What is the urban heat island effect?

By: Jane McGrath  | 
New York's tall buildings and asphalt help capture the sun's light and results in warmer temperatures.
Eric Meola/Iconica Collection/Getty Images

If you can't take the heat, get out of the city! If you turn on the local weather report, you'll probably notice an odd trend. Temperatures are often a few degrees higher in cities than they are in their surrounding rural areas. This temperature discrepancy is the result of a bizarre phenomenon known as the urban heat island effect.

As the name implies, the effect turns cities into urban heat islands. According to the U.S. Environmental Protection Agency, temperatures in U.S. cities can get as much as 10 degrees Fahrenheit higher than their surrounding areas. Normally the temperature disparity is not quite that significant, but even a few degrees can make a huge difference. The additional demand for air conditioning in the summer leads to higher energy bills, which increases greenhouse gas emissions from power plants.


Perhaps the worst result of the heat island effect is the number of heat-related deaths. Although damage-causing storms get the most media attention, the National Oceanic and Atmospheric Administration reports that extreme heat kills more Americans each year than tornadoes, hurricanes, floods and lightning combined [source: NOAA]. So what creates this urban heat? And how can city planners reduce it?

How it Happens: The Physics Behind the Urban Heat Islands

City building roofs and asphalt are often dark-colored, which helps drive the urban heat island effect.
David Zimmerman/UpperCut Images/Getty Images

To understand these heat islands, we first need to understand a few simple rules of physics. Most importantly, we should understand that objects can absorb and reflect light.

In fact, the color of an object depends on what kind of light it reflects. For example, a green object reflects green light and absorbs all the other visible colors of light. Darker colored objects are excellent absorbers of light. In fact, black surfaces absorb almost all light. On the other hand, lighter colored surfaces do not absorb much light at all -- rather they reflect almost all of it.


So what does the absorption of light have to do with heat? When an object absorbs light, it converts that light to thermal energy, and emits it back out as heat. So, because black objects absorb more light, they also emit more heat. That's why wearing a black shirt on a hot, sunny day will only make you hotter. The black shirt absorbs light and emits it as heat onto your skin. Wearing a white shirt, on the other hand, will help reflect the sunlight and keep you cooler.

The Effects of Heat Energy on Urban Temperatures

The rate at which an object can reflect solar radiation is called its albedo [source: Budikova]. The bigger the albedo something has, the better it reflects radiation. Traditional asphalt has a low albedo, which means it reflects radiation poorly and instead absorbs it.

When we build and expand cities, we tend to erect buildings with dark surfaces and lay down asphalt pavement. The buildings and the pavement absorb a significant amount of light and radiation and emit it as heat, warming the city. This heat absorption is why the temperature difference between cities and rural areas is highest a few hours after sunset. Cities hold on to more heat for a longer period of time than rural areas do [source: EPA].


Rural Plant Life Reinforces Urban Air Temperatures

Scientists believe that vegetation plays a large part in keeping an area cool through a process called evaporative cooling. Evaporation is when liquid turns into gas. Plants take in water through their roots and depend on it to live. But after the plant is done with it, dry air absorbs that water by turning it into gaseous water vapor. The air provides the heat that drives this process, so during the process, the air loses heat and becomes cooler.

We experience the same type of thing when we sweat -- when air hits your sweaty skin, it absorbs the moisture and cools the air around you [source: Asimakopoulos]. Since building a city means replacing vegetation with structures, the city loses the evaporative cooling advantages of vegetation.


Other factors also contribute to the effect. For instance, cars and air conditioners, which are ubiquitous in urban areas, convert energy to heat and release this heat into the air. Now that we know what's causing this phenomenon, let's learn the steps to reduce it.

Techniques to Reduce Urban Heat Islands

Green roofs, like this one atop Chicago's City Hall, help offset the urban heat island effect.
Tim Boyle/Staff/Getty Images

Luckily, since we know what causes the urban heat island effect, we can control it to a significant extent. Certain techniques reduce the demand for air conditioning and reduce energy bills.

Because the dark surfaces and low albedo of man-made structures heat urban area, the logical solution would be to reverse this trend. City planners may do this by painting structures white, or other light colors. This basic technique goes a long way in reversing the extreme heat caused by these surface temperatures.


Bright Coatings Can Reduce Heat Waves

However, some people don't like the idea of a glaring, all-white city. Low-reflectivity coating offers an alternative and comes in non-white colors. These kinds of coatings reflect invisible radiation without reflecting all light [source: Synneffa]. So, they keep an object relatively cool without sacrificing its dark color.

Certain high-reflectivity coatings can also be applied to asphalt. Asphalt chip seals and emulsion sealcoats are two such examples that treat asphalt to make its surface more reflective [source: EPA]. The processes reduce the albedo factor of asphalt, which is a major contributor to the urban heat island effect.

Less Heat From the Roof to the Street

One fad that's gaining popularity is the installation of green roofs atop city buildings. This solution doesn't have anything to do with color. A "green roof" harnesses the evaporative cooling effect that cities so desperately need, offsetting the urban heat island effect to an extent. And since plants absorb carbon dioxide, these roofs tackle air pollution as well.

Several other methods help reduce the urban heat island effect as well. For instance, roof sprinkling is another evaporative cooling solution. Sprinklers on the roof wet the surface so that the air around it cools through evaporation [source: Asimakopoulos]. Urban planners also set up traditional parking lots where trees and vegetation grow. Tall trees not only contribute to evaporative cooling but also provide much-needed shade.


Lots More Information

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More Great Links


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