That will be a great trip. The Colorado Trail stretches almost 500 miles from Denver to Durango. It winds through the Rocky Mountains in Colorado, traversing six wilderness areas and eight mountain ranges at heights of up to 13,334 feet. In fact, most of the time you聮ll be 10,000 feet or higher. That means nights can be chilly, especially if you聮re still on the trail after mid-August.
Western Mountaineering Alpinlite Super Sleeping Bag

That said, it won聮t be frigid. If you聮re a cold sleeper, a bag rated to 20 degrees would be more than adequate聴warm enough for cold nights, but not so toasty you聮ll poach when the temps are higher. At the pricier end for a bag such as this you聮ll find Feathered Friends estimable Swallow ($349; featheredfriends.com), rated to 20 degrees with a water-repellent EPIC shell. It weighs just two pounds and is an excellent bag. Western Mountaineering聮s Alpinlite Super ($385; westernmountaineering.com) weighs just a touch less than the Swallow and offers a little roomier cut than most mummy bags. For something more affordable, REI聮s Sub Kilo +20 bag (rei.com) sells for $239 but is on sale now for $160. It uses good-quality down, has an excellent polyester shell, and weighs one pound, thirteen ounces.
Otherwise, a bag rated to around 30 degrees should do nicely. I like Marmot聮s Arroyo ($249; marmot.com), an 800-fill down bag that weighs one pound, eleven ounces, uses Marmot聮s excellent silicon-finished nylon shell, and has a snug-fitting hood. Or, Mont-Bell聮s U.L.SS. Down Hugger #3 ($270; montbell.com) is a super-light (one pound, seven ounces) bag that uses Mont-Bell聮s proprietary stretchy shell to gently 聯hug” you as you sleep. That helps reduce cooling that occurs when you thrash around and force warm air out of the bag. I聮m using this Mont-Bell bag on some trips this spring and really like it. Sierra Designs聮 Osage ($179; sierradesigns.com) uses slightly less fluffy down and heavier materials; but the result is a 30-degree bag of reasonable cost that still weighs only two pounds, four ounces.
Note that I聮ve mentioned all down-fill bags. For a trip such as yours I think that聮s the ideal material;聴light and warm. One synthetic option is the North Face Cat聮s Meow ($169; thenorthface.com), which uses Polarguard Delta fill, has a temp rating of 20 degrees, and weighs two pounds, ten ounces.
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