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Breakdown: Why cities are often warmer than rural areas

Published: Sep. 12, 2021 at 10:43 AM CDT|Updated: Sep. 12, 2021 at 10:44 AM CDT
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MEMPHIS, Tenn. (WMC) - In cities, the air, surface and soil temperatures are almost always warmer than in rural areas. This effect is known as the Urban Heat Island (UHI).

Urban areas, where these structures are highly concentrated and greenery is limited, become “islands” of higher temperatures relative to outlying areas. These pockets of heat are referred to as “heat islands.” Heat islands can form under a variety of conditions, including during the day or night, in small or large cities, in suburban areas, in northern or southern climates, and in any season.

An urban heat island occurs when a city experiences much warmer temperatures than nearby rural areas. The difference in temperature between urban and less-developed rural areas has to do with how well the surfaces in each environment absorb and hold heat.

When you visit a big city, you won’t see many plants. Instead, you’ll see sidewalks, streets, parking lots and tall buildings. These structures are usually made up of materials such as cement, asphalt, brick, glass, steel and dark roofs.

These materials are often very dark colors—like black, brown and grey. A dark object absorbs all wavelengths of light energy and converts them into heat, so the object gets warm. In contrast, a white object reflects all wavelengths of light. The light is not converted into heat and the temperature of the white object does not increase noticeably. Thus, dark objects—such as building materials—absorb heat from the sun.

Dark surfaces--whether a black t-shirt or an asphalt street--absorb the sun's heat, while...
Dark surfaces--whether a black t-shirt or an asphalt street--absorb the sun's heat, while lighter colored surfaces reflect heat from the sun.(NASA/JPL-Caltech)

If you travel to a rural area, you’ll probably find that most of the region is covered with plants. Grass, trees and farmland covered with crops, as far as the eye can see.

Plants take up water from the ground through their roots. Then, they store the water in their stems and leaves. The water eventually travels to small holes on the underside of leaves. There, the liquid water turns into water vapor and is released into the air. This process is called transpiration. It acts as nature’s air conditioner.

The UHI is strongest during dry periods, when the weather is calm and skies are clear. These conditions accentuate the differences between urban and rural landscapes. Cities are distinguished from natural landscapes by their form: that is, the extent of the urban land cover, the construction materials used, and the geometry of buildings and streets. All of these factors affect the exchanges of natural energy at ground level.

Much of the urban landscape is paved and devoid of vegetation. This means that there is usually little water available for evaporation, so most available natural energy is used to warm surfaces. Construction materials are dense, and many – particularly dark-colored surfaces like asphalt – are good at absorbing and storing solar radiation.

A review of research studies and data found that in the United States, the heat island effect results in daytime temperatures in urban areas about 1–7°F higher than temperatures in outlying areas and nighttime temperatures about 2–5°F higher.

Climate Central created an index to evaluate the intensity of urban heat islands and applied it...
Climate Central created an index to evaluate the intensity of urban heat islands and applied it to 158 cities across the U.S. By measuring the type of land cover in each city, from greenspace to paved areas, and factoring in building height and population density, our research team adapted the Sangiorgio model to estimate how urban heat island intensity varies across cities.(Climate Central (climatecentral.org))

Humid regions (like the Mid-South) and cities with larger and denser populations experience the greatest temperature differences. Research predicts that the heat island effect will strengthen in the future as the structure, spatial extent, and population density of urban areas change and grow.

To cool down urban heat islands, some cities are ‘lightening’ streets. This is done by covering black asphalt streets, parking lots, and dark roofs with a more reflective gray coating. These changes can drop urban air temperatures dramatically, especially during the heat of summer.

Planting gardens on urban rooftops can also help to cool down the city, too! In fact, a study in Los Angeles, California, calculated that changes like these would be enough to save close to $100 million per year in energy costs!

To help cool the heat island, builders can use materials that will allow water to flow through. These building materials—called permeable materials—promote the capture and flow of water, which cools urban regions.

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