When we think of Westerns, we usually think of a movie shot out in the desert, with baking heat on all sides, sand beneath the feet of the cast, and a few wooden buildings making up the town everybody lives in. "Tombstone" decided to break with that tradition and give us something a little darker, including rainstorms. They probably wish they hadn't bothered.
Making it rain in a movie isn't the easiest thing in the world if the weather doesn't help you out. You need a rigged-up sprinkler system to keep the water flowing. You also need to make sure it covers the whole scene you're recording, which is where the producers of "Tombstone" fell down. You can clearly see in long shots that only the areas closer to the camera are getting rained on, and the road ahead is bone dry.