If it's a short rainstorm, and they stay dry, they will certainly be fine.I worry about sudden, unexpected weather - like short but heavy rainstorms. But yes, they seem to be pretty comfortable in the weather outside.
Even if it lasts for several days, they will probably be fine if they stay dry.
At any age, chicks can huddle up and make it through a short time at a slightly lower temperature.
The older they get, the less it bothers them, so they do fine with larger temperature swings for longer stretches.