Agreed with Deacon. Most rain jackets will repel water for a while until the coating wears off or the fabric is worn/torn. However, good rain jackets are designed not only to repel water but also to trap and hold the air between the jacket and body to increase the insulation value. If the rain jacket is purchased to small, say as a "medium" in summer, additional clothing layers for cold weather will reduce the insulation value quickly. In a full day (or multiple days) rain it's difficult if not impossible to stay fully dry so the rain jacket should be able to help keep you warm to offset the ever-present threat of hypothermia in the cold weather months.