Snow Cover: More information
The maps and bar graphs show how Northern Hemisphere snow cover for different years and months compares to averages for the period 1966 to 2015. The maps show snow cover duration, expressed as the number of days a grid cell is snow covered for each month for each year. On the maps, areas with longer than average snow cover duration are indicated in blue (positive anomalies). Areas with shorter than average snow cover duration are indicated in red (negative anomalies). The maps of anomalies help show where changes in snow cover are strongest.
The bar graphs indicate the mean snow cover extent for the Northern Hemisphere for a given month. The graph helps to illustrate hemisphere-wide changes for each month.
Snow cover plays a key role in regulating the global climate system. It is the most reflective of all natural materials occurring at the Earth’s surface, typically reflecting more than 50% and up to 90% of solar radiation. It is an efficient insulator that influences the temperature of tundra soils. Seasonal snow cover also acts as a natural reservoir, storing solid precipitation in winter and releasing it during the spring and summer melt, affecting the discharge regimes of Arctic-draining rivers.
Looking at the bar graphs, changes in snow cover extent have been most notable in April, May and June. In these months, most of the Northern Hemisphere’s snow cover is in the Arctic; much of the snow at lower latitudes has already melted away. The strongest declines in Arctic snow cover extent have occurred in May and June.
The maps show spatial patterns of anomalies of snow cover duration. There are no large anomalies in Arctic latitudes in winter months. Much of the Arctic at this time is so cold that precipitation is nearly always in the form of snow, and the snow, once fallen, rarely melts. However, both positive and negative anomalies can be seen at lower latitudes, reflecting variability of the passage of winter storms over the North American and Eurasian continents as well as widespread melt events. Large anomalies in snow cover start to appear in the Arctic after May. Negative anomalies are prevalent over Alaska and Siberia after about 2005. By July and August, much of the snow cover has melted. In September and October, both positive and negative anomalies can be seen. An increase in the number of days with snow cover might indicate an increase in the frequency of early autumn snow events, which are related to an increase in the area of open ocean as a result of declining sea ice cover.
The data shown here are from the NOAA Climate Data Record (CDR) produced by the Rutgers University Global Snow Lab. This record is based on satellite-derived maps of snow cover produced by NOAA. This record of snow cover is the longest continuous environmental remote sensing data set. Snow maps were initially produced on a weekly basis. Since the late 1990s, snow maps have been produced daily. The Global Snow Lab has gone to great efforts to make the record of snow cover consistent through time. The CDR consists of weekly snow cover maps showing a pixel as snow covered or snow free. More information is available from Rutgers University Global Snow Lab.
Robinson, D.A. and A. Frei. 2000. Seasonal variability of Northern Hemisphere snow extent using visible satellite data. Professional Geographer 51, 307-314.
Derksen, C. and Brown, R. 2012. Spring snow cover extent reductions in the 2008-2012 period exceeding climate model projections.Geophys. Res. Lett. 39, L19504, doi:10.1029/2012GL053387.