The University of Arizona

Urban Heat Island: Raising City Temperatures

By Zack Guido | The University of Arizona | September 15, 2008

The urban heat island (UHI) amplifies local temperatures and principally occurs when natural landscapes are converted to urban areas. Higher temperatures occur because dense concentrations of materials like asphalt and buildings absorb more heat during the day and release it more slowly at night than natural ground cover such as soil and vegetation (Figure 1).

illustration of an urban heat island

Figure 1. Surface and atmospheric temperatures vary over different land use areas.
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Credit: Environmental Protection Agency

Both Arizona and New Mexico have recently experienced rapid urban development to accommodate population growth. This increase in infrastructure has produced much warmer temperatures in urban areas. For example, between 1949 to 2005 in Tucson, the average annual minimum temperatures increased by 5.4 degrees Fahrenheit, with 3.6 degrees of this increase attributed to the UHI effect.1 Since population growth is projected to continue in both states (Figure 2, below), the UHI  may intensify as already developed urban areas expand outward and upward.

Although the UHI is created by land-use changes, global warming enhances it. Future predictions of summer temperatures in 2100 are as much as 10 degrees Fahrenheit warmer than present and will subject people to greater risks of excessive heat exposure and increase demand for energy and water.

In the Southwest, the impacts of the UHI are already felt by many people and animals. This has instigated extensive research into:

graph of projected Arizona and New Mexico population

Figure 2. Projected population growth for Arizona and New Mexico, 2010 - 2030.
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Credit: Joe Abraham, Institute of the Environment, The University of Arizona

Characteristics of UHI

As a result of urban development, urban areas have experienced a more rapid increase in temperature than the surrounding rural, agricultural, and wildland areas (Figure 3)2. Some general characteristics of the UHI are as follows.

  • Average minimum temperatures increase more than maximum temperatures.
  • Average nighttime temperatures increase more than the average daytime temperatures.
  • Urban areas experience year-round increases in the minimum daily temperatures.
  • The UHI causes longer daily warm periods and shorter daily cool periods.
  • Compared to rural areas, the duration of the hot season is extended in urban areas.
  • Temperatures can substantially vary between urban neighborhoods—the amount of vegetation plays a significant role in damping the UHI.3

Impacts on humans

Graph of increasing temperatures in Casa Grande National Monument

Figure 3. The figure illustrates larger increases in daily minimum temperatures at urban measuring stations in Phoenix than at the rural stations located at Casa Grande National Monument. Casa Grande National Monument is located between the two cities.
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Credit: Zack Guido, CLIMAS, The University of Arizona

  • The UHI causes people to experience uncomfortable, high temperatures for longer periods of the day; high temperatures arrive earlier and stay later in the day than they do in rural areas. This not only increases discomfort, but exposes people to higher risk of heat illnesses. For example, 50 people died in 2007 from heat-related illnesses in Maricopa County, Arizona4.
  • Higher temperatures increase the use of air conditioning and evaporative coolers which increases energy costs and water use.
  • Research suggests that the distribution and abundance of insects and disease vectors have increased as a result of the UHI. For example, at Sky Harbor International Airport in Phoenix, the number of days with temperatures between 59 and 100 degrees F—the temperature range in which insects survive—has increased by about a month since 1948. Accompanying this temperature increase has been an increase in the duration of arthropod (e.g. spiders) activity5.

Impacts on agriculture

  • Urban warming will likely increase agricultural pests. This is a consequence of the increasing number of days when temperatures fall between 59 and 100 degrees F, temperatures suitable for insects.5
  • The UHI has changed the sowing and harvest dates for cotton near Phoenix. For example, at Sky Harbor International Airport, the average planting date between 1997 and 2000 occurred 14 days earlier, and the average harvest date was 22 days earlier than at a rural location 100 miles west of Phoenix.5
  • Research in Phoenix suggests that late-season heat stress reduces the quality and quantity of cotton.5

References

  1. Comrie, A.C. 2000. Mapping a wind-modified urban heat island in Tucson, Arizona. Bulletin of the American Meteorological Society, 81(10):2417-2431.
  2. Brazel A. J., N. Selover, R. Vose and G. Heisler. 2000. Tale of two climates—Baltimore and Phoenix urban LTER sites. Climate Resources, 15:123–135.
  3. Harlan, S.L., et al. 2006. Neighborhood microclimates and vulnerability to heat stress. Social Science and Medicine, 63:2847–2863.
  4. Personal Communication with Anton Haffer, National Weather Service in Phoenix, Arizona, May 20, 2008.
  5. Baker, L.A., et al. 2002. Urbanization and warming of Phoenix (Arizona, USA): Impacts, feedbacks and mitigation. Urban Ecosystems, 6:183-203.