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Historical Variations in Snow Cover
This section deals with the past variability of snow cover extent (SCE) and snow cover characteristics (e.g. first and last dates of permanent snow cover) using both satellite and in situ (surface-based) data sources. The main focus is on Canada and North America, but data are also included for the Northern Hemisphere. Satellites have provided the capability for monitoring snow cover extent over the Northern Hemisphere over the past ~30 years. Surface-based snow observations go back to the early 1900's in some countries, but the spatial coverage of the networks is usually limited to populated areas. In North America, the network of historical snow depth observation in sufficiently dense to permit continental-scale SCE to be reconstructed back to the early 1990s for the November-April period (Frei et al, 1999; Brown, 2000).
The following material is organized into two major sections. The first section presents time-series information on continental-scale snow cover variability for North America and the Northern Hemisphere derived from satellite and reconstructed information, and regional-scale information on variability in snow cover characteristics for 8 regions of Canada based on the daily snow depth network (from 1955). The second section provides information on trends in continental-scale and regional-scale snow cover. Either scroll down the page or choose an option from the dropdown list.
References
- Brown, R.D., 2000: Northern Hemisphere snow cover variability and change, 1915-1997. J. Climate, 13, 2339-2355.
- Frei, A., M.G. Hughes and D.A. Robinson. 1999. North American snow extent: 1910-1994. Int'l. J. Climatology, 19, 1517-1534.
1) Time Series Information
a) Annual Variations in NA Seasonal SCE from Satellite Data, 1972-:
The following plots show the annual variation in seasonal (Winter-DJF, Spring-MAM, Summer-JJA & Fall-SON) snow covered area (million sq. km) over the North American continent (including Greenland), as estimated from satellite imagery. The data were obtained from the NOAA Climate Prediction Center. Note that values after May 1999 are experimental.
b) Running 12-month Means of Northern Hemisphere SCE, 1972-:
This plot provides 12-month running means of satellite-derived SCE for North America, Eurasia and the Northern Hemisphere. The 12-month mean removes the seasonal cycle and highlights longer-term variability in the data series. Over the 1973-1999 period, annual snow cover has exhibited significant decreases over both continents. Data supplied by Dr. David Robinson, Rutgers U.
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c) North America - Satellite and Reconstructed SCE, 1915-2000:
Annual variability of North American (NA) Snow Cover Extent (SCE) is shown below for the period from 1915 to 2000. The plots were derived from a combination of station (1915-1971) and satellite (1972-2000) information. Snow cover extent variations during the pre-satellite period were derived by gridding daily snow depth observations from Canada and the U.S.A., and computing the area with snow depths greater than or equal to 2 cm using a fixed grid network. The computed snow cover index agreed closely with satellite estimates of NA snow cover variations over the 1972-1992 period for the six month period from November to April.
The plotted open diamonds depict anomalies for the station-derived snow cover index; solid diamonds are SCE anomalies derived from the NOAA satellite data; and the heavy solid lines are the result of passing a 9-year binomial filter to the combined station (1915-1971) and satellite (1972-2000) anomaly series. Standardized anomalies were computed with respect to a common 1972-1992 reference period. For further information see Brown (2000) "Northern Hemisphere snow cover variability and change, 1915-1997" J. Climate, 13, 2339-2355 .
d) Canadian Regional Snow Cover Variability, 1955- :
The following regional series were generated by averaging normalized values of snow cover statistics from all stations within a defined area. The lat/long boxes used to define each region are provide at the bottom of Regional Trend Summary Table. A wide range of snow cover properties are being monitored to determine how the snow cover is responding to a changing climate. Note that the regional series have been filtered to more easily differentiate the various regions.
e) Eastern and Western Canada:
2) Snow Cover Trend Information
a) Contour Maps of Trend in North American Monthly SCE, 1972-2000:
These maps provide contour plots of trend in NOAA satellite-derived snow cover since 1972 (see example for May). The maps were created by contouring gridpoint "Student t-test" values of the linear trend in snow cover fraction over the 1972-2000 period. Values < -2 (red-brown colours) indicate significant local reductions in snow cover, whilst values > +2 (dark blue) indicate significant local increases in snow cover during the 1972-2000 period. Values were only computed for areas where 2/3 of the values in the 1972-2000 period were > 0 or < 100% snow covered.
Monthly Plots:
b) Contour Maps of Snow Cover and Snow Depth Trends for Canada (1946-1995):
Trends were calculated from station data for the period from 1946 to 1995. Increasing trends are shown as cross-hatched contour lines and decreasing trends as regular lines. The filled squares show the locations of the stations included in the analysis (they were required to have at least 40 years of data). Dark shading is used to highlight areas where changes are locally significant (probability p < 0.05). Light shading is used to highlight areas where station exhibited marginally significant changes (0.05 < p < 0.10).
Seasonal Snow Cover Duration:
Monthly Snow Depth (November to April only):
c) Trend in Continental Snow Cover Extent (SCE):
The table below presents trends in NA Monthly SCE for three different periods: the entire period of available information (1915-), the period of satellite coverage (1972-) and the last 10 years. Note how results change based on the length of the period selected. The trends are computed from reconstructed SCE up to 1971 and from NOAA satellite-derived values from 1972. Trend units are million sq. km per year. Reconstructed values are only available for the 6 month period from November-April (updated August 2000). Statistically significant values (0.05 level) are highlighted / asterixed.
| Month |
1915-2000 |
1972-2000 |
1991-2000 |
| January |
.0032 |
-.0140 |
-.0558 |
| February |
.0018 |
-.0292 |
-.0012 |
| March |
.0000 |
-.0274 |
.0309 |
| April |
-.0004 |
-.0405 |
.0000 |
| May |
n/a |
-.0445** |
.0352 |
| June |
n/a |
-.0569** |
-.0570 |
| July |
n/a |
-.0584** |
-.0333 |
| August |
n/a |
-.0426** |
-.0167 |
| September |
n/a |
-.0200** |
-.0617 |
| October |
n/a |
-.0170 |
-.0750 |
| November |
.0111** |
.0082 |
-.2850** |
| December |
.0071 |
-.0045 |
-.1100 |
d) Trends in Canadian Regional Snow Cover:
Summary Table of trends in Canadian regional snow cover over the period 1955/56 to 1998/99. Includes First Date of Permanent Snow Cover, Last Date of Permanent Snow Cover, Maximum / Mean / Standard Deviation / Median Snow Depths, and the Number of Days during the Year when the Snow Cover Duration (SCD) exceeds 2 cm. Methodology and definition of regions are included below the table. | |
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