Why is it usually considerably warmer in the city than surrounding suburbs, especially at night? Just this morning, for example, the low at Reagan National Airport near the District was 36, while Dulles Airport – 25 miles to the west – bottomed out at 27. These kinds of temperature discrepancies are due to a phenomenon known as the urban heat island effect.
An urban heat island (UHI) is produced within a city center when development adds heat sources into the surrounding air. The added heat comes from tightly packed infrastructure, highly heat-absorbent surfaces like asphalt, and all the vehicles on the road.
The above image shows vegetation coverage for Washington, D.C. using near-infrared (heat) sensors. The rates of heat transfer for trees, plants, and shrubbery occur slower than many thermally conductive urbanized structures.
Notice the dense area of light blue coloring in the center of the image showing downtown D.C. The U.S. Capitol building can be seen surrounded by the Botanic Garden facing the National Mall that stretches towards the Potomac River. Just outside the central urban corridor of downtown D.C., vegetation can be seen in red throughout the map where cooler temperatures persist, with one notable exception: south of the District along the west banks of the Potomac River another small light blue area stands out. This is the airport, Reagan National, that records D.C.’s official temperature observations.
Local temperature variations
Every sizable city will have at least some impact from the urban heat island effect. The greatest impacts from the urban heat island effect occur in the largest cities with the highest populations. Temperature variations between the District and rural suburbs are often easily recognizable.
The following chart (below) compares the UHI effect for Reagan National Airport and Dulles.
The above graph compares minimum daily temperature trends between the years 1980 and 2012 for Reagan National Airport (blue) and Dulles International Airport (red). The trend lines show that D.C. tends to have higher minimum daily temperatures than Dulles. Urbanization has been increasing around both locations and, thus, it can help explain the increasing temperature trend, along with climate change.
Dulles Airport – change in land use over time
Reagan National Airport – change in land use over time
UHI effects on March 6, 2013
Surface and air temperatures between two nearby locations tend to have high fluctuations during the winter season. During the Snowquester storm last March, the variation of air temperatures around the region made all the difference in precipitation-type. Around midday on March 6, Reagan National Airport recorded an air temperature of 37 degrees with rain while Dulles International Airport was reporting 33.1 degrees with snow. Although warm easterly winds off the Atlantic Ocean were occurring at the time precipitation was falling, the urban heat island effect may have impacted the magnitude of regional surface temperature differences that day.
Picture the metro area without all the asphalt, concrete and brick structures to absorb the solar heat during daylight hours. Had the surface temperatures in the District been just a few degrees cooler on March 6, snow flakes falling through the atmosphere may not have melted so readily. Dulles’ higher elevation, relative to Reagan National, also probably helped keep it cooler.
Thunderstorms over urban heat islands
Some urbanized heat islands absorb enough energy from the sun to rapidly transfer and release thermal air currents into the atmosphere during the summer season. The warm buoyant air rising from the city can occasionally develop localized low pressure systems. Increased evaporation from the surface over an urban heat island allows greater amounts of water vapor particles in the atmosphere to coalesce into puffy cumulus and vertically-stacked cumulonimbus clouds. Thunderstorms that then develop can have high precipitation rates from the excessive moisture in the air.
Related: Urban rain overview (via NASA)
Surface temperature measurements using a thermal infrared thermometer
One way of measuring the urban heat island effect locally is using a thermal infrared thermometer. This device takes the temperature of a surface by measuring the radiated reflectivity from that surface. The Sun emits continuous short-wave solar radiation towards the Earth, heating terrestrial surfaces. The heat from these surfaces re-radiates some of that energy outward as long-wave infrared radiation. The mathematical difference between the absorbed and emitted energies from a surface can be converted into a temperature value.
Concrete temperature observations were higher than temperature values recorded in the grass for each measurement made at the same locations.
There was a slightly larger temperature range during the night when overall temperatures were lower between the concrete and grass readings.
The D.C. region is just one of several large metropolitan areas in the United States that is impacted by the UHI effect. Uncertainty still remain on how a UHI differs from one city to the next, how a UHI impacts local weather patterns, and how the urban heat island effect may be better identified.
The author of this post, Brendan Richardson, is Capital Weather Gang’s fall intern.