This post has been updated to cover all 80-degree or higher lows in Washington through early August 2011. Graphs will be updated to include this year once it is apparent there is no risk of further 80-degree lows.
The once rare 80F+ low has been appearing more often of late. 2011 has easily turned into the leader of the pack on consecutive days and overall totals. Prior to 2011, there have never been more than two 80 degree or higher low temperature days back-to-back at Washington. A previous record streak of two days happened most recently in 2010, when DCA recorded 80F for a low on the 7th and 8th of July.
From July 21-24, 2011 D.C. recorded its warmest stretch of lows ever, with four days in a row 80F+. Temperatures actually spent about 128 hours above 80F, from 7:52 a.m. on July 20 through just after 4:00 p.m. on July 25. In the climate records, this will be counted as a four-day stretch — even though the hours match up to five days — due to the 80F morning low of the 25th not standing through midnight. As an encore performance, 80-degree plus lows returned for the final three days of July 2011, bringing the record total to seven for the month and year.
The warmest low on record at D.C. is 84F and it occurred on July 23rd and July 24th, 2011 as well as on the 16th of July in 1983. There have been 42 days (including the seven in 2011) 80F+ lows going back to when daily records I have access to begin in 1872. Before 1930, there were only 3, and they all happened in 1876. The sample and averages are still quite small, with a 1930-2010 average of 0.4 days per year and a 30-year average of 0.7. But looking at the graph, and now considering 2011, one gets the sense these high-end low temperatures are becoming more of a norm, if also still erratic.
The trends do continue, and in some ways are more apparent thanks to increased data samples, into categories such as 75F+ lows and similar. In an urban environment like D.C. the question arises whether the increased frequency of such lows points to larger climate change or, more simply, a growing urban heat island effect. It is worth at least noting that Baltimore’s climate record includes much more numerous 80F+ readings during the earlier history where D.C. lacks them. In part, this may be due to the Baltimore station initially being in the city prior to moving to a more rural setting at BWI.