The map shows the annual snowfall (in centimetres) based on the 30-year period 1941-1970. Snowfall is measured by inserting a ruler into the new snow at several points to obtain its depth and to estimate the degree of drifting or scouring that has occurred. The water equivalent of snowfall for most climatologically stations is estimated by simply assuming the freshly fallen snow has a density of 0.10 gram per cubic centimetre. On the average, this is a sound approximation over large parts of the country, but variations from 0.05 to 0.15 are common from storm to storm, and in the drier regions the average density is probably closer to 0.08 gram per cubic centimetre. Since 1961 the Nipher shielded snow gauge has been used at principal stations to derive the water equivalent of snowfalls. Snow collected in the gauge is melted to obtain this value. At remote stations snowfall is measured in terms of water equivalent by automatic and storage gauges, such as the Fischer and Porter Gauge and the Sacramento