This map shows the median date of snow-cover onset (defined as the first date with 14 consecutive days of snow cover greater than 2 centimetres in depth) computed over 18 winter seasons (1979 to 1997). In areas with permanent or semipermanent snow cover (for example, Arctic ice caps) or in areas with irregular or ephemeral snow cover (coastal British Columbia), researchers were unable to compute the median values. The main feature of the map is the rapid southward extension of snow cover over Canada during the September to December period. The moderating influence of Hudson Bay can be seen over northern Quebec, where snow cover starts later than in the equivalent latitudes west of Hudson Bay. The date of snow-cover onset has important implications for animals that rely on snow cover for shelter, as well as climatic influences on air temperature, ground freezing and lake ice formation.