Bags fall into three basic categories.
Summer bags are suitable for temperatures of about 30°F and higher. They’re lightweight (because less insulation means less weight), and they pack down tiny (often as small as a cantaloupe). Summer bags often have full-length zippers, which allow you to zip them almost completely open for ventilation (or to use as a quilt) when the night gets really steamy. Most summer bags are simple sacks without too many bells and whistles. You won’t have much need for a hood or a draft collar (see below for descriptions of features) on hot July nights, for instance, so why spend the money on things you don’t need?
Ideal for temperatures of about 20°F and above, these bags are best suited for spring and fall trips, as well as summers in the high mountains when temps can dip below freezing at night. Good three-season bags have added features to combat colder temperatures like cinch-able hoods, draft collars, and zipper draft tubes (see below for descriptions of features).
These puffy cocoons, good for about 20°F and below, have all the features of a three- season bag (cinch-able hoods, draft collars, zipper draft tubes), but are beefed up with more insulation. Winter bags are always bulky to pack, so you’ll want to buy a good compression stuff sack to help you tame it.