The Hottest Time of the Year

The second half of July and first half of August is traditionally the hottest time of the year for much of the United States.  Washington, D.C. hottest temperature on record (106°) has occurred twice: once on July 20, 1930 and August 6, 1918.  The National Weather Service is responsible for keeping weather records for the United States.

The Nation’s Capital averages 36 days of 90°/+ temperatures in a given year, with the majority coming during meteorological summer from June 1 through August 31.  So far this year, Washingtonians have sweat through 27 days of 90° heat.  The record for highest number of 90° days in a given year is 67 that occurred twice (1980, 2010), while the lowest such total was seven most recently in 1905.  Although there were 58 days that reached or exceeded 90° in Washington, D.C. last year, only five such days occurred through July 4.

Residents of the DC Metro Area are well-positioned to experience another summer with a higher than average number of 90° days.  A “heat wave” is defined as any stretch of three or more consecutive days with temperatures of at least 90°.  The longest heat wave so far this summer has been six days.  However, the current heat wave, that began July 17, is poised to continue through at least next Monday, which would make it the longest heat wave of 2017.  The longest heat wave of 2016 was 13 days from July 21 through August 2.  

Last summer was the third hottest on record in the Nation’s Capital.  This summer has seen a significantly higher number of 90° days to this point (26) than last year (16).  However, there were also four days of triple-digit heat in the Nation’s Capital last year and there haven’t been any so far this year.

While no 100° heat has occurred yet this summer, there will be several chances to reach the century mark during the next several days.  Whether or not this happens, this summer has already had the tenth hottest June on record (average monthly temperature of 77.3°) and the sixth warmest July (82.8°) through July 19.    

Record Highs (Degrees Fahrenheit)  Source: NWS

July 20

DCA: 106° (1930) --> Tied with August 6, 1918 for hottest DC temperature on record
IAD: 101° (1980)
BWI: 102° (1930)

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July 21

DCA: 104° (1926)
IAD: 101° (1991)
BWI: 104° (1930)

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July 22

DCA: 103° (1926)
IAD: 105° (2011)  --> Hottest temperature on record at Dulles Airport
BWI: 106° (2011)

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July 23

DCA: 102° (2011)
IAD: 99° (2011)*
BWI: 102° (2011)*

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July 24

DCA: 101° (2010)
IAD: 99° (2010)
BWI: 101° (2010)

*= also occurred in previous years

Summer’s Midway Point

Nationals Park, Washington, D.C.

Now that we are at the unofficial halfway point of the 2017 summer season, we can reflect upon how this summer compares to average so far.  Meteorological summer runs from June 1 through August 31.  True to form, the month of June and the first half of July have featured plenty of above average temperatures.

June 2017 finished 2.1° warmer than average in the Nation’s Capital and included 8 days of at least 90°.  Last month was also exceptionally dry in Washington, D.C. with only 1.13” of rain at National Airport.  That made for the third driest June on record in the Nation’s Capital.  The dry month of June that Washingtonians experienced has been part of a larger weather pattern with 19 of the last 23 months being drier than average since August 2015.  Had May not been appreciably wetter than average, DC residents would have endured a Top 15 driest first half of the year.

July has gotten off to a hotter and wetter than average start in Washington, D.C. with 12 days of 90° heat through July 16.  There have been a total of 23 days of 90° so far this year.  By comparison, the Nation’s Capital didn’t experience day 23 day of at least 90° until July 26 last year.  There has already been 3” of rain so far this month in the Nation’s Capital and that’s just under the total of 3.13” that occurred in all of last July.

The relative humidity has made it feel much worse.  Wind direction plays a critical role in both temperature and relative humidity in the Nation’s Capital.  Winds on the hottest days in Washington, D.C. tend to be westerly, meaning they blow from west to east, while the more humid days have southerly winds.  

There was a good example of just how big a difference that a change in wind direction can make in temperatures and relative humidity this past Friday.  On July 14, the 1 PM temperature at National Airport was 92° with a dewpoint temperature of 78° and a south wind of 10 mph.   However, at 2 PM, the winds shifted to W at 16 mph with gusts to 23 mph.  The change in wind direction helped temperatures jump to 97° and the dewpoint drop to 71°.

Weather Quiz

True or False.  Three of Washington, D.C.’s five warmest July’s on record have occurred since 2010.

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Answer to June 15 Weather Quiz question.

True.  On June 15, 1994, the 101° temperature set a record high for the date, but was June’s third hottest temperature on record according to the National Weather Service.

Hottest Weather of the Summer?

Washingtonians are in the midst of the hottest weather since last August.  Today is poised to be the hottest day of the current heat wave with temperatures reaching the upper 90s in much of the DC-Baltimore corridor.  A “heat wave” is defined as any stretch of three or more consecutive days at or above 90°.  So far this year, Washingtonians have sweat through 20 days of at least 90°.  The Nation’s Capital didn’t experience day 20 of 90° last year until July 23.  

Sultry conditions in the DC Metro Area are common during the summer months.  Making the above average temperatures even worse is the high relative humidity that will help create a dangerous heat index of greater than 105° in many spots this afternoon.  That’s prompted the National Weather Service to issue a heat advisory for areas near and east of I-95 in the Mid-Atlantic Region today.

Heat advisories have been fairly recent occurrences in the last 10 years, with multiple hot summers and heat waves (particularly from 2010 – 2012 and in 2016).  Washington, D.C. averages 36 days of at least 90° in a given year, while the record is 67 such days.  For example, there were 58-90° days last year.  Days with temperatures of at least 95° are less common and 100° is rarer still in the Nation’s Capital.  That’s what makes the current heat wave so significant.  

Since weather measurements for the Nation’s Capital are taken at National Airport on the Potomac River, the wind direction sometimes plays an important role in temperatures.  If the wind blows out of the south at National, then that’s off the water and can keep temperatures a little cooler than they would be elsewhere.  A south wind would contribute to slightly higher dewpoints and relative humidity.  On the other hand, drier air heats up and cools off more than humid air.  So if the air is slightly drier, than the chances increase for 100° temperatures at National Airport.  While today’s record high at National Airport is 100° from 1954, the last time it was 100° there was August 15, 2016.

Area residents know that very humid conditions are common in this part of the country during the summer months.  That can also contribute to heavy rainfall.  Following the third driest June on record, DC had 2.27” of rain so far this month.  That’s more than half as much as the 3.13” of rain that occurred during all of last July.  More scattered showers and thunderstorms are likely tomorrow across the Mid-Atlantic Region.  Although this weekend will be less humid, 90° temperatures and the current heat wave can continue through Sunday.