Snow that persists for more than a year on the glacier is called firn. We have taken several longer cores (20-30 m) to better understand the transition from snow to ice. One characteristic of firn is the prevalence of thick ice layers. These form when summer meltwater percolates into firn that is below the freezing temperature (0 degrees C). As the water freezes in it releases heat, warming the firn up. Once all of the firn is at the freezing point additional meltwater stays liquid and presumably flows out of the glacier, much like groundwater flows downslope.