As title says.
The requirement of a tempest/rain for being able to cast this spell, I feel, is really unneccessary and unfun.
I don't think its a spell that much better to say 'Fireball'. But if you think it is, how about you make a compromise and make it work all the time, maybe at reduced damage, and at greater damage durings storms.
I don't think I've ever seen someone cast this spell.
The last time I have seen someone cast this spell he hit me for 70 damage in one cast. I believe it does 1d10/level when the rain condition is met or you are underground. Seems really good.
But a cool suggestion might be a way for Druids to create rain.