Sonic Ultra Zip Top runs on Dryflo(TM) Air fabric with laser-cut vent holes at the back of the neck and underarm panels, placing airflow exactly where a long climb generates heat. A deep center front zip opens further for extra venting when the pace picks up, and Microactive seams stay soft against skin instead of chafing under a pack strap. Set-in sleeves keep the fit clean through every arm swing. What Our Gearhead(R) Experts Are Saying:"I've run three-hour mountain days in this without swapping it out or noticing it start to smell. It dries between climbs almost as fast as I can drink water, so I'm never carrying a spare."