SOTC: Northern Hemisphere Snow

We all associate snowstorms with cold weather, but snow's influence on the weather and climate continues long after the storm ends. Because snow is highly reflective, a vast amount of sunlight that hits the snow is reflected back into space instead of warming the planet. Without snow cover, the ground absorbs about four to six times more of the Sun's energy. The presence or absence of snow controls patterns of heating and cooling over Earth's land surface more than any other single land surface feature.

In many locations in recent decades, temperatures have risen while precipitation levels have remained largely the same. Satellite data have confirmed that average snow cover has decreased, especially in the spring and summer. Where snow cover is disappearing earlier in the spring, the large amounts of energy that would have melted the snow can now directly warm the soil.

In terms of spatial extent, seasonal snow cover is the largest single component of the cryosphere and has a mean winter maximum areal extent of 47 million square kilometers, about 98 percent of which is located in the Northern Hemisphere.

Northern Hemisphere Weekly Snow Cover and Ice Extent, February 8-14, 2010Northern Hemisphere snow extent map: Week of maximum snow extent (50.7 x 106 km2) for the period 1979 to 2011 (image from 8-14 February 2010). Image from Northern Hemisphere EASE-Grid Weekly Snow Cover and Sea Ice Extent Version 4 product.

Snow cover is an important climate change variable because of its influence on energy and moisture budgets. Snow cover accounts for the large differences between summer and winter land surface albedo, both annually and inter-annually. Snow may reflect as much as 80 to 90 percent of the incoming solar energy, whereas a snow-free surface such as soil or vegetation may reflect only 10 to 20 percent. A warming trend results in decreased snow cover. With the resulting decrease in reflected energy, absorption of solar radiation increases, adding heat to the system, thereby causing even more snow to melt. This is the classic temperature-albedo feedback mechanism; it is a "positive feedback" because it reinforces itself. Surface temperature is highly dependent on the presence or absence of snow cover, and temperature trends have been linked to changes in snow cover (Groisman et al. 1994).

In addition to the albedo effect, snow cover represents a significant heat sink during the melt period of the seasonal cycle due to a relatively high latent heat of fusion. As a result, the seasonal snow cover provides a major source of thermal inertia within the total climate system, as it consumes large amounts of energy with little or no fluctuation in temperature as snow crystals melt into water.

Northern Hemisphere snow cover extent by monthAnnual Cycle of Northern Hemisphere snow cover extent: This graph shows the annual cycle of snow cover, from September through August, for the years 1967-2013. Median extent appears in black. Greater ranges of extent variation appear in lighter shades of gray, with the entire range of extents appearing in the lightest shade. Snow cover extent for September 2013 through January 2014 appears as a red line. In the early 2013-2014 winter, Northern Hemisphere snow extent rose well above the 1967-2013 median before falling slightly below it, but did not fall outside the 1967-2013 range. Image by Andrew Slater, National Snow and Ice Data Center, University of Colorado, Boulder, based on data from the Rutgers Snow Lab.

During the past four decades, satellite remote sensing has provided valuable information on hemispheric-scale snow extent. Since 1966, the National Oceanic and Atmospheric Administration (NOAA) has produced weekly snow extent maps for Northern Hemisphere land surfaces using visible-band satellite imagery (Robinson and Frei 2000). Because snow has such a high albedo compared to other surfaces on Earth, snow-covered areas appear much brighter in satellite imagery than most other surface types.

Remote sensing data sets from the microwave portion of the electromagnetic spectrum can also be used to derive snow cover maps, with the added benefit of being able to "see" through clouds. When snow covers the ground, some of the microwave energy emitted by the underlying soil is scattered by the snow grains; therefore, when moving from snow-free to snow-covered land surfaces, a sharp decrease in emissivity indicates the presence of dry snow.

These remote sensing data sets are derived using different types of analyses and separate regions of the electromagnetic spectrum, yet their results are strikingly similar. Both visible and passive microwave data sets show similar patterns of inter-annual variability, and both consistently indicate maximum snow extent that exceeds 40 million square kilometers for the Northern Hemisphere.

In the Northern Hemisphere spring (April through June), snow cover extent is mainly over the Arctic, where snow blankets the ground for up to nine months a year. The timing of springtime snow melt is particularly important in terms of spring river runoff, permafrost thaw, and the length of the growing season. Springtime snow cover extent has historically fluctuated over three- or four-year cycles, but recent observations have shown long-term snow extent declines. A study published in 2012 found an overall drop in snow cover after 1967, with an acceleration of the decline rate after 2003 (Derksen and Brown 2012).

June snow cover and September sea ice extentsJune snow cover and September sea ice extents: This graph shows sea ice and snow cover extents for the Northern Hemisphere from 1979 to 2012. The thick lines are 5-year running means. Note that the snow cover extent is for June (red) while the sea ice extent is for September (gray). Although both experienced overall declines, June snow cover decline exceeded September sea ice decline. Image adapted from Derksen and Brown 2012.

Northern Hemisphere spring seasons set multiple records for low snow cover extents after the year 2008.  North America saw three record-low extents between June 2008 and June 2012, and Eurasia set a new record-low June extent each of those years. The springtime extent decline continued in 2013. North America experienced its fourth-lowest extent in June 2013, and Eurasia experienced a record-low extent in May 2013, according to the 2013 Arctic Report Card.

April-June 2013 snow-cover days anomaly mapsSpring 2013 snow-cover days anomaly maps: Areas that experienced up to 100 percent fewer snow-covered days than the long-term average (1981-2010) appear in brown, and areas that experienced up to 100 percent more snow-covered days than the long-term average appear in blue-green. Image courtesy NOAA, based on data provided by Dave Robinson and Tom Estilow, Rutgers Snow Lab.

Last updated: 6 February 2014

See Also

Snow Cover: Time series maps from NSIDC's Satellite Observations of Arctic Change

NSIDC's Snow Glossary: General and scientific terms related to snow