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. 200 grams Primaloft® Gold Eco Series synthetic insulation. Solarcore® Aerogel insole provides underfoot superior warmth while being extremely flexible and ultra-thin. Waterproof full grain leather upper, complete with a waterproof membrane seals out water and lets moisture escape.  Vibram® Icetrek outsole designed to provide the improved grip on very cold, icy, or snowy surfaces. Vibram traction lugs specifically designed to increase traction and shed debris with each step FEATURES: • Heavy-duty, Waterproof, YKK zipper for easy on and off • Bellows tongue keeps out debris • Protective and abrasion resistant rubber toe cap • Cleansport NXT™ treated for natural odor control • FloatPro™ Foam midsole for lightweight comfort that lasts