Kristi Agosto, right, holds her son Vincent, 1, as Katie Parker, left, and her 1-year-old son Jack try to cool off at Beverly Triton Beach in Edgewater, Md., on July 3. (Michael Robinson Chavez/The Washington Post)

(This story, first published Thursday evening, was updated Friday midday and Monday morning.)

Monday morning update: It remained at least 80 degrees in Washington from 8 a.m. Monday, July 2 to 11 p.m. Friday, July 6, for a total of 111 hours. The temperature finally dropped to 79 degrees at 11:59 p.m. Friday night. It ranked as the 4th longest streak at 80 degrees or higher on record in Washington.

From Friday…

After seven straight days with highs above 90 degrees, Washington’s week-long heat wave is ending. The heat combined with the humidity has been punishing, feeling as hot as 110 degrees. But the round-the-clock lack of relief from the heat has proved most notable.

For 100 hours now, starting at 8 a.m. Monday, the temperatures has remained at or above 80 degrees. This lack of cooling, and string of sultry nights, ranks among the most extreme on record.

Thursday marked the third straight calendar day with temperatures remaining above 80 (unless the temperature falls unexpectedly into the 70s before midnight), which rank among the top seven longest stretches on record, dating back to 1872.


Top 10 longest daily streaks of minimum temperatures of 80 or higher in Washington. (xmACIS2)

All of the 10 longest periods above 80 degrees have occurred since 1999, and eight out of 10 since 2010.

In 2013, the temperature remained at least 80 for 138 hours. The second longest streak, in 2016, it remained that warm for 133 hours. The third longest streak in 2011 lasted 128 hours. The current streak is likely to end around 108 hours, Friday evening.

Because of low temperatures of just 80 Tuesday and Wednesday, Washington notched record warm minimum temperatures both days. Tuesday’s record surpassed the previous mark of 79 set in 1898 and 2002, while Wednesday’s muggy low tied 2002.

Lows of 80 degrees and higher, now commonplace, were once very rare. They occurred just 26 times from 1872 to 1999 or about once every five years. Since 2000, they’ve happened 37 times or twice every year on average.


(Ian Livingston)

The proliferation of these extremely high low temperatures is the expected result of climate warming in Washington, both from urbanization and increasing greenhouse gas concentrations in the atmosphere.

The high low temperatures have consequences as they signify limited nighttime cooling which, for people without access to air-conditioning, can be extremely taxing on the body and the cause of heat-related illness.

A big cold front sweeping through the Washington region Friday will put an end to this extended sauna-like stretch. Much lower humidity and cooler temperatures arrive this weekend. Low temperatures will sink into the 60s and even upper 50s in some of the Washington region’s cooler areas.

Ian Livingston contributed to this report.