InTrans / Apr 07, 2008

Hot enough to fry an egg

Go! Magazine

When the summer heat is sizzling and pavements are too hot for bare feet, it feels like it could actually work to fry an egg on the street or sidewalk. During the day, pavements can reach 150º F (66º C). The exteriors of buildings heat up too. In cities, especially downtown areas where there are lots of tall buildings, the temperature can be up to 10º F (5.6º C) warmer than surrounding regions. This phenomenon is called a heat island.

How to grow a heat island

According to the U.S. Environmental Protection Agency (EPA), heat islands form when natural land cover such as trees and grass are replaced with buildings, pavements, and other parts of a city’s built infrastructure. Humans contribute too. By using air conditioning, operating factories, and driving vehicles, we add waste heat to the air.

Without trees and vegetation providing shade and evaporation, natural cooling is lost. All that warm air gets trapped between tall buildings, making people feel hot and uncomfortable, so they crank their air conditioning even higher. That generates more waste heat, air pollution increases, and the cycle continues. During the summer, living in a heat island is tough. Poor air quality can make people sick. So can the heat. Some people can even die from the heat.

Sketch of an urban heat island profile with location on the horizontal axis and late afternoon temperature o the vertical axis
Urban heat island profile

Despite all these negative summertime effects, you’d think there might be some benefit for heat islands in cold climates. And there are. Wintertime benefits include less need for energy to heat buildings and help melting snow and ice on roads. But according to the EPA, the negative summertime effects generally outweigh the wintertime benefits.

Pavement’s role in a heat island

In cities, pavements cover a lot of ground. Sidewalks, parking lots, and streets can take up about 1/3 of the total land surface in urban areas. Pavements in Chicago, Ill., Houston, Texas, Sacramento, Calif., and Salt Lake City, Utah occupy between 29 and 39 percent of these cities’ total land surface. In Houston, 60 percent of that’s for parking. Pavements contribute to heat islands in 2 main ways:

  1.  Compared to vegetation, they absorb and store more heat.
  2. Most pavements are solid and impervious to water, so they provide less cooling due to evaporation.

Cool it

Researchers are testing several different ways to help heat islands cool off. They’re using vegetation and testing “cool” materials for buildings and pavements.

Trickle down

One way to make pavements cooler is to make them porous. Instead of a solid slab that allows nothing to pass through it, the pavement is built with small spaces or voids in it. The spaces let water drip through the pavement into the ground and take advantage of the cooling effect of evaporation. Also, porous pavements generally cool faster than regular pavements by convective airflow—that’s when the heat rises off the pavement. An example of convective airflow is the heat mirages you see in the distance on a highway.

Catching some rays

Another way for a pavement to be cooler is to make it reflect more solar radiation. The ability to reflect short-wave radiation (mostly visible light) is called solar reflectance or albedo. A pavement with a higher albedo reflects more solar energy and absorbs less. Albedo is basically associated with color. Lighter colors have higher albedo and darker colors have lower. So a white object will reflect more solar rays than a black one. The same is true for pavements. Lighter-colored pavements reflect more light than darker ones. A new concrete pavement (the white/light gray stuff) has an albedo of 35–40 percent while a new asphalt pavement (the black stuff) has an albedo of 5–10 percent. It’s not as simple as saying, hey, why don’t cities with this heat island problem just pave everything in concrete. That’s because over time (roughly 5–10 years), concrete pavements get darker from tire marks, dirt, and oil, and asphalt pavements get lighter due to the asphalt oxidizing. So their albedo values get closer.

Frying an egg

To be thoroughly cooked, eggs need to reach 160º F (71º C), so hot pavement probably isn’t going to do the trick. One problem with pavement as a cooking surface is that it doesn’t conduct heat as well as metal—like a frying pan.

Egg frying on sidewalk
Frying an egg

To learn more about frying eggs in places other than a frying pan, take a look at Everyday Mysteries: Fun Science Facts from the Library of Congress.

By Rema Nikalanta

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