July heat in Washington was unprecedented

Jason Samenow
Weather editor July 31, 2011

Relentless and punishing, July’s heat was unrivaled in 140 years of Washington, D.C., weather record-keeping. The July temperature averaged 84.5 degrees at Reagan National Airport — Washington’s official weather station — more than a degree above July 2010 and July 1993, which previously held the mark for hottest month.

July’s heat piled onto a torrent of recent hot-weather records and attained all sorts of unenviable hot-weather milestones.

Jason is currently the Washington Post’s weather editor. A native Washingtonian, Jason has been a weather enthusiast since age 10. View Archive

The high temperature was at least 90 on 25 occasions, the most on record. When the July 29 temperature hit 104 degrees, it was the highest reading since 105 on August 17, 1997, and it tied for the fifth-hottest in the books. On July 22, the heat index — a measure of the combined heat and humidity — reached 121 degrees, the highest level since 122 on July 16, 1980.

Amazingly, the month also had eight record days for warm low temperatures, including seven when the temperature failed to fall below 80 degrees. Four of those days came consecutively (from July 21 to 24), the longest such stretch on record by two days.

Twice (July 23 and 24) the District tied for its warmest all-time low temperature of 84 degrees.

And the heat was off the charts at other weather stations in the region.

On July 22, Dulles International Airport soared to 105, its hottest temperature since records began in 1963. That same day, Thurgood Marshall Baltimore-Washington International Airport sizzled to 106, the second-hottest temperature in Baltimore’s weather records, which began in 1871. Also on the 22nd: The water temperature at Little Falls on the Potomac River surged to a bath water-like 96 degrees, higher than any reading since measurements began in 1988.

The heat milestones reached in July add to numerous stunning warm weather records during the past 18 months.

In 2010 and 2011, the District has experienced its warmest spring, two of the top three hottest Junes, the hottest June day, the hottest two Julys, the hottest summer, the earliest 100-degree reading in a day, the longest stretch of temperatures above 80 degrees, the longest uninterrupted stretch above 100, and the hottest days so early and late in the season.

What’s the cause?

During both of the past two summers, a sprawling area of hot high pressure — sometimes referred to as a heat dome — has stagnated over an expansive area extending from the south central United States into the mid-Atlantic. The reason for its placement and persistence is difficult to pin down, but it might be linked to La Nina, a Pacific weather pattern that has waxed and waned over the past year.

But long-term temperature trends in our region suggest man-made factors are almost certainly playing a role in the heat’s intensity.

Since the late 1800s, the average number of days per year at or above 90 has increased from 25 to 36, almost 50 percent. The average number of days per year with low temperatures at or above 75 has increased from about three to 14 in this same span. Washington closed out July with 13 such days consecutively.

There were seven days in July with low temperatures of 80 or higher. During the entire period from 1871 to 1930, only three such days occurred, all in 1876.

The unusual nighttime warmth also has occurred across the country. During the past 30 days, the nation has experienced nearly 6,000 record warm low temperatures and just more than 400 record cold low temperatures.

The two primary suspects for the warming trend are urbanization and the increasing concentration of greenhouse gases in the atmosphere from burning fossil fuels. Regardless of the reason, evidence is mounting that Washington’s summer climate is turning markedly less hospitable.

The U.S. Global Change Research Program projects substantial increases in the frequency and intensity of extreme heat across the United States because of the buildup of greenhouse gases in the atmosphere.

Although a general warming trend is forecast, each successive year will not necessarily be warmer than the prior one. Cool summers are still likely to occur. As recently as 2004, Washington experienced just 11 days reaching 90 or higher.

Greenhouse gas warming and urbanization, however, have loaded the dice for more summers like the current one, and quite plausibly worse.

The Capital Weather Gang’s Ian Livingston contributed to this report.

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