Heat island. Sound like an exotic summer vacation destination? You may be surprised to learn that you either live in or very near to one and don't even realize it. Basically, a heat island is a metropolitan area whose air temperatures are a lot warmer than the rural areas surrounding it. (Doesn't sound very exotic anymore does it?)  

The heat island effect can occur year-round, during the day or night, but it is particularly noticeable during the summer when it elevates temperatures that may already feel excessive.

Why Cities Are Warmer

Heat islands form as trees and vegetation are replaced by asphalt and concrete when constructing office buildings, parking lots, and roads to accommodate growing populations. These man-made surfaces absorb and store (rather than reflect) the sun's heat. On a hot, sunny summer day, roofs and pavement surfaces can heat up to 50-90ºF hotter than the air! 

Eventually, these heated surfaces radiate their heat up into the atmosphere, causing air temperatures at the city center to elevate. According to the U.S. Environmental Protection Agency (EPA), the average annual air temperature of a city with a population of 1 million or more can be as much as 5ºF (3ºC) warmer than that of its surrounding rural areas--and that's just during the day. At night, the differences between average urban-rural temperatures can reach up to 22ºF (12ºC). This is because rural areas cool off faster at night than cities, which retain much of the heat collected by roads and buildings. (The greatest urban-rural temperature difference is often observed 3 to 5 hours after sunset.)

Another thing that contributes to a city's heat surplus is the lack of vegetation. Trees naturally provide cooling through shade and the evaporation of moisture from their soil and leaves.

Much More than 'Just Warmer Temperatures'

So city structures cause increased daytime temperatures and reduced nighttime cooling...what's so earth-shattering about that? Besides the fact that heat islands destroy natural microclimates, they also impact the environment, your purse strings, and your quality of life in the following negative ways: 

• Increased energy consumption. Higher temperatures in summer increase energy demand for cooling, which in turn can strain the power grid, resulting in potential brownouts and black outs.

 

• Higher air pollution levels and greenhouse gas emissions. In able to keep up with the increased energy demand, utility companies rely on the burning of additional fossil fuels at their power plants in order to harness more power. This results in greater emissions of heat-trapping greenhouse gases, such as carbon dioxide (CO2). Higher air temperatures also promote the formation of ground-level ozone.

 

• Greater risk for heat-related illness. Warmer days and nights can cause general discomfort and exacerbate heat waves and excessive heat events - all of which widens the window of time you're in danger of suffering from heat cramps, heat exhaustion, and heat stroke. Higher air pollution levels also cause breathing difficulties for those with asthma and other respiratory conditions.

Is Your Home a Heat Island?

Do you live in an urban heat island city? If so, what actions (planting trees and vegetation, green roofs, cool roofs, cool pavements, etc.) is your community taking to reduce the heat island effect? To find out, visit the EPA's Urban Heat Island Community Actions Database.

 

Sources:

The Environmental Protection Agency (EPA). The Heat Island Effect: Heat Island Impacts. Accessed July 27, 2014. http://www.epa.gov/heatislands/impacts/index.htm

UCAR Center for Science Education. Urban Heat Islands. Accessed July 28, 2014. http://scied.ucar.edu/longcontent/urban-heat-islands