Certain spells are a bit hard to do. I try to stray from weather spells. They seem easy, but once you think about it, the weather is an earthly thing. The weather is the way it is for a reason. If it rains, it rains. I have found that there is a very small chance that a small thing like me can change something so vast such as the weather. I leave mother nature to that sort of thing.
But don't be discouraged. There are many other spells out there. As well, have you read up on how to properly perform a spell? Most seem simple, but having full concentration and a relaxed mind also helps.
^^this link here is a great place to start.
Again, Don't fret, just keep trying!~