Seasonal variability of Northern Hemisphere snow extent using visible satellite data

In this paper we use a satellite-derived data set to explore spatial and temporal variations of snow extent across Northern Hemisphere continents during the last three decades. These weekly visible-wavelength satellite maps of Northern Hemisphere snow extent produced by the National Oceanic and Atmospheric Administration constitute the longest consistently-derived satellite record of any environmental variable. We document the considerable intra-annual variability of snow extent, and show that during each month, fluctuations over relatively small areas are responsible for the majority of the year-to-year variability. Regions that cover less than 6% of Northern Hemisphere lands north of 20 degrees N explain 62%-92% of the interannual variance across the continents. On average, snow was more extensive across both Eurasia and North America from the 1970s to middle 1980s than during the late 1980s to late 1990s. During late winter, spring and summer, snow extent has decreased since the middle 1980s, while during fall to middle winter, snow extent has remained relatively constant. Accurate information on continental snow extent is critical for weather and hydrologic forecasting; for understanding hemispheric-scale atmospheric circulation, thermal variations, and regional snow extent; and for using snow as a credible indicator of climate variability and change.