Philadelphia’s Climate in the Early Days

January, 1790 was a remarkable year in the northeastern US for several reasons.  It was less than one year into George Washington’s first term, and it was one of the warmest winter months on record.  Fortunately for science, a diligent Philadelphia resident named Charles Pierce kept a detailed record of the monthly weather from 1790 through 1847, and his record is archived by Google Books.  Below is his monthly report from that book.

JANUARY 1790 The average or medium temperature of this month was 44 degrees This is the mildest month of January on record. Fogs prevailed very much in the morning but a hot sun soon dispersed them and the mercury often ran up to 70 in the shade at mid day. Boys were often seen swimming in the Delaware and Schuylkill rivers. There were frequent showers as in April some of which were accompanied by thunder and lightning The uncommon mildness of the weather continued until the 7th of February.

Compare that to January, 2009 with an average temperature of 27F, 17 degrees cooler than 1790.  One month of course is not indicative of the climate, so let us look at the 30 year period from 1790-1819 and compare that to the last 10 “hot” years.

From Charles Pierce’s records, the average January temperature in Philadelphia from 1790-1819 was 31.2F.  According to USHCN records from 2000-2006 (the last year available from USHCN) and Weather Underground records from 2007-2009, the average January temperature in Philadelphia for the last ten years has been 29.8 degrees, or 1.4 degrees cooler than the period 1790-1819.  January, 2009 has been colder than any January during the presidencies of Washington, Adams, Jefferson, or Monroe.  January 2003 and 2004 were both considerably colder than any January during the terms of the first five presidents of the US.  …