Urban sprawl contributes to extreme heat events.

Stone, B, JJ Hess and H Frumkin. 2010. Urban form and extreme heat events: Are sprawling cities more vulnerable to climate change than compact cities? Environmental Health Perspectives http://dx.doi.org/10.1289/ehp.0901879.

Urban planners take note: the way a city grows and develops influences how often it has heat waves.

For a number of reasons, cities are always warmer than rural areas – a condition attributed to the urban heat island effect. But, according to a study that examined heat patterns over 50 years, the urban areas that sprawl have twice as many days with extreme heat when compared to cities that grow more compactly.

The higher temperatures, in turn, may contribute to increased illness and death in certain groups of people who are more vulnerable to hot weather.

Urban sprawl refers to a city and its suburbs that expand over large areas of land. It is often associated with a lower density of people and buildings; low mixed land use; a reliance on cars with subsequent increases in air pollution, accidents and injuries; and other related health and social effects.

Extreme heat is a significant cause of sickness and death. On average, about 400 people in the United States die each year from high temperatures. Deaths can skyrocket over just days or weeks during summertime heat wave events. Vulnerable populations include the elderly; the very young; those who are isolated; and people with psychiatric, respiratory and heart disease.

Although temperatures – and the number of extreme heat days – have been increasing in U.S. cities, urban sprawl's contribution to the heat waves is not well understood.

In this study, researchers compared a city’s degree of urban sprawl and the number of extreme heat events that occurred between 1956 and 2005. The urban sprawl in 53 U.S. cities was characterized based on the density of the city, the street network and the mix of land use types within the metropolitan area. Sprawling cities included Atlanta, Ga., Raleigh, N.C. and Tampa, Fl. Those with the least sprawl included Colorado Springs, Colo., Providence, R.I., and Allentown, Pa.

Extreme heat days for each city were calculated from temperatures gathered from the National Climatic Data Center (NCDC). They were defined as any day where the apparent temperature – a combination of heat and humidity – was in the top 15 percent of all recorded temperatures for that city between 1961 and 1990.

Extreme heat events increased in all the cities studied during the five decades. The researchers found that overall, the number of extreme heat days in major U.S. cities increased by 0.2 days per year – or, on average, about 10 days per city – from 1956 to 2005. The cities with the highest levels of urban sprawl experienced an increase in the number of extreme heat events by almost 15 days compared to an increase of less than 6 days in the most compact cities.

These results suggest the need for urban planners and public health officials to work together to implement measures that would reduce the unintended consequences of urban sprawl. The authors offer several concrete ideas, such as incorporating green space, using reflective surfaces on roads and buildings and encouraging mass transit systems.