The Sea To Summit Spark 15-Degree Sleeping Bag keeps our pack light and offers plenty of reliable warmth for backpacking trips through the early parts of winter. The lofty 850-fill down has a hydrophobic Ultra Dry Down treatment to maintain warmth in cold, damp conditions. The bag is exceptionally warm, especially considering that it weighs just over two pounds, and the mummy shape maximizes thermal efficiency on cold, frosty mornings. The three-quarters length zipper is protected by a down-filled draft tube to eliminate cold spots, and the hood and footbox are protected by a DWR coating to ensure the down stays lofty even if our tent gathers condensation.