I can't fully understand how he manages to, but Bud Jeffries has a lot of equipment stored outside, inculiding kettlebells. Rust creeps on almost every item made of metal shown in his videos. I never kept anything outdoors and I won't until forced to. If you expose something like a kettlebell to the atmospheric agents, even humidity alone, bad things are in order to happen to the object, no matter what. It may not be this year, may not be next, but sooner or later they're gonna rust. Powder coating as suggested by
@natewhite39 could slow down the process a bit, but they'll eventually end up like all the others, one day.
If you have no choice but to store them outside,
@ShawnM's plastic bag solution would be a good idea, but I'd also put them in some kind of conainter. I would buy it thick enough to provide a sufficient insulation from the heat and, if you don't find it in whithe or green colours, I would paint it in one of the two. If memory serves me well, the first one absorbs the least ultraviolet rays and the second one "reflects" them.
Still, I'd go to bed with tears in my eyes because I know they're bound in a dark place and they are probably looking for me. Mom/dad/wife/husband would ask "What's up, my dear?", and I would yell to them "It's nothing!", but I'd be very sad and enraged. Outside storing of kettlebells can be potentially very dangerous. Do it only if it is your last choice.