Heat island effect
The heat island effect is a phenomenon where urban areas experience significantly higher temperatures than their surrounding rural regions. This temperature disparity arises from urban infrastructure, including buildings and roads, which absorb and retain heat more effectively than natural landscapes. On hot, sunny days, urban surfaces can reach temperatures that are 50 to 90 degrees Fahrenheit higher than the air temperature, contributing to an average city temperature that can be 10 degrees warmer than rural areas. This effect is particularly pronounced during the day and less so at night, with wind further mitigating temperature differences.
While the heat island effect can provide some advantages during colder months by preventing snow accumulation, it poses serious challenges during heat waves, leading to increased air conditioning use and associated costs. This heightened energy demand can strain power grids and worsen air pollution. Furthermore, the warmer urban environment can adversely affect wildlife and alter local weather patterns. To mitigate the heat island effect, strategies such as using reflective building materials and increasing urban vegetation are recommended. Implementing these solutions requires collaborative efforts from communities and government agencies to create effective change, particularly as climate change exacerbates these temperature fluctuations, making the issue increasingly urgent.
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Heat island effect
Summary: The heat island effect is the phenomenon of urban areas tending to be hotter than the surrounding rural areas.
The heat island effect occurs in heavily populated, urban areas. These “islands” are hotter than surrounding, less populated areas because of the heat absorbed by buildings, roads, and other artifacts of the human-built urban landscape. On hot, sunny days, surfaces such as roofs and pavement can have temperatures 50 to 90 degrees Fahrenheit higher than the air temperature. Moist or shady surfaces, by contrast, remain close to air temperature. The average temperature of a city can be as great as 10 degrees higher than in the surrounding rural areas. This temperature difference tends to be larger during the day than at night and smaller when the area is windy.
Most buildings and asphalt surfaces absorb more energy from sunlight than land covered with plants do. The energy from the sun takes the form of heat, and the heat from the buildings and streets in turn heats up the surrounding air. Cities are also bulky, so they take longer to cool down, and they block the ground from cooling, too. Vegetation, by contrast, not only absorbs less heat but also cools the air. Plants take in water that they need to live through their roots, and some of this water evaporates into the air, cooling it.
Consequences
During cold weather, the higher temperatures of urban heat islands are beneficial and may even make it easier to keep streets clear of snow. During hot weather, however, the heat island effect leads to a heavy use of air-conditioning, and people tend to stay indoors instead of going outside to enjoy the city. During a heat wave, when the temperature rises across a whole region (such as in the Chicago heat wave of 1995, which killed about 700 people), even a few degrees can make the difference between life and death for those who cannot afford access to air-conditioning. Indeed, more deaths each year result from heat than from tornadoes, floods, hurricanes, and lightning combined, according to the National Oceanic and Atmospheric Administration.
In addition to being expensive, air-conditioning creates its own problems. Air conditioners are energy intensive, and the electricity they use comes from power plants, which release carbon to produce the extra power, adding to air pollution and emissions that exacerbate climate change. The demand for air-conditioning on the hottest days of the year is so large that power grids have to plan extra capacity to handle it, leading to larger power systems and larger costs, if not power outages at times of peak demand. On hot days, given a choice, people would prefer to travel in air-conditioned cars rather than walking or taking the bus, increasing gasoline consumption not just for air-conditioning but also to fuel personal vehicles. Refrigeration costs are also higher when the air surrounding the unit is hotter.
The higher temperatures also affect wildlife by raising the temperature in nearby bodies of water. Many species are less adaptable to changes in temperature than humans are. The heat island effect can also alter weather patterns, wind patterns, how and where rain falls, and how humidity moves through the air to create clouds and fog. Higher temperatures can also make it more likely that chemicals already present in the air will react to form more dangerous compounds, such as low-level ozone or nitrous oxide. However, the higher temperatures may make low-level tornadoes less likely and increase the length of the growing season. Thus, the heat island effect makes it more difficult to predict the weather and complicates the task of disentangling the effects of changes over time in solar heat absorption from climate change due to other causes like carbon emissions.
What Can Be Done?
There are two main ways to reduce the heat island effect in an area, and neither can be implemented quickly. Building surfaces can be constructed with materials that absorb less light, and more vegetation can be added. The proportion of the light that is reflected from an opaque surface is called that surface’s albedo, and that proportion can vary widely. For instance, fresh asphalt has an albedo of 0.04, new concrete 0.55, and fresh snow 0.8–.9, on a unitless scale from 0 to 1. Using light-colored materials, then, such as concrete instead of asphalt, or covering a surface with particular paints, can have a big impact. A report from Lawrence Berkeley National Laboratory found that planting three trees per house and resurfacing about two-thirds of pavements and rooftops with reflective surfaces could reduce the temperature in downtown Los Angeles by 2 to 3 degrees Celsius. This change would have about the same impact on smog exposure there as eliminating on-road vehicle exhaust and would save about $500 per air-conditioned home per year, according to the report.
Any of these solutions requires coordinated effort by many property owners to solve a shared problem, just as the shared problem results from the actions of many. Community and national governments may get involved in changing laws about what kinds of building materials may be used. Standards set by the Leadership in Energy and Environmental Design (LEED)—an internationally recognized green building certification system developed by the US Green Building Council—may also be helpful in changing the types of building materials used.
Scientists noted that the heat island effect will become a much more serious problem as regions warm due to global warming and climate change. For example, the city of Chicago could see an estimated thirty more days per year above 100 degrees Fahrenheit. Because of temperature increases, the US government encouraged cities to invest in green infrastructure, such as by planting additional trees and developing green rooftops.
Bibliography
Akbari, H. Energy Saving Potentials and Air Quality Benefits of Urban Heat Island Mitigation. LBNL-58285. Berkeley, CA: Lawrence Berkeley National Laboratory, 2005.
"Heat Island Effect." Environmental Protection Agency, 2024, www.epa.gov/heatislands/. Accessed 1 Aug. 2024.
Klinenberg, Eric. Heat Wave: A Social Autopsy of Disaster in Chicago. Chicago: University of Chicago Press, 2003.
National Oceanic and Atmospheric Administration. “Heat Wave: A Major Summer Killer.” http://www.nws.noaa.gov/om/brochures/heat‗wave.shtml.
"Reduce Urban Heat Island Effect." Environmental Protection Agency, 17 Apr. 2024, www.epa.gov/green-infrastructure/reduce-urban-heat-island-effect. Accessed 1 Aug. 2024.
US Green Building Council. “What LEED Is.” www.usgbc.org/DisplayPage.aspx?CMSPageID=1988. Accessed 1 Aug. 2024.