Arcum's Weathervane — ruling 1

Adds the snow or removes the snow supertype to or from a land. Adding snow doesn’t overwrite other supertypes.

Arcum's Weathervane — ruling 2

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.

Arcum's Weathervane : règles, fonctionnement et interactions (MTG)

Arcum's Weathervane is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

, : Target snow land is no longer snow. , : Target nonsnow basic land becomes snow.

How it works : Arcum's Weathervane

Original English text — Wizards of the Coast / Scryfall

#1October 15, 2006· Wizards of the Coast
Adds the snow or removes the snow supertype to or from a land. Adding snow doesn’t overwrite other supertypes.
No other card is named in this ruling.
#2October 15, 2006· Wizards of the Coast
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.
No other card is named in this ruling.