Snowfall
{2}{U}
Enchantment
Cumulative upkeep {U} (At the beginning of your upkeep, put an age counter on ~this~, then sacrifice it unless you pay its upkeep cost for each age counter on it.)
Whenever an Island is tapped for mana, its controller may add an additional {U}. If that Island is snow, its controller may add an additional {U}{U} instead. Spend this mana only to pay cumulative upkeep costs.