First depends on what outside surface you are playing on, If it's Hard courts (tarmac etc) and it's wet forget it, take up badminton or squash!
If you are playing on astro / shale / clay then you can still play but you may have to do two things, change the type of tennis balls being used and decrease the tension of your strings.
The tennis balls when damp or wet can weight twice as much as normal, so your normal hit will only go half court!
So you need to change to a ball that doesn't collect water and / or decrease you racket tension by 2 Ibs to compensate for the heavier ball.
Tennis balls that are playable when wet or damp are:
Tretorn Micro X
and the best option: Slazenger Wimbledon Ultra Viz Hydroguard
Hope this helps!