Changing Northern Hemisphere Snow Seasons

Spatial and temporal patterns in the onset, offset, and length of the snow season across Northern Hemisphere continents are examined for the period from 1967 to 2008. Full snow seasons (FSS) and core snow seasons (CSS) are defined based on the consistency of snow cover within a location over the course of the cold season. Climatologically, the seasonal onsets of FSS and CSS progress more rapidly across the continents than the slower spring northward offset. Average Northern Hemisphere FSS duration has decreased at a rate of 0.8 week decade (5.3 days decade) between the winters of 1972/73 and 2007/08, while there is no significant hemispheric change in CSS duration. Changes in the FSS duration are attributed primarily to a progressively earlier offset, which has advanced poleward at a rate of 5.5 days decade. A major change in the trends of FSS offset and duration occurred in the late 1980s. Earlier FSS offsets, ranging from 5 to 25 days, and resultant abbreviated durations are observed in western Europe, central and East Asia, and the mountainous western United States. Where regional changes in CSS were observed, most commonly there were shifts in both onset and offset dates toward earlier dates. Results indicate that it is important to pay close attention to spring snowmelt as an indicator of hemispheric climate variability and change.