My neighbour owns a roofing company and in the middle of the summer heat doing our shingling a few years back all of them were wearing heavy winter coats. When asked about it they said it actually made them cooler and also kept the sun off their skin. My cousin not believing this tried it and apparently initially you're a lot hotter but as you start to sweat it cools back down again(as long as there is a breeze). You also keep yourself insulated from the direct sun.