I am writing a novel and want the maximum amount of accuracy as possible. There is a scene in which a group of archers lay an ambush, but it starts raining heavily. I want to know to what degree this decreases the lethality of the attack.
If anyone has any general information on ancient archery and how well or poorly ancient archers would fare when firing SHOOTING in heavy rain, that would be greatly appreciated.
Please, please, please, do not write a novel where archers "fire" anything!
Archers did not "fire". There is nothing that burns or ignites in the process of "shooting" or "loosing" arrows from a bow.
"Firing" is something that is done by artillerymen or musketeers, using firearms, not something that is done by archers using bows.
It is an absolute anachronism to call shooting or loosing arrows "firing", or to have some fictional commander give the order, "Fire!" to a group of archers.
I know that all too many authors have committed this grievous sin in the past.
Please do not add to their number.
It would definitely help if you could be more specific on what 'Ancient Roman' archers are. The bows and bowstrings they'd be using depends on where they're from (I'm assuming they're foreign auxiliaries)
Sinew backed Eastern bows can be affected by moisture but that's more a process that takes months of humid exposure, not something that happens in a day because of some rain. So no real worries there.
The feather fletches on arrows don't do very well in the rain at all. They basically go floppy and don't stabilize arrows nearly as well. Also they cause far more drag which slows down arrows. So accuracy and speed (and consequently range and penetration) drops. Back in the day they used to use leather/parchment fletches too but I don't know how they're affected by rain.
Whether the bowstrings are affected depends on the material. Sinew bowstrings stretch when wet, and part of what holds a sinew string together is that sinew contracts as it dries, holding the fibers together. So if it gets wet enough the string might fall apart entirely. Linen and silk strings were used by the English but that's centuries later, so I'm not sure that's relevant. Anyway they too tend to stretch but don't really fall apart. Which direction the arrows deflect depends on which side of the bow the archers put their arrows and that again depends on their culture.
Bows and arrows are actually quite complicated to tune, the strength of the bow, the weight of the arrowpoint and stiffness of the shaft have to match and the length of the string brings it all together. If the length of the string stretches longer, shots will be weaker and arrows will actually bounce off the bow instead of flying straight.