Dates of formation and loss of snow cover

The map shows isolines on four different dates: 1) the day of the year when there are less than or equal to 2.5 centimetres of snow that occurs and remain absent for 7 or more days; 2) the standard deviation of the mean date of snow cover loss; 3) the day of year when greater than or equal to 2.5 centimetres of snow occur and remain for more than 7 days; and 4) the standard deviation of the mean date of snow cover formation. The beginning and end of the winter season in Canada is usually identified with the formation and disappearance of snow cover in autumn and spring respectively. The duration of snow cover varies considerably from year to year in the southern fringes of the country and adjacent to the east and west coasts. However, even in northern continental areas early and late snowfalls which leave an ephemeral snow cover are common. The beginning and end of the Canadian snow season is the earliest and latest dates when 2.5 centimetres or more snow was measured. However, this definition is difficult to apply to the Prairie Provinces, where snow cover is relatively shallow and is subjected to sudden melting and new accumulations at the season extremities. For the Prairie Provinces, the definition can be modified by specifying that the initial cover of 2.5 centimetres (or more) should remain for at least 7 days, and that the date of loss would occur at the beginning of a 7-day period with less than 2.5 centimetres on the ground in spring. This definition was adopted for the present map. Snow cover data have been regularly measured at principal meteorological stations since 1941. The present map is based on the 1955-1972 period. Unfortunately, records prior to 1955 are not available in the form required for the selected computer analysis. To generate data for the map, a total file of over 500 meteorological station records was accessed and the means, standard deviations, and other arithmetic data were calculated. The mean dates of formation and loss and their standard deviations were plotted on a 1 : 5 000 000 scale orographic base map to guide isopleth interpolation.