's been installed - and this was a nuisance warning anyway (clearly recognizable as such).