Generally, the annual snowpack at GLEES is established in November and lasts into July. Figure 10.1 is the 1987-91 recession curve of the snow-covered area fraction versus degree days. About 20% of the area consists of rocks, which are usually blown clear of snow, and trees. The trees may hide some of the snow in the aerial photographs that were used to develop the curve. The remnant area fraction of snow cover represents the permanent snowfield on the cliff face overlooking the lakes.