Winter

Winter is the fourth and last season every year, and the most different season compared to the other three. In winter, tiles turn into snow tiles reducing movement and crops tiles will not generate any income. Winter is the only season in which frostbite can occur.

Terrain changes
In winter, all tiles with 1 or more humidity gain the snow effect. Any ground unit that moves in winter will have 1 reduced movement if there is a snow tile along their path (multiple snow tiles to not reduce this further).

Grass tiles that have snow no longer contribute to niceness, reducing the power gain of cities, towns, farms and outposts.

In winter, both frostbite and bonedrought can occur on tiles if global warming has progressed far enough. However, due to their mechanics it is unlikely for both to appear on the same tile.

Crops tiles do not generate any income in winter, but farms can still cultivate tiles that contain snow.