The phrase “the heat is on” is a common one, but in reality it is also common across D.C. and urban areas in general, compared to places even 10 miles outside of the District. Known as the Urban Heat Island effect, there are several causes for this extra heating; one being, dark surfaces absorb significantly more sunshine, which causes roads and buildings to heat more than suburban and rural areas throughout the day. A second reason would be that materials commonly seen in urban areas, such as concrete and asphalt, significantly differ when it comes to absorbing and radiating heat, compared to rural areas. A third major reason is the lack of evapotranspiration, through trees, in urban areas. Evapotranspiration is defined as the process by which water is released from the land into the atmosphere by evaporation, so as the sun heats the surface of the Earth, water evaporates from plants and re-enters the atmosphere. A study done a few years back, by the U.S. Forest Service, found that U.S. cities are losing 36 million trees each year, and with that type of decrease in vegetation, cities are losing a lot of shade and cooling that trees would normally bring to an area.
Other causes of the Urban Heat Island effect, also known as UHI, are tied in with tall buildings. Buildings provide multiple ways to absorb sunlight, along with helping to block the flow of wind, which inhibits cooling and prevents pollutants from dissipating. Add in the fact that heat from tons of automobiles, air conditioning, industry, and high levels of pollution are ways of not only raising urban temperatures, but also increases ozone concentrations, and now you can see why cities are warmer than its surrounding regions. For most cities, the difference in temperature between the urban and surrounding rural area is largest at night, but there is usually a discernable difference during a summer day, as well. The typical temperature difference is several degrees between the center of the city and surrounding fields. The average afternoon temperature of a city with 1 million people or more can be some 2 to 5°F warmer than its surroundings during the day, while in the evening, the difference can be as high as 22°F!
To see how the above estimates apply to our local area, I took a deep dive into meteorological summer for three locations: D.C., Beltsville, Maryland and Vienna, Virginia. I picked these three areas, because they are only separated by at most 20 miles. Meteorological summer refers to the months of June, July and August, as opposed to astronomical summers which are based on the position of the Earth relative to the sun, and that changes year to year. In my research, I found that the D.C. average high temperature during the summer months is 90°F,compared to 87°F for Beltsville, MD and 84°F in Vienna, VA. So a 3-6°F spread in daily summertime highs can be seen 10 to 20 miles from the District. Digging further, D.C. hit triple-digit heat 12 times in the month of June, 28 times in the month of July and 19 times in August, with the hottest thermometer reading of 106°F! While Beltsville, Maryland saw only one 100°F day in June, the region saw triple-digit heat 15 times in July and 9 times in August. Over the last 74 years, since records were kept in Vienna, Virginia, never had there been a 100°F day in June or July and only 1 day in August in 2015, when the temperature topped out at 100°
Now when you see a weather forecast, and the daily Almanac has the District hotter by a few degrees over the surrounding region, you’ll know what goes into why the thermometer reacts the way it does. If someone tells you that they can cook an egg on the top of a car in the middle of summer, it may not be all that it is cracked-up to be, but then again you may want to believe them if they live in a city.