Hypothermia can be a cause of death. Your citizens can get hypothermia when the temperature drops below 45 F if they cannot access a house containing firewood or coal. Clothing will increase the time a citizen can survive without going to a house to warm up, but citizens with clothing can still die of hypothermia.
Citizens will first try to go to their own home to warm up. If their home does not have any fuel or they are homeless, they will instead go to the nearest home. Because of this, citizens on the "march of death" can be saved by being made homeless.
Citizens on the verge of getting hypothermia are marked with a snowflake above their heads. Houses might also have a snowflake above them, indicating that they are lacking fuel and cannot warm citizens.