Snowfall

National Climatic Data Center List of Annual Snowfall for Various Cities

Note that the leftmost data column indicates the number of years of data used to calculate the averages. They are generally NOT the same years used to calculate the 30 year averages that the National Climatic Data Center also determines. The 30 year averages are running averages updated every 10 years, adding the most recent decade and dropping the oldest decade. They are more indicative of shorter-term climatological trends than averages calculated for longer periods of time such as 60 or 70 years.


When is the average snowfall not the amount to expect?

For most people, the average represents a value obtained by taking a set of values, adding them, then dividing by the number values. In stricter statistical terms this is the "mean". An average can be any of several measures of central tendency, including the "median" or the "mode". The median is a value such that half the numbers are above that value and half are below it. The "mode" represents the most frequently occurring value. The median is often a better representation of an "expected value" than the mean since the mean is more sensitive to the affects of outlining or extreme values.

If the mean seasonal snowfall for a location is 60 inches but the median is 65, then the majority of the seasons in the climatological record experienced above average snowfall. You would thus "expect" to get 65 inches per season rather than 60 but you would also expect to have an occasional year where snowfall is severely lacking. These much lower values skew the average down from the amount that would typically occur.

The opposite condition as just previously described occurs if the median were 55 rather than 65. Most seasons would feature below average snowfall with a fewer seasons producing well above average snowfall.

The "Frequently Asked Questions" section on the National Climatic Data Center's Climate Normals page provides a good discussion about what normals really are and how they are not necessarily indicative of the weather you should expect. See in particular the entries entitled "What is a Climate Normal?" and "Is a Normal the Climate You Would Expect?".


Probability of a "White Christmas"

The National Climatic Data Center has produced a map showing the probability of a white Christmas. A white Christmas is defined as one with one inch or greater of snow depth on the ground. What a rip-off if you live in the southern United States.