The highest low temperatures in Washington, D.C.: A look at 80F+ overnight readings

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.

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High and low temperature by year at Washington, D.C.

Ok, I am being unbalanced with D.C. climo of late. It’s a mindless task when bored, and I recently finished a compilation. Here’s a graph (to scale, if not clearly marked as such) of high and low temperatures by year in Washington, D.C. The change in highs has been quite minor though the range is equally small. Lows, however have seemingly continually pressed upwards throughout recorded history.

The 81-2010 average for the highest temperature of the year was 98.8, with the long-term average coming in at 98.3. The 81-2010 average lowest temperature was 9.9, while long-term is 7.3.

This post is part of a Washington, D.C. climatology series, both on and off site, that will be maintained at semi regular intervals.

100 degree days at Washington, D.C., Baltimore, Md. and Dulles, Va.: Averages, extremes and what to look for

This article has been updated to cover all 100F+ readings observed through August 1, 2011 when National Airport hit 100F and the other locations remained in the upper 90s.

While the Washington, D.C. to Baltimore, Md. area averages about one 100-degree day each year, it is not a super common occurrence, and stretches of years without them are — or, someday, where? — to be expected. Yearly numbers and a few other facts for both Washington and Baltimore follow, as does a quick look at the atmospheric setup for the area’s most extreme of extreme heat.

Washington, D.C. averages 1.2 days of 100F+ or above as of the 1981-2010 climate period. The long-term average is less than one day per year, with 109 days (including five so far in 2011) at D.C. that have hit or topped 100F. The highest temperature ever at D.C. was 106F, and it was reached on August 6, 1918 and July 20, 1930. A breakdown of all days 100F+ at D.C. is heavily skewed toward the lowest numbers. 44% hit 100 on the nose, 22% made it to 101, 17% to 102, 7% to 103, %6 to 104 and 2% each to 105F and 106F. The most recent “super heat” temperature came on July 29, 2011 when D.C. reached 104F, the highest temperature observed there in over a decade and tied for 5th hottest all time.

July 19-22, 1930 make up the longest string of days 100F+ in Washington at four, also featuring the hottest temperature on record. Washington has dealt with three other 100F+ streaks of three days, most recently in 1993. The most 100-degree days in any year was 11 in 1930, when two major streaks of 100F+ lasted for the record four days and three days (with a two day break in between) each. For 10 years in a row, in 1888 through 1897, no 100 degree or higher readings were recorded at Washington. The longest streak of sub-100F years in recent times was the seven years from 1970 through 1976.

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