The world follows a season cycle that progressively shifts the importance of resources and drives agriculture. Climate is partially dependent on the season, as is weather, though both have a random aspect.
Winter is considered to be the harshest season in the game, because of the low temperature and inability to farm. Thus, Food supplies tend to decrease, and your citizens could freeze to death during the harsh months. Strong stores and production of Firewood is a necessity, as is housing, and will help to prevent hypothermia. Warm clothing keeps your citizens warm longer when they are outside their house, but they will still freeze to death without proper housing.