People from what I've heard have problems with PTW's in the rain or high humidity due to the electronics inside. This is only anecdotal though.
Playing in snow (or cold weather) isn't advised as you run the risk of cracking your mechbox due to the cold.
Rain in a regular AEG should be fine as long as you don't have any gaping holes where a significant amount of water is able to get into the gun.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE