One hundred miles and 30 days later, the Black Diamond Rhythm Tee only shows one sign of distress: a tiny tear caused by a tree branch in the backcountry of Golden, Colorado. I’ve been running in this tee — a wool shirt designed for high-output activity — over the last month and I’m positive I’ll continue reaching for it in all seasons (but winter) for biking and hiking, too.

This isn’t like other wool shirts. My first thought when I was handed the Black Diamond Rhythm Tee was that it was just a textile swatch — it’s that light. But, no matter its weight (or lack thereof), it still performs like wool, which is a good thing, especially if you’re running in the Colorado foothills, where days often start brisk and quickly become scorchers. Wool shirts have a broad thermal range to keep up with this type of weather — they keep you warm when it’s cool and vice versa — and, bonus, they’re less likely to chafe than polyester shirts.

The Rhythm Tee came to be as part of a collection at Black Diamond created with input from alpine runner and athlete Joe Grant, known for biking to all of Colorado’s 14ers and summiting each one. Needless to say, the Rhythm tee is made for high-output and sweat-inducing activities. “I always try to choose gear that I can forget about when I’m out in the elements and that doesn’t hinder the experience,” Grant says. “The Rhythm Tee is soft to the touch, very light and breathable, and has plenty of stretch.”

To gain that stretch, Black Diamond uses an innovative yarn called Nuyarn in the Rhythm tee, in which Australian Merino wool is loosely wrapped around a nylon core, giving it unsurpassed stretch and rebound. So, when the shirt is pulled, the wool straightens out but doesn’t stretch out, allowing it to return to its shape once the core rebounds. This construction also traps tiny air between the core and wool, delivering more warmth from less material, and keeping the wool fibers intact, so it’s softer against the skin.

On a recent long run, I wore the Rhythm Tee around and atop a table mesa in Golden — picture huge hills with steep sides and a mushroom flat top. The trails were full of hikers, bikers, runners (and rattlesnakes), all out to soak up the sun and warm air. The 700 feet of incline to the top had me sweating almost immediately, and by mid-run, beads of it were visible on my chest. The shirt never felt bogged down by the moisture or stuck to my skin, except between my back and my running vest, a fairly common issue (and no fault of the shirt’s) due to a lack of space between the two.

Just as quickly as I started sweating, the weather conditions changed. As I descended the trail into the shadow of the mesa’s wall, the temperature dropped about twenty degrees. Yet, the light-and-thin wool kept me warm enough to avoid any uncomfortable shivering as my sweat dried. After my two-hour run, I stopped at a bar in town for a post-run drink. And, by the time I finished my tea and limeade concoction, the inside of the shirt was dry. The shirt also tamped down some minor annoyances that stack up over time: the close fit kept it from flapping in the breeze and its stretch freed me from a tug of war with a full hydration vest.

After sub-five-mile runs, I’ve hung the shirt by an open window, hopped in the shower and found it dry by the time I finished. I can usually wear it multiple times before even thinking about washing it because wool resists odor-causing bacteria. It may be the only running (and travel) shirt I ever need.

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The Woolmark Company provided this product for review.

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