What is the phenomenon of the urban heat island effect?

Can't stand the heat? It's time to leave the city! If you tune into the local weather broadcast, you may notice an intriguing pattern: Cities often record temperatures that are several degrees warmer than the rural areas nearby. This temperature variation is due to the unusual phenomenon called the urban heat island effect.

As the term suggests, cities become like urban heat islands. According to the U.S. Environmental Protection Agency, urban temperatures can soar by up to 10 degrees Fahrenheit more than surrounding rural regions. Though the difference is usually not as extreme, even a small rise in temperature can have a significant impact. The added demand for air conditioning during the summer drives up energy costs, resulting in higher greenhouse gas emissions from power plants.

One of the most concerning outcomes of the heat island effect is the number of fatalities caused by extreme heat. While storms that cause damage grab more media attention, the National Oceanic and Atmospheric Administration reports that extreme heat claims more American lives annually than tornadoes, hurricanes, floods, and lightning combined [source: NOAA]. What causes this urban heat, and how can city planners mitigate it?

The Science Behind Urban Heat Islands: How It Happens

To grasp the concept of heat islands, we must first look at some basic principles of physics, particularly how objects can absorb and reflect light.

The color of an object determines the type of light it reflects. For instance, a green object reflects green light while absorbing other visible wavelengths. Dark-colored objects are highly efficient at absorbing light, with black surfaces absorbing almost all visible light. In contrast, lighter objects reflect most of the light that hits them, absorbing very little.

But how does light absorption relate to heat? When an object absorbs light, it transforms that light into thermal energy and radiates it as heat. Black objects, absorbing more light, emit more heat. That's why wearing a black shirt on a hot day makes you feel even hotter—it absorbs light and radiates it as heat. A white shirt, by contrast, reflects light and keeps you cooler.

The Impact of Heat Energy on City Temperatures

The term albedo refers to how well an object can reflect solar radiation. A higher albedo means better reflection of radiation. Traditional asphalt has a low albedo, meaning it absorbs more radiation rather than reflecting it.

As cities grow, we often construct buildings with dark surfaces and lay down asphalt, both of which absorb a considerable amount of light and radiation. This absorbed heat is emitted as warmth, causing urban areas to become warmer than rural regions, especially after sunset. Cities retain heat longer than rural areas [source: EPA].

How Rural Vegetation Contributes to Urban Air Temperature Regulation

Experts suggest that plant life plays a significant role in cooling areas through a process called evaporative cooling. During evaporation, liquid water turns into gas. Plants absorb water through their roots, but once they have used it, the surrounding dry air turns it into water vapor. This process cools the air as the heat energy is absorbed during the change from liquid to gas.

Sweating provides a similar effect — when sweat on your skin is exposed to air, it evaporates, cooling the surrounding air [source: Asimakopoulos]. The expansion of cities often replaces vegetation with buildings, eliminating the natural cooling effect that plants provide through evaporation.

Other elements also play a role in this effect. For example, vehicles and air conditioners, which are common in cities, convert energy into heat and release it into the atmosphere. Now that we understand the causes, let's explore the ways to mitigate this effect.

Methods to Mitigate Urban Heat Islands

Fortunately, now that we understand what drives the urban heat island effect, we can take significant steps to control it. Various methods can help reduce air conditioning use and lower energy costs.

The heat caused by urban environments is often due to the dark surfaces and low albedo of man-made structures. To counteract this, city planners could consider reversing the trend by using lighter colors, like white, for buildings and other structures. This simple yet effective strategy can help mitigate the extreme temperatures caused by these surfaces.

Reflective Coatings Can Help Combat Heat Waves

For those who are not fond of a glaringly white city, low-reflectivity coatings provide a solution. These coatings come in various colors and reflect invisible radiation without bouncing all visible light back. By doing so, they can maintain the aesthetic of a darker-colored structure while keeping it cooler.

High-reflectivity coatings can also be applied to asphalt to reduce heat. Treatments such as asphalt chip seals and emulsion sealcoats make the asphalt surface more reflective, which in turn helps mitigate the urban heat island effect by lowering the albedo of these surfaces.

Less Heat Transfer From the Roof to the Streets

One trend that is becoming increasingly popular is the addition of green roofs on city buildings. Despite what the name suggests, the term 'green roof' is not about color. Instead, it refers to roofs that utilize the cooling effects of evaporation, which can help reduce the urban heat island phenomenon. Moreover, these roofs also combat air pollution as plants absorb carbon dioxide.

There are several additional strategies for alleviating the urban heat island effect. For example, roof sprinkling is a method where sprinklers are used to wet the roof, causing the surrounding air to cool as the water evaporates [source: Asimakopoulos]. Urban planners also implement traditional parking lots with trees and plants, where tall trees offer both shade and further evaporative cooling.