Arcum's Weathervane

[FranAvalon]

I'm not sure if the problem is with Arcum's Weathervane or other cards that care about the Snow supertype. Here's what happened:

With a Ronom Serpent on the battlefield, I turned one of my opponent's lands into a snow land, but the Serpent still couldn't attack. Then I turned all my lands into nonsnow lands, and I didn't have to sacrifice the Serpent.

Using Thermokarst on the opponent's land that I'd made into a snow land in the previous example, I didn't gain life. But using Thermokarst on one of my lands, which should have no longer been a snow land, I did gain life.