Snow cover data refer primarily to the presence and total depth of a snow cover on the surface of the earth. This is in contrast to data concerning characteristics of freshly fallen snow. Snow cover depth is increased by the occurrence of freshly fallen snow, but it is decreased by melting, wind action and settling. Two maps on this plate show the mean dates of the occurrence of the first and last snow covers of one inch (2.54 cm) or more. These dates are not necessarily the average dates of the beginning and ending of a continuous snow cover since the snow cover may form and later disappear once or several times during a winter season. A third map showing the mean annual number of days with a snow cover of one inch (2.54 cm) or more only includes those days on which there was a snow cover. For the last map, the mean annual maximum depth of snow data were obtained by averaging the maximum depth reported for each snow season of record. Snow cover data are mainly based on the ten-year period from 1941 to 1950 inclusive.
