The three maps on this plate show the mean annual number of days with measurable precipitation, the mean annual number of days with measurable snowfall, and the variability of annual precipitation. A day with sufficient measurable precipitation (a precipitation day) is considered as a day on which the recorded rainfall amounts to one one-hundredth of an inch (0.0254 cm) or more, or the snowfall measured is one-tenth of an inch (0.254 cm) or more. At any one location the annual precipitation may vary considerably from one year to the next. This variability of annual precipitation is expressed in terms of the coefficient of variation. This coefficient is obtained by dividing the standard deviation of the annual precipitation by the mean annual precipitation.
