VestPacs are designed to provide the best possible performance for every situation. The revolutionary 3D Airmesh™ technology adapts to take advantage of any environment. In hot climates, moisture evaporates from the mesh, actively cooling both your body and your H2O. When the temps drop below freezing, the VestPac becomes an UnderPac™, a breathable mid-layer worn under the jacket. This insulates your core while utilizing body heat to keep the water reservoir from turning into an ice block.