To determine what makes a jacket exceptionally warm, we need to consider several factors insulation type, fabric technology, and overall design. Insulation, which can be natural (like down) or synthetic (like polyester), plays a critical role in trapping heat. Down insulation, renowned for its incredible warmth-to-weight ratio, is filled with the fluffy clusters of duck or goose feathers. On the other hand, synthetic insulation offers excellent moisture resistance and is hypoallergenic, making it suitable for a wider range of conditions.