With 25 years of Merrell's women's-specific fit, the Siren 4 Thermo is uniquely engineered to cradle the distinct contours of female feet, providing comfort, support, and stability even on the longest hikes with 200 grams of added insulation for warmth and protection. Now equipped with SOLARCORE® insulation, lightweight insulation material initially designed for NASA, you never have to worry about warmth. The all-new medial zipper provides easy on/off, a new improvement made directly from customer feedback. Merrell waterproofing on the upper will keep your feet dry. With Vibram® IceTrek® underfoot, traction remains consistent under the wettest, most slippery conditions. For the first time ever incorporating Vibram® traction lug for enhanced grip on uneven terrain.