Thursday, February 21, 2013

Colder Early Winters in Fairbanks?

Were only a month away from the vernal equinox now, with possible sunshine increasing at nearly seven minutes a day. Nonetheless, today we'll look back to the dark days of early winter. Fairbanks has had a run of cold early winters, or maybe better, early winters the past few years have featured more cold weather than before that. Is this something new? One way to assess this is simply to examine the number of cold days. Here I've plotted the numbers of days October through December with low temperatures of 20 below or lower. I chose 20 below because temperature that "warm" should not be impacted too much by the urban heat island growth. 
What we see here is this past early winter, with 36 days of lows of 20 below or lower before New Years Day, was the third highest total since 1930. The red line represents the 5-year running median, which is now back up to values common in the mid 1990s and during the past negative PDO phase (transitions between phases marked by the black lines). The plot of lowest absolute temperature during early winter does not paint the same picture:
This clearly shows that absolute low temperatures have been increasing. Prior to 1980, temperatures of 50 below or lower did not occur every early winter, but were by no means uncommon. Since 1980…just one year, 1999. This however must, in part, be attributable to increase in urban effect, which is greatly amplified by the formation of ice fog at temperatures colder than -35F. 

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