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Is D.C.’s weather station reading way too hot?

The sunset over Reagan National Airport and Crystal City. (Joseph Gruber via Flickr)
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(Update, 10:30 a.m. Thursday: Pat Michaels, former Virginia state climatologist and now director of the Center for the Study of Science at the Cato Institute, has posted a very interesting and entertaining commentary on this matter: Too Hot in Washington: A Climate Mystery?)

Original post from 12:11 p.m. Wednesday

Since at least 2014, the temperature readings observed at Reagan National Airport, the District’s official weather station, have behaved in a perplexing and, perhaps, suspicious manner.

We have long understood Reagan National to be one of the warmest spots in the region as it sits near sea level adjacent to the Potomac River and is surrounded by urban infrastructure — in a so-called “heat island.” But, over the past 19 months, its temperatures have registered even warmer compared to surrounding areas than usual.

No one, to our knowledge, has provided a satisfying explanation for the airport’s temperature behavior.

At times, when other local temperature readings have been cooler than normal, Reagan National’s measurements have been warmer than normal.

Take, for example, July. Reagan National logged a warmer-than-normal month by about 2 degrees, while Washington Dulles International Airport was about a degree cooler than normal and Baltimore-Washington International Marshall Airport was near normal. Robert Leffler, a retired National Weather Service climatologist who staffs a cooperative weather observing site in Damascus, also reported July to be about a degree cooler than normal.

[Recap: July was toasty and juicy in Washington, D.C.]

Reagan National’s 14 days at or above 90 degrees in July, by far, exceeded the count at any other station in the D.C. area, not to mention the broader Virginia-Maryland region. (Locally, the U.S. National Arboretum had 11 days reaching at least 90 degrees, BWI seven, Upper Marlboro seven, Patuxent River four, Manassas four, Dulles three, Beltsville three, Oxon Hill two and Damascus one.)

[D.C. just reached its typical annual number of 90-degree days with plenty of summer to go]

Reagan National’s warmest low temperature in July of 82 degrees was about as warm as anywhere on the East Coast, according to Leffler, who compiled a quick comparison of Reagan National’s low temperature compared to other locations.

“[Reagan National’s] highest daily minimum of 82 was the highest I found in a quick check, equal only to Miami, Fla., one of the east’s notoriously hottest places for morning temperatures,” Leffler reported. “[It] was warmer than all the other notoriously uncomfortable eastern cities including Tampa, Fla. (81), New Orleans, La. (80) Charleston, S.C. (78), and Savannah, Ga. (76).”

But what suggests a potential problem with the temperatures measured at Reagan National is that they are sometimes out of step with the overall climate patterns affecting the local region.

A given climate pattern should more or less have the same effect on weather stations in the D.C. region. If it’s a cool pattern, it should produce below normal temperatures at local reporting stations — not above normal temperatures as we have seen several times at Reagan National since January 2014. Monthly temperature differences from normal or “anomalies” between neighboring stations should be comparable.

From the mid-1980s to December 2013, Reagan National and Dulles had fairly similar monthly anomalies, almost always within a degree of one another. When Reagan National had a warmer-than-normal month, so did Dulles most of the time. Again, this is what one would expect.

But from January 2014 through July this year, Reagan National’s monthly temperature difference from normal or “anomaly” has averaged 2.2 degrees above Dulles’ – a fact pointed out to me by former Virginia state climatologist Pat Michaels. That is a bit of a red flag signaling Reagan National’s temperature may be reading too warm.

Another possible red flag? A Web site that analyzes Reagan National’s temperatures compared to neighboring sites concluded they have been reading 2.6 degrees warmer than expected over the past year and “are not within an acceptable error range.” Perhaps not coincidentally, those 2.6 degrees are very similar to the difference in the monthly temperature anomalies between Reagan National and Dulles.

But before we can conclude, based on this evidence, that Reagan National’s temperature readings are reading too hot, there is a complicating matter — which is the historical temperature record.

While Reagan National and Dulles had comparable monthly temperature anomalies from the mid-1980s to the end of 2013, the airports exhibited divergent temperature anomalies prior to the mid-1980s. Just as in the period spanning the past 19 months, Reagan National often registered a monthly temperature anomaly 2 to 3 degrees higher than Dulles in the early 1960s to the early 1980s.

Similarly, between the early 1960s and late 1970s, we see huge and head-scratching differences in the number of 90-degree days between the airports, like we’ve seen recently.

We are not sure what this all means.

It’s possible that certain weather regimes lend themselves to wider disparities in local temperature anomalies — although it is bizarre that some periods of time show basically no differences while others have big differences.

It’s possible there are (and have historically been) problems with the calibration of temperature sensors that could lead to differences. But Steve Zubrick, the science operations officer at the National Weather Service forecast office in Sterling, Va., says the airport sensors get checked every 3 months.

It’s possible changing environments around the temperature sensors could influence their readings. But “to my knowledge, the environment around the [Reagan National] temperatures sensor has not changed much over the last 2 years, if at all,” Zubrick says.

And so we remain perplexed.  The matter has practical implications for the integrity of historical weather records and the legitimacy of the reports of Washington, D.C.’s temperatures by digital and broadcast media.  More analysis of this situation is needed, so stay tuned.

More from the Capital Weather Gang

Should Reagan National remain D.C.’s official weather station?

Mr. President, Move Washington’s weather observations to the White House

Weatherman-in-chief and first forecaster: The White House is now doubling as a weather station

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