Category: "Weather and Climate"

Frost Days and Ice Days: Declining Numbers over the Century

January 17th, 2013

David Easterling recently reported in BAMS** that the number of frost days per year is decreasing over the US. A frost day is a day in which the minimum temperature goes below the melting temperature of ice (32 F or 0 C). This doesn't sound good for a frost observer like me. Moreover, the largest decrease, a value of 2.6 fewer days per year per decade, is in my area, the Pacific Northwest. Will frost soon be a threatened species?

His analysis and presentation was based on 1948-1999 data, and moreover, he averaged over multi-state regions. I'm more interested in my local area, King County, Washington, and would like to see both the longer-term trend and variability. I quickly sent him an email, requesting more info. But I was impatient and sought the raw data myself. Following the info in his article, I found the data online***. An added plus: at many stations, the data now goes back another 50 years. With a few simple commands typed into an Excel spreadsheet, I determined the longer-term trend for a station near the University of Washington stadium in Seattle (the only such site in Seattle).

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