Maybe it was black ice? No in all seriousness pure rainwater in a non polluted area is very clean. I inspect my water tanks once a month and they are crystal clear. Additionally I have a filter to further insure its safe.
i mean, i tested portland snow. but it was snow that fell from the sky and should be in a sense pure, but it's not. I figured snow was always fine if you take from what doesn't touch the ground or anything. But it still had a lot of nasty stuff in it.