One of the most common questions that comes up on this topic is 'Can fire-type pokemon be used in the rain?' People also ask if it hurts them, does damage to them, will they refuse to come out of the pokeball? Or is it just fine and dandy to use them when it's wet out? Like most issues, this is not a black and white matter; we cannot give you a 'yes' or 'no' in reply. We can give you some guidelines and examples:
While fire types do not (and should not) like water in general, they *do* need to drink so their dislike of it is not to the point of a phobia. If the water is "in control" (for example: If they're drinking out of a lake and thus can get away from it when they want) then it's more okay to them than something like rain (which cannot be escaped and can only be hidden from under some cover. It's still raining, it's still waiting for them to leave their protection: it's not something they can control).
For the most part, rain will not seriously harm a fire-type pokemon; rain might sting, it might burn, or it might just be really annoying, but for most healthy fire-types it will do no lasting (physical) harm. There are some special cases where wet weather can hurt or even kill, but it is rare (Example: A wounded or sick Charmander out in the rain with no cover for its tail flame.).
As a guideline, the more evolved and more healthy a pokemon is, the better it will resist water (Ash's Charizard's tail actually went under water in the Charizard Valley episode.) That being said, let's remember it does not take much of a threat of pain or death to cause an aversion to form. Generation after generation of Charmander and Growlithe and Vulpix and such has grown up learning that they are weak to water (learning the easy way *and* the hard way), and that water is a Bad Thing. With few exceptions, fire types should fear or dislike getting wet (and rain/uncontrolled water). Drinking is okay, getting a bath or going out puddle-jumping is not.
Type plays a part in this as well. The more "fiery" a type is (Ponyta and Magmar vs Growlithe and Vulpix), the more vulnerable that pokemon is -- which means the fear and hatred of water/rain/getting wet should rise. Fire types and rain/uncontrolled water is the most obvious of the "pokemon type/nature" parings. For other types just use your head. Oddishes should not be pyromaniacs. Squirtles should not get off on licking their fingers and sticking them into electrical outlets. ...but this does not mean that a Bellsprout and a Charmander cannot be friends. It might take longer, and there could be a lot more mistrust at first, but in time a friendship is fine and dandy.
If you have any questions, contact a staff member.