The Urban Heat Island (UHI) Effect

What is the Urban Heat Island Effect?

It is no secret that the impervious surfaces – the asphalt, concrete, bricks and steel – that make up most of the physical infrastructure of our cities are much better at absorbing and retaining heat than natural areas, particularly those covered with vegetation. Vegetation and the soils underneath are, however, better at retaining moisture, which helps to cool these surfaces through evaporation or transpiration.

In addition to the hot, dry impervious surfaces retaining and emitting heat, urban areas are hotbeds for the generation and emission of waste heat. Waste heat is the byproduct of energy conversion, such as when fuel is burned inside a car engine or when electricity or gas is used to heat or cool a building. In the summer, when energy demands are typically the highest, waste heat from cars, trucks, and air conditioning units can be significant in densely populated areas. The end result is that dense urban centers become a “heat island,” surrounded by a cooler countryside. This is known as the Urban Heat Island (UHI) effect.

Urban heat island effect in Atlanta, GA
Thermal image of “Hotlanta” (Atlanta, GA). Large impervious surfaces appear the hottest (white). Source: New et al. (2014).

The U.S. Environmental Protection Agency (EPA) estimates that the UHI effect may result in a 1.8–5.4°F (1–3°C) temperature difference between a city of 1 million and the adjacent rural hinterland. During the evening, the temperature difference may be even greater; the city may be as much as 22°F (12°C) warmer than the surrounding countryside. The UHI effect therefore exhibits a very distinct diurnal (daily) pattern. Impervious surfaces heat up quickly during the day (particularly dark surfaces like asphalt), causing surface temperatures to rise much higher than in vegetated areas, including parks within the city. The temperature of the air at the surface, however, remains largely unaffected during the day due to convective winds moving warm air upwards.

At night, the situation is reversed. The impervious surfaces act as a powerful heat reservoir, slowly emitting heat energy throughout the night, warming the air above. The convective winds that mixed the air during the day usually subside at night, potentially resulting in a thermal inversion in which warmer air is trapped under a layer of cooler air above. Under these conditions, areas with more impervious surface coverage will remain warmer than those with less. Therefore, at night, the UHI effect (i.e. the temperature difference between the city and countryside) is often more pronounced both in terms of surface temperatures and low-level air temperatures.

urban heat island diagram
The urban heat island (UHI) effect is often most pronounced late in the day and evening.

Effects of the UHI

UHI smog in Beijing
The UHI can aid the formation of photochemical smog, such as in Beijing, China. Source: green_kermit, Flickr.

While any additional heat during the hottest weeks of the summer can be a great discomfort to many of us, the UHI effect can have outright deadly consequences for some. One consequence is the direct effect of temperature on human health. Perhaps in part due to global climate change, extreme heat waves in recent years have plagued large urban areas around the world. The European heat wave of 2003, for example, was the worst heat wave on the continent in almost 500 years, killing some 40,000 people. While the UHI effect does not cause wide-spread heat waves, it can certainly exacerbate already high temperatures in urban areas and reduce the natural cooling effect at night. As global temperatures continue to rise, the UHI will likely become more pronounced.

Higher temperatures can also affect our health indirectly by enhancing the emission and formation of certain air pollutants. Higher temperatures typically result in greater energy consumption, used to cool homes, business and cars. If the energy we use to stay cool is derived from fossil fuels – oil, coal, natural gas -, then the byproduct is not only additional waste heat (adding to a positive feedback cycle), but also the release of additional air pollutants such as particulate matter, sulfur dioxide, and nitrogen dioxide among others. Furthermore, higher temperatures aid in the formation of ground-level ozone, which is a major component of photochemical smog. In high enough concentrations, these pollutants can cause or contribute to a variety of serious health problems, including heart disease, cancer, asthma, and strokes. These effects are most pronounced among the young, elderly, and those already at risk for cardiovascular or pulmonary disease.

While higher temperatures within cities during the summer months are generally bad for people, the UHI does seem to be good for plants. Research conducted by NASA showed that, among 70 cities in eastern North America, the higher temperatures and additional rainfall attributed to the heat island effect increased the growing season within the cities’ zone of influence by about 15 days.  ”If you live in a rural area and drive regularly into the city, and if you pay attention to vegetation, you will see a difference in the growing seasons in early spring and late autumn,” said Xiaoyang Zhang, a lead author on the study.

Mitigating the UHI

One of the most often discussed strategies for dealing with the UHI is reducing the effect of impervious surfaces by either increasing their albedo (reflective capacity),  covering them with cooler vegetated surfaces (i.,e. “green roofs”) or replacing them altogether with pervious surfaces wherever possible. The albeto of many impervious surfaces, particularly rooftops, could be enhanced by simply painting these surfaces white. Light surfaces reflect incoming solar radiation more effectively than dark surfaces, thereby reducing the amount of heat they absorb. This strategy has been used in many parts of the world, including Greece for example, for centuries.

green roof chicago city hall
Green roofs, such as the one atop Chicago’s city hall, can help mitigate the urban heat island.

Vegetated, or “green,” rooftops have become increasingly popular in recent years. If a building can support the extra weight, vegetated roofs offer a number of benefits, most notably a reduction in rooftop temperature due to the greater mass of plants and soil acting as insulation and cooling due to evapotranspiration. The cooling effect provided by the green roof reduces building energy consumption as much as 50 to 90 percent. Vegetated roofs also absorb and transpire water, reducing runoff during periods of heavy rainfall. The plants also help to mitigate air pollution by filtering airborne particulates and absorbing carbon dioxide. Local wildlife often take refuge on vegetated rooftops, providing additional habitat otherwise lost to urbanization.

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