Normally when it rains after a homa, yagna or any religious activity, it is considered auspicious ("devatas are pleased with the ritual; hence its raining"). However, while performing Kumbabhishekam (consecration) of temple, if the rain water falls directly on the vigraha (idol), it is customary that a a fresh kumbabhisheka need to be performed.
Above are based on customs and may vary from place to place. Related to above, I have two questions: a) Why rain is considered auspicious but rain water is not in some cases?
b) What is purity of rain water for direct use in rituals (direct use = capture rain water and use it directly in the rituals; rain water hasn't joined river, lake, etc.)