Washington D.C. has the sixth most intense urban heat island in the U.S., out of 60 cities investigated in a report by Climate Central.

The study finds that on average, the urban area of D.C. is 4.7 degrees warmer during summer than its neighboring, rural areas since 2004. In the past ten years, the difference in temperature between the city and surrounding rural areas has ballooned to as much as 21 degrees.

D.C. also ranked fourth in the list of most intense overnight urban heat islands, at a whopping 7.1 degrees warmer than surrounding stations, on average.

Reagan National Airport was used as the weather recording location representing Washington D.C.

The study also found that the temperature has been rising faster in heat islands than surrounding locations since 1970. Baltimore-Washington International Airport, which is about 10 miles outside Baltimore city as the crow flies, has the third fastest-growing urban heat island effect out of the 60 locations studied. On average, it is warming at a rate of 0.66 degrees per decade faster than cooler, surrounding areas.

The top two cities with the fastest growing urban heat islands are Columbus, Ohio (0.84 degrees faster per decade), and Minneapolis, Minn. (0.77 degrees faster per decade).

An urban heat island is what you end up with when materials that absorb large amounts of solar radiation (e.g. concrete, asphalt, and shingled roofs) replacing grasses and trees, which naturally cool their surroundings with shade. Urban materials add to the increase in temperature during daylight hours by radiating a larger amount of heat than their other, natural counterparts would. After the sun sets, urban zones continue to pump heat back into the air, keeping the temperature warmer through the night than surrounding, green areas.

Washington D.C.’s growing heat island effect has been documented here at the Capital Weather Gang, and it doesn’t come as a surprise that these airports (DCA and BWI) made the top ten lists.

Looking at Reagan National, development has continued to erode what little green space exists around the airport. The area around BWI has seen significant development in the last few decades.

While at first blush it may appear that an increase in urbanization and thereby heat island effect could be blamed for the global warming trend, numerous independent studies on global temperature have showed that urban heat islands are not biasing the measurements.

In particular, a recent study looking into the impact of urban heat islands on the historical climatology network found that “while UHI and other urban-correlated biases are real (and can have a big effect), current methods of detecting and correcting localized breakpoints are generally effective in removing that bias. Blog claims that [the impact of urban heat islands] explains any substantial fraction of the recent warming in the US are just not supported by the data.”

In other words, the practices that scientists employ to remove any urban heat island bias from the data is working, and the resulting trend does not include any meaningful measurement of urban growth.

Climate Central also published an interactive graphic in which you can explore the urban heat islands of the other cities in the study.