Machshirin 4:2

If rain fell on a person, ki yutan doesn’t apply, even if he was a primary source of impurity (av hatuma). If he shook the water off, then ki yutan applies. Let’s say that a person stood under a rain spout to cool himself or to wash off. If he’s ritually unclean, the water is also unclean; if he’s ritually clean, then ki yutan applies to it.

Machshirin 4:3

If a person overturned a bowl against a wall in order to wash it off (from the rain spout), ki yutan applies; if to protect the wall (from the rainwater), ki yutan does not apply.