Dangerous July Heat in the DC Metro Area (Source: WUSA9) |
The “heat index” is what
people feel when air temperatures are combined with relative humidity. It will be between 105° and 115° each
afternoon through Sunday and that’s as dangerous as it gets. The last
time any triple-digit heat occurred in Washington, D.C. was in August 2016. That was also the last time DC residents experienced triple digit
heat on consecutive days (August 13 – 15).
DC’s hottest temperature
in all of 2018 was 98° on July 16. Meanwhile, so far this year the
hottest observed temperature in the Nation’s Capital was 96° on June 29.
Temperatures are poised to exceed that on each of the next three days. It’s also worth noting that having three
consecutive days of high temperatures ranging from 95° to 99° is also rare in
the Nation’s Capital. The last time that occurred was July 19 – 22, 2017.
Although weather records
in Washington, D.C. date back to the early 1870s, they have been kept in two
different locations. Since National Airport opened during World War II,
weather measurements have been made there. Prior to that, they were made
downtown. This is significant since National Airport’s location on the
Potomac River has a moderating influence on temperature. For example, if
a south wind is blowing at National then the wind is blowing off the
water. During the summer that keeps temperatures slightly cooler than
areas away from the River.
It’s also interesting to
note that DC’s hottest overall temperature of 106° occurred twice prior to
National Airport opening: once on July 20, 1930 and again on August 6,
1918. Not only was that a downtown temperature, but Washington, D.C. was
much less developed then. There wasn’t
as much of an urban heat island effect at the time, making those days of
triple-digit heat even more impressive. Today,
overnight low temperatures don’t fall as much as they used too. That’s the result of a much stronger urban
heat island effect as the Nation’s Capital has become more developed combined
with the fact that National Airport is located on the river.
That said it will be
interesting to observe how high temperatures go the next three days. Regardless of how hot it gets and whether or
not any records are set, this weekend’s weather will be dangerously hot.
Record
High / High Low Temperatures (Source: National Weather Service)
July 19:
Washington, D.C., National Airport (DCA): 102° (1930) / 81° (2013)
Dulles Airport (IAD), Sterling Virginia: 98° (1977) / 77° (2013)
Baltimore, MD (BWI Airport): 103° (1930) / 80° (1942)
July 20:
DCA: 106° (1930) – DC’s hottest July temperature on record / 82° (2015)
IAD: 101° (1980) / 75° (2015 – has also occurred in previous years)
BWI: 102° (1930) / 80° (1930)
July 21:
DCA: 104° (1926) / 82° (1987)
IAD: 101° (1991) / 77° (1987)
BWI: 104° (1930) / 80° (1930) – BWI’s hottest July “low” temperature
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