Greenland’s surface melt is a primary contributor to global sea-level rise. However, the leading causal mechanisms are still unclear. In this project, we use decade-long measurements from automatic weather stations in the most melt-susceptible regions on the Greenland Ice Sheet to identify dominant energy components and associated physical processes for surface mass loss. Surprisingly, large melt events, such as the one in 2012 during which almost the entire Greenland Ice Sheet experienced surface melt, contribute only 2% to total surface melt since 2007. The day-to-day variability of normal-rate melt events is dominated by sensible heat exchange (31%) and shortwave radiation (28%). They are likely caused by katabatic winds, a gravity-driven downslope wind that can be forceful and often associated with clear skies. In Greenland, grounds are usually colder than the air above as sunlight gets reflected by snow and ice. These katabatic winds enhance vertical mixing to bring down warm air aloft to heat grounds. With katabatic winds occurring during clear skies, they coincide with periods of increased shortwave radiation. Since high-melt regions along the ice sheet margins will typically have relatively steep slopes, these downslope winds will continue to have a large impact on Greenland’s surface melt in the future.