Hi! Sylvia here from The Butterfly Place. As James said, best not to let out in the rain as the butterflies need to be able to warm their wings to fly off. However, if you release them and it rains later, don't worry: they'll find a place to roost if they need to.
You shouldn't release them in the rain http://www.rainbowsendfarm.biz/HowToRelease.htm
Thanks! Your answer is awaiting moderation.