Coarse blocks are a widespread ground cover in cold mountain areas. They have been recognized to exert a cooling influence on subsurface temperatures in comparison with other types of surface material and are employed in man- made structures for ground cooling and permafrost protection. The contrast in heat transfer between the atmosphere and the ground caused by thermally driven convection in winter and stable stratification of interstitial air during summer is usually invoked to explain this “thermal diode” effect. Based on measurements and model calculations, we propose an additional cooling mechanism, which is independent of convection, and solely functions based on the interplay of a winter snow cover and a layer of coarse blocks with low thermal conductivity. The thermal conductivity of a block layer with a porosity of 0.4 is reduced by about an order of magnitude compared to solid rock. We use a simple and purely conductive model experiment to demonstrate that low-conductivity layers reduce the temperature below the winter snow cover as well as mean annual ground temperatures by comparison with other ground materials. Coarse block layers reduce the warming effect of the snow cover and can result in cooling of blocky surfaces in comparison with surrounding areas in the order of one or several degrees. The characteristics of this mechanism correspond to existing measurements.