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Why the City Is So Much Hotter Than the Suburbs or the Country


If you’re one of the thousands of sunburnt Athfest revelers who continued sweating long into the weekend nights, you can blame the rock ‘n’ roll, but you should also blame the Urban Heat Island effect.

When sunlight reaches our atmosphere, some radiation is reflected back out to space, some passes through and is reflected by the earth’s surface, and some is absorbed. That absorbed solar radiation is stored as heat, which re-radiates as infrared; this is an important component of the greenhouse effect. If you’ve ever walked past a west-facing brick wall on a summer evening and felt its heat on your bare skin, that’s infrared (thermal) radiation. It’ll smack you right in the face if you step out onto an unshaded asphalt parking lot on a hot summer day.

As humans change the pattern of land use from forest and grass to buildings and hardscape, we change the reflective and thermal properties of our surroundings. Brick, concrete, asphalt, dark roofs—they all absorb solar radiation and have the thermal capacity to hold a lot of heat (think about a pizza stone or cast iron pan). Trees and grass are not very highly reflective, but rather than simply absorb and hold heat, they contribute to evapotranspiration: using the sun’s energy to convert liquid water to water vapor.

Urban heat islands have higher summer surface and air temperatures, day and night, than their suburban and rural surroundings. This leads to more energy use for air conditioning, refrigeration and, presumably, sweet tea and ice production. More energy means more fossil fuel consumption, which means more greenhouse gas production, which indirectly exacerbates the urban heat island effect. Higher temperatures and sunlight accelerate the chemical reactions that turn motor vehicle exhaust and gasoline vapors into ground-level ozone, which contributes to respiratory problems that keep more people inside (refer back to air conditioning).

Storm runoff from impervious surfaces causes water quality problems on its own. But hot hardscapes warm up the runoff as it flows towards storm sewers, increasing its temperature substantially before it gets to the Oconee rivers or their tributaries. Besides all of the erosion and contaminants that this runoff brings with it, the elevated temperature can really throw aquatic ecosystems off balance.

Take a look at your favorite online satellite imagery site, and you’ll notice that there are a lot of white roofs on the large commercial and industrial buildings around Athens and the UGA campus. The primary reason for these is to reflect sunlight and directly reduce cooling demands—and is the reason why Georgia Power offers a rebate for them—but they are also excellent mitigators of the heat island effect. On the other hand, developers continue to lay vast, treeless expanses of black asphalt parking lots and wide roadways. (I defy you to stand for three minutes in the middle of the Epps Bridge Centre parking hellscape.)

Athens certainly has an impressive tree canopy, and it’s important to keep it that way. It will be a shame when the mature trees on Clayton Street are cut down when Athens-Clarke County replaces the sidewalks, for example. (The city says they can’t be moved or saved and is planting new trees.) Encourage your government to push for planted road medians. If you have trees of your own, take care of them, and consider planting young ones to fill in when the old ones die. Even medium-height shrubs planted near buildings, especially on the west face, can mitigate the heat island effect. Manicured grass lawns aren’t particularly helpful, but thicker, taller landscapes are.

If you are in a position to replace a roof, look for cool roof options: white membranes for flat roofs, and metal or reflective shingles for sloped roofs. New materials should be Energy Star rated and have a high SRI (Solar Reflectance Index).

Whether you are a commercial real estate magnate or just looking for the right material for your driveway, consider cool paving materials. There are several options, including asphalt mixed with or coated with high reflectance additives, permeable concrete, permeable pavers (vegetated or non-vegetated), fly ash concrete and good ol’ gravel. It is important to apply these options wisely, though—a reflective paved area next to a building may simply reflect heat into the structure.

Have a question for the Greensplainer? Email news@flagpole.com.