Thursday, February 19, 2009

Washington, DC Snow: Statistics of Low-Frequency Events

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Image (click to enlarge): Frequency of 1" or more of snow in Washington, DC by date in February, 1930-2007, CapitalClimate chart from NWS data

Much attention has been given in the global warming deniosphere to rare events such as the recent UK snows and an all-time record low temperature in Maine. Aside from the facts that
  • snow doesn't necessarily correlate with temperature
  • climate is a long-term average in which all of the extremes are reduced to a small residual,
the occurrence of rare events is statistically random, within the limits of physical possibility. The general rarity of snow in the Nation's Capital doesn't prevent the snow day fanboy population in and around the Beltway from agonizing over the prospects for the white stuff and speculating over when to finally stick a fork in winter and get a life. Generally, this occurs by early June, at which time discussion turns to the outlook for the following winter.

Clearly, by late February the average temperature trend has begun to turn upward, but what is the effect on snow chances? The chart shows the frequency of 1" or more of snow falling on any given date in February over the last nearly 80 years (1930-2007) in Washington. The peak of 9 occurrences occurs near the middle of the month on the 11th, but the preceding and following days have frequencies of 2 and 3, respectively. On the other hand, by the end of the month, snow has been observed 5 times on the 28th, which is 5 times as often as the mere 1.4" which fell on Feb. 21, 1993.

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