2-Season Summer bags are suitable for temperatures of about 30 degrees 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 open them wide to use ventilate (or use as a quilt) when the night get 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) on hot July nights, for instance, so why spend the money on things you don’t need?