CRYSYS - CRYosphere SYStem in Canada

LEARN MORE ABOUT SNOW!

Voltaire's celebrated observation in 1759 that Canada was merely a few acres of snow was not far off the mark (except for the area!). Canada is indeed a snowy country with many parts of the country experiencing snow cover for 5-6 months of the year (see figure 1 below). The structure and dimensions of snow cover are highly variable in both space and time, depending on a host of factors such as the location of storm tracks, weather conditions between storms, wind action, surface topography and vegetation cover.

Figure 1: Mean duration of snow cover (days) over the 1972-94 period, as computed from satellite-derived maps of weekly snow cover extent. Source: R. Brown, Environment Canada (data supplied by D. Robinson, Rutgers University).

Why is Snow Important?

A snow cover is important for several reasons. Firstly, it represents a major store of water which is released in the spring melt period. Knowledge of how much water is contained in a snow cover (the Snow Water Equivalent, or SWE) and the rate at which it melts is critical information for flood forecasting, agriculture and for the optimal management of water resources. Secondly, snow is an excellent insulator which prevents the soil from freezing to great depths. For example, the mean January air temperature at Goose Bay, Labrador is -16.4 deg. C, while the mean January soil temperature at 5 cm depth is only -2.1 deg. C. The difference of more than 14 deg. C is due to the insulating effect of snow. Thirdly, because of its high surface reflectivity (80-90% for new snow) and insulation properties, a snow cover dramatically changes the energy exchanges between the surface and the atmosphere. Numerous studies have shown that mean air temperatures are typically 5-10 degrees C colder when a snow cover is present.

An extensive snow cover also allows cold air to move large distances without experiencing much warming. This feedback effect means that winters with larger than normal snow cover tend to be colder than normal. Because of these reasons, changes in snow cover conditions have widespread ecological and economic implications across Canada (e.g. snow industries (skiing and snow removal), agriculture (changes in snow cover & depth affect frost penetration and soil moisture recharge in the spring), water resource management (changes in the timing and amount of water from snow melt) and ecosystems (alteration of the sub-nival environment for plants and animals).

How is Snow Measured?

The large spatial variability in snow cover makes it extremely difficult to obtain reliable estimates of how much snow is on the ground. Regular daily snow depth measurements are made at several hundred climate and weather observing stations across Canada. However, these sites tend to be concentrated in populated areas at lower elevations and they are only point estimates. To improve the spatial representativeness of snow cover measurements and to obtain information on snow water equivalent for water resource planning, many government agencies and hydro-power authorities take regular (usually at two-weekly intervals) measurements of snow depth and mass over a series of points termed a "snow course". Canadian snow courses also tend to be concentrated in southern regions of Canada, but there is less low elevation bias than the daily snow depth network. An overview of the methodology and techniques for ground-based measurements of snow cover is provided by Woo (1997).

Measuring snow using snow courses is a labour intensive process and only provides a small sample of the snow cover over an area. Hence, major use of active and passive microwave satellite data has been undertaken due to their all-weather ability to map snow water equivalent over selected types of terrain. Satellite data provides more spatial information, but the length of record is typically much shorter than in situ sources. Passive microwave data are available from 1978 onwards for snow cover studies.

Figure 2 below shows a map of SWE (Snow Water Equivalent) for the Canadian prairies as derived from passive microwave data. This product is used extensively by water resource managers across the prairies. Canadian scientists are currently working to extend this method to other environments such as forest, where snow cover microwave signatures are more difficult to interpret.

Figure 2: Snow water equivalent derived from SSM/I satellite data for the Canadian Prairies, February 11, 1994.

Is Snow Cover Changing in Canada?

Like all components of the climate system, snow cover exhibits considerable variation from one year to the next in response to the natural variability of atmospheric circulation patterns which affect both snowfall and temperature. Historical and satellite data suggest that snow cover over Canada increased during the 20th Century, reaching a maximum in the mid-1970's. The 1980s and 1990s were characterized by a rapid reduction in snow cover over many regions of Canada, particularly in western Canada. The largest changes have been observed in the spring period where snow cover has been disappearing at a rate of 1-2 days per year over parts of western Canada. Further discussion of this topic is provided on the Snow Synopsis page of the new SOCC website.

What Effect will Global Warming have on Canadian Snow Cover?

Snow cover is expected to decrease significantly over much of southern Canada in response to global warming (see the SOCC's future response of snow for more detailed information). This would have significant impacts on agriculture, hydroelectricity generation, and snow-related industries. However, it is not so clear what other aspects of the snow cover may do. For example, Brown (2000) documented a long-term increase in cold season SWE over North America during the 1915-1997 period in response to increasing snowfall. Warming would also be accompanied by increased frequencies of mixed precipitation and rain-on-snow events which have implications for snowmelt, snow depth and snow density. Effective monitoring of these properties requires the integration of both in situ and remotely sensed data.

Further Snow Information on CRYSYS Education Pages:

• Is snow cover changing in Canada?
• Animation of mean seasonal variation in snow (grey) and sea ice extent (white) over North America. The animation was derived from visible and microwave satellite data provided by NSIDC.
• Temporal and Spatial Evolution of Snow: Compaction and Metamorphism (CRYSYS Education Guide).
• CRYSYS Photo Gallery: Lots of snow photographs from around the world!

Some Useful References:

• Brown, R.D., 2000: Northern Hemisphere snow cover variability and change, 1915-1997. J. Climate, 13, 2339-2355.
• Woo, 1997: A guide for ground based measurement of Arctic snow cover.
• Groisman, P. Ya, T.R. Karl, and R.W. Knight, 1994: Changes of snow cover, temperature and radiative heat balance over the Northern Hemisphere. J. Climate, 7, 1633-1656.
• Goodison, B.E. and A.E. Walker, 1993: Use of snow cover derived from satellite passive microwave data as an indicator of climate change. Annals of Glaciology, 17: 137-142.
• Robinson, D.A., K.F. Dewey and R.R. Heim, 1993. Global snow cover monitoring: an update. Bull. Am. Meteorol. Soc., 74,, 1689-1696.
• The Handbook of Snow, Gray and Male (eds.), Pergamon Press, 1981.
• The Climates of Canada. David Phillips, Supply and Services Canada, 1990.

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