Each space can be affected by Snow.
Snow occurs in Winter when a space has 1 or more Humidity and in other seasons when a space has 4 Humidity.
Snow slows the movement speed of ground units by 1. This effect does not stack; a unit with a movement speed of 3 can move 2 spaces even if both its starting space and all of the spaces it moves to are affected by Snow.
At night, Snow reduces the positive effect of a Grass tile on the growth of nearby City, Town, Farm and Outpost tiles to 0.