Arcum's Weathervane

• WWizards of the Coast2006-10-15
  Official Ruling
  Can’t be used on a dual land to add the Snow Supertype even though it is a basic land subtype. You can use it on any land to remove the snow supertype.
• WWizards of the Coast2006-10-15
  Official Ruling
  Adds the snow or removes the snow supertype to or from a land. Adding snow doesn’t overwrite other supertypes.

Rulings via Scryfall