More often than less, it thunders when its raining anyway. You say that your spell works only if there's a drizzle or rain. So, my guess is that the thundering is happening on its own. It's not your spell that's making it happen.
I've always had mixed feelings about the outcome of weather spells. While a part of me believes that you can manipulate it, logic tells me you cant. But let us assume that we can affect the weather. In such a case, is it wise to mess with nature's agenda, just because you can? Have you thought about the consequences before you pick a weather spell just for fun?