A particle-scale investigation of outdoor machine-made snow density: The role of critical droplet diameter

Machine-made snow has been indispensable for ski resorts and winter sports, consuming large amounts of water. The machine-made snow density is important for ski resort resiliency facing climate change, water conservation, and the safety of skiing sports. This study presents a particle-scale investigation of machine-made snow density variation during snowmaking and proposes a conceptual analysis considering the effect of critical droplet diameter (dcr) for machine-made snow. First, outdoor snowmaking experiments are conducted under different weather conditions and atomization performances. Experimental results indicate that the snow density of machine-made snow is influenced by a combination of droplet diameter and air temperature. The snow density increases with the Sauter Mean Diameter at constant environmental conditions. The snow density increases as environmental temperature increases when the atomization performance is constant. Then, particle-scale characterization suggests that there is a critical droplet diameter that determines the percentage of frozen and unfrozen droplets and, therefore, influences the snow density. Frozen droplets form the structure of the snowpack. Unfrozen droplets fill the voids of snow packing and increase snow density. Finally, a theoretical calculation of a single droplet freezing process is employed to analyze the influence of weather conditions, heat transfer time, and droplet diameter on the critical droplet diameter. The calculation results suggest that decreasing air temperature, humidity, and solar radiation and increasing ground clearance of the snowmaker increase the critical droplet diameter and lower the snow density. The effect of humidity, solar radiation, and ground clearance on snow density is more remarkable at higher temperatures.