The answer is linked to Riddick's vision and the telescopic goggles he uses to magnify the distant ground. Living things seem to give off an aura (we are not ever made clear if it is heat, or some other radiation signature) he is sensitive to. After seeing the rain and the proliferations of auras, he decided it was time to leave the planet.
Explicação:
Living on the planet for a year or so, gave Riddick ample time to learn the lay of the land, but living on the side of the mountains he did, there were few incursions of the hibernating creatures where he lived. The creatures were always active where he was because they lived in the pools from the runoff and as far as he could tell, there was no rain there. Once he crossed the mountains, he discovers much to his dismay, large and very stable storms formed and would bring out of the ground thousands of these hibernating beasts.