Well, most people are going to tell you that you cannot make it rain. It is impossible to mess with the natural order like that. Others will say it is possible, but unethical. You are hurting the planet by messing with the natural oder. There are a very few who claim they can manipulate the weather and they have yet to show evidence of these claims.
The best advice I can give is rather than trying to force something that won't work, learn to work with what you have. If you are needing the rain for a spell, try adjusting the spell to use river water or something more readily available.