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If the 1981-2010 temperature normals for Reagan National Airport are taken at face value, DCA temperatures on average bottom out during January 12th-17th (at a mean of 35.7 degrees), then rise gradually (to 35.9 degrees on January 21st, 37.2 degrees on February 5th, and peak between July 12th-21st at 80.0 degrees). However, January daily temperatures during this period -- and even more so during the most recent 30 years, 1984-2013 -- appear inconsistent with those normals. During the past 30 years, the first 14 days of January at DCA have each averaged above the 1981-2010 normals, whereas January 19th-23rd have each averaged below those normals. This may be illustrated by comparing January 13th to January 21st average temperatures during the past 30 years at DCA, as well as looking at the historical DC temperature record for those two days and the coldest average day of the year. For the first 112 years of official DC daily temperature history -- 1872-1983 -- January 13th was the coldest January day, at 33.3 degrees on average; and January 21st was the second warmest January day, at 36.8 degrees on average. The January temperature peaked on the 22nd at 37.2 degrees on average during 1872-1983, with the annual mean temperature bottoming out on February 5th at 32.6 degrees on average. However, during 1981-2010, the mean temperature at DCA on January 13th was 38.7 degree on average, decreased sharply to an annual minimum of 32.2 degrees on average on the 21st, and rose about halfway back to 35.3 degrees on average on February 5th. Further, the disparity with 1872-1983 has widened during 1984-2013, with January 13th the warmest January day of this most recent 30-year period at 40.3 degrees on average, January 21st the coldest day of the year at 32.1 degrees on average, and February 5th the coldest February day (29 years through 2012) at 35.9 degrees on average. To recap, January 13th has gone from being the coldest January day at 33.3 degrees on average during 1872-1983 to the warmest January day at 40.3 degrees on average during 1984-2013 -- an increase of 7.0 degrees. January 21st has been the mirror image of January 13th -- going from the second warmest January day at 36.8 degrees on average during 1872-1983 to the coldest day of the year at 32.1 degrees on average during 1984-2013 -- a decrease of 4.7 degrees. Finally, February 5th has gone from being the coldest day of the year at 32.6 degrees on average during 1872-1983 to merely the coldest February day at 35.9 degrees on average during 1984-2012 -- an increase of 3.3 degrees. The increase in temperature on February 5th between these two periods has been pretty clearly within the range of what would be expected in an era of warming temperatures, as February temperatures 1984-2012 have been 2.9 degrees warmer on average than during 1872-1983. But what about the January temperature record, and in particular, the contrast between January 13th and 21st during these two periods? Is that merely an aberration, or is a more fundamental change taking place that is not reflected in the current temperature normals?
