I can't say if rain makes it harder to light a fire. I've seen that lighting a fire next to an existing fire never seems to fail (a procedure used for preparing soil for agriculture), and Privateer is probably correct about lighting fire on top of ashes. It can also be mentioned that it's not a good idea to place food to be cooked on old ashes, as the food gets burnt, and standing on old ashes (e.g. when preparing soil) is a lot warmer than standing elsewhere (but not harmfully so). I suspect it might be possible to use this ash heat effect when it's very cold and you need to e.g. treat an elk hide, but haven't tried it myself.