Synthetic bags don't usually give as much warmth per gram as a down bag but they do the job in a range of conditions, including damp ones. Buying a bag is a play-off between weight, temperature rating, and the contents of your wallet. There are also a range of fabrics, shapes, and even sizes to choose from, so you don't have to carry around more sleeping bag than the length of your legs demands. Features to check include a snug fitting hood and neck baffle to stop the heat escaping at the top.
