Frostbite

Each space can have a Frostbite marker.

Frostbite occurs in Winter on spaces without buildings or trees on them if at least 20% of the map is covered by chaos, on spaces with 1 building or tree on them if at least 40% of the map is covered, etcetera.

If Frostbite is present on a space, it lowers the hitpoints of tiles and ground units in that space by 1 (down to 1), making them more vulnerable to damage.