These snug jackets keep you super warm and are feather-light. They’re either filled with feathery down or a synthetic version, and the principle is simple. The air throughout the loose filling creates a layer of insulation between the outside cold and the warmth of the body – and also traps body heat. The greater the air barrier between the body and the cold, the more effective the insulation. Pay attention not just to the filling, but also to the quality of finishing on the neck, arms and hips – if you choose a high-quality product, you’ll get the best possible cold protection.