One of the most contentious issues in the weather community isn't about climate change or the downward spiral of the quality of The Weather Channel in recent years, but rather about the location of Washington D.C.'s official weather observation station. It may sound silly to those outside of the D.C. area, but it's a real issue that affects weather reporting for the nation's capital.
To understand what's going on with D.C.'s weather station, it's important to understand the urban heat island effect. An urban heat island is the warming of air temperatures in dense city centers due to human modification of the Earth's surface. Anyone who's visited a major city during the summer knows how much hotter the downtown areas are than the suburbs. Asphalt parking lots, sidewalks, concrete buildings, reflective glass, tar roofs, vehicles, and miles and miles of roadways significantly warm up a city's air temperatures — sometimes 5 to 10 degrees higher than the surrounding areas, leading to the formation of an urban heat island.
Washington D.C.'s official weather reporting station — technically called an ASOS (Automated Surface Observing System — isn't actually located in the city. The ASOS is located at Washington National Airport (DCA) in Arlington, VA. The station itself, which has operated continuously at the airport since the 1930s, sits inside the red circle in the map above. It's about 600 feet away from (and 13 feet above) the Potomac River and 800 feet from Runway 01/19, which is the most heavily used runway at the airport.
The station is often criticized by meteorologists and weather enthusiasts for being wildly unrepresentative of Washington D.C.'s real weather conditions, and the data backs up that claim. An excellent example of both the urban heat island effect and the poor location of Washington's weather station was seen on March 4 of this year. Temperatures across the Mid-Atlantic dipped down to near or below zero that morning, but the official reporting stations at both Washington National Airport and at the harbor in Baltimore, MD both reported temperatures almost 20 degrees warmer than surrounding areas.
The Washington Post's Capital Weather Gang often writes about the controversy surrounding the ASOS location at DCA. A post by Jason Samenow in December 2012 makes note of an email he received from climatologist Robert Leffler showing how skewed D.C.'s temperatures are due to the location of the ASOS at the airport. During the entire month of December, DCA only reported low temperatures at or below freezing 4 days of the month. Compare that to the 13 days with lows at or below freezing at the National Arboretum 5 miles away from DCA in Washington D.C. itself, and the 12 days at or below freezing at Baltimore-Washington International Airport. This put Washington D.C. on par with Baton Rouge, LA and Jacksonville, FL for number of days with low temperatures at the freezing point.
Temperature readings aren't the only thing affected by the station's location. Snow totals in D.C. are often much lower than the rest of the area. A great example of this was the blizzard that struck the Mid-Atlantic in February 2010. As seen in the snow total map above, the storm dropped 2 to 3 feet of snow across the region, with accumulations around 2 feet common in the immediate Washington D.C. area. DCA officially recorded 17.8 inches of snow from the storm, while totals in Arlington proper just a mile or two to the west clocked in between 20 and 24 inches. American University in D.C. itself recorded 27 inches of snow, with totals over 20 inches common in the District.
What contributes to this discrepancy? At Washington National Airport, specifically, its proximity to the city, airport terminals, tarmac, runways, and the warm Potomac River all serve to moderate temperatures and keep snow totals to a minimum. Due to the high heat capacity of water, areas right around oceans, lakes, rivers, and ponds tend to stay warmer at night and cooler during the day due to the influence of the water itself.
The National Weather Service (NWS) has strongly resisted any efforts to move the official Washington D.C. ASOS away from Washington National Airport to a more representative location for the city's weather, asserting that the station is up to NWS standards. "Our equipment there is calibrated and checked quarterly for accuracy by NWS technicians and reports 24/7. It is sited properly by the NWS policy that governs proper siting of NWS observation equipment," Steve Zubrick told the Capital Weather Gang in an August 2012 post regarding the possibility of moving the ASOS to a different location someday.
Even though Washington D.C.'s official weather station isn't actually in the city itself, and it's often largely unrepresentative of the city's actual weather, it doesn't seem that the station is going to move anytime soon. Keep this in mind the next time you check the weather in our nation's capital.
[Image via AP / Map of DC via Google Earth / Snowfall total map via National Weather Service]