June 27, 2010 - Day 9 of an 11 day heat wave. The high at D.C. was 99. Photo by author.

Even though the average high temperature in Washington never quite reaches 90 degrees (topping out at 89 during a 16-day stretch in July), anyone who’s spent time here in the summer knows it’s a pretty common occurrence.

As of the current (1981-2010) climate period, D.C. averages 36 days of 90-degree or higher temperatures during the full calendar year. The month most likely to see numerous 90-degree days is July, which is followed closely by August. A few years have witnessed only a little more than a handful of days reaching 90F or above, while others – like 2010 – seemed to never stop spitting them out.

Keep reading for more information about 90-degree days in D.C....

90 degree occurrences by month

The earliest 90-degree day on record for D.C. is March 22, 1907 when it was 90 degrees. The next day it hit 93. All three March days at or above 90 occurred that year. The latest 90F+ occurence happened October 11, 1919 with a temperature of 90. More recently, D.C.’s latest 90F+ was on October 9, 2007 when it hit 94.

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July is the most prolific producer of 90 degree or higher days, coming in with right around 14. The two other months of meteorological summer – August and June – average around 10 and seven respectively. The summer average is 31, accounting for 85% of the typical number in any year at D.C. For monthly 90F+ extremes, see the D.C. climate series monthly analyses for April, May, June, and July. Other months to come…

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Based on the historical record from 1872 through 2010, the late June through mid-August timeframe is “prime time” for 90-degree days in D.C. The graph above is a raw percentage, and would probably do well with some smoothing, but for purposes here it will suffice. The “peak” or “peaks” are pretty evident in July. Only one day in the sample – July 16 – has seen more 90-degree days than sub 90-degree days, with 73 over a 139 year period.

90 degree days by year

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When examined over time, the full-year average of 36.4 days (36 when rounded) 90F+, reveals considerable range year-to-year. This is evident not only in the most recent 30-year climatology period, but throughout the entire history of observations. On the ends of each extreme are the 67 90F+ days in 1980 and 2010 and the seven such days in 1886 and 1905. Lest we think years with few 90F+ days are a thing of the past, D.C. witnessed one of its lowest yearly 90-degree days as recent as 2004 with 11.

Heat waves

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By common definition, a heat wave is three or more consecutive days at 90 degrees or higher. During the 1981-2010 climate period, D.C. averaged just under five (4.7) heat waves per year.The extremes on this scale are the 11 heat waves in 1991 and the zero in 1886, 1907 and 2004. Keep in mind, this metric masks some important facts, as it treats all heat waves the same even though a heat wave can be three days long or many more than that.

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When it comes to the typical length of the longest heat wave of the year, the current 30-year average is just shy of nine (8.9) days. The longest 90F+ streaks happened in 1980 and 1988 when a full three weeks in a row of 90s occurred in Washington. Two other years came in with 18 days in a row – one in 1872, and more recently in 1999. 1886, 1907 and 2004 all tied with the shortest streaks in a year at two days each.

Long term trend in 90 degree days

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Around the beginning of the 20th century, D.C. averaged just over three weeks of 90-degree days per year. Climate averages peaked (at least thus far) in the 1961-1990 period, when Washingtonians expected 37 days of 90 degrees or greater. Since then, the numbers have tailed off quite slightly, mainly due to inconsequential (when rounded) decreases in the July and August averages.

-View the source data behind the graphs above (Excel)

This post, as the climate series which covers each month of the year for Washington, D.C., will be updated from time to time, either to keep it current or to add new information. Is there anything you would like to see that is not currently here?

Article last updated: July 19, 2011.