"David Brooks" wrote
Does engaging the alternate air shut the external static source off? If not,
it's only a partial test.

I have never seen an alternate static source on a light GA airplane
that shut the external static source off. Every one I have seen
simply vents the static system to the cabin. It is intended ONLY as a
means to deal with static source blockage, such as by ice, not with
intermittent effects such as streaming water.

Michael