"CLEAR SKIES, CALM WINDS, lows in the upper fifties." As these words echo across much of the lower 48 this month, thoughts turn to the backcountryand a spur-of-the-moment overnighter in whatever wild amphitheater happens to be available. (Sorry, Central Park doesn't count.) Thankfully, the new ultralight warm-weather sleeping bags make snoozing happily under the stars a snap. The sacks on the following pages start at an ultra-wispy 17 ounces and max out at 32, which is still less heft than a bunch of grapes. They're also highly packableseveral squish down to the size of a Nalgene bottleand take full advantage of sheer nylon shell fabrics. Of course, gossamer bags demand extra care: Flop a synthetic- or down-filled ultralight over a snaggy limb, for instance, and you're asking for a fill-spewing gash. But user-friendly touches like simple collar-and-hood cinch adjustments, high-contrast zippers that are easy to find, and hang loops for drying make these bags as smart as they are light. Pop one in a daypack, toss in a ground pad, and add some water, a headlamp, a bagel for dinner, and an orange for breakfast. Seize the day, sure, but nab a comfy night at the same time.