I think you can use Fireball in a drizzle but not heavy rain. If you attempt Fireball in a heavy rain, your bat guano would turn into muddy soup and the spell will not be completed. However, if a creative party member wants to hold a make-shift umbrella over your bat guano while the orcish archers are firing their arrows at the two of you… Likewise, any good soaking, such as falling out of a boat or swimming across an underground lake, would render your bat guano temporarily or even permanently unusable. Of course, the material components of Fireball are bat guano or sulfur. Wet sulfur might not be much of a problem. Unless your player specifically asks for sulfur, always assume the character has opted for bat guano. Players casting at 3rd level should know better and depriving their characters of bat guano will make everyone miss out on all the fun. (see below)