During the month of February, between 46 and 53% of the land area of the northern hemisphere is snow covered. Continental snowpacks act as chemical reservoirs; pollutants can accumulate in the pack over the entire winter and are released during a relatively short spring melt period. Interactions between the snow and the atmosphere can change the quantities of different chemical species stored in the snow. In addition, snow can retain chemical species to be incorporated into major ice sheets and large glaciers. The record of atmospheric chemistry preserved in this way is important in understanding the past history of the atmosphere.