What Is an Urban Heat Island?

An urban heat island is any city that experiences higher air temperatures than its surrounding rural areas. (The word “island” isn’t literal but, rather, an analogy for the isolated hotter temperatures.) 

Most cities experience the urban heat island effect to some degree. However, cities in population-dense regions and humid climates (think Los Angeles and the southeastern United States) experience the effect more intensely.

According to the U.S. Environmental Protection Agency (EPA), city centers generally measure 1-7 degrees F hotter during the daytime and over 2-5 degrees F warmer at night than their less-developed neighbors. However, as the National Weather Service noted on Twitter in February 2021, temperature differences of more than 20 degrees aren’t uncommon.

With heat stress projected to be twice as large in cities compared to surrounding rural areas by the mid-21st century, according to a 2017 study in the journal Geophysical Research Letters, the urban heat island effect will only increase in future decades.

What Causes a Heat Island Effect?

Trees and other vegetation act as nature’s air conditioners by providing shade and evaporating water from their soil and leaves. Heat islands form when natural landscapes are replaced by asphalt, concrete, and stone used to construct roadways, buildings, and other structures.

These man-made materials absorb, store, and re-emitting the sun’s heat more than the natural landscape would. As a result, surface temperatures and overall air temperatures climb increase. The mere hustle and bustle of city life (traffic, factories, and dense crowds) also generates waste heat, which further exacerbates the heat island effect.

While the heat island effect is typically considered to be a summer phenomenon, it can be felt during any season, including winter, and at any hour of the day. That said, it is most noticeable after sunset, when pavement and other city surfaces release stored-up heat from earlier that day.

The effect is also strongest when there are clear skies and calm winds, since these conditions maximize the amount of solar energy reaching city surfaces, and minimize the heat that’s carried away, respectively.

The Impact of Urban Heat Island Effect

City-dwellers may consider higher temperatures an unavoidable part of city life (along with noise, light pollution, and the occasional rodent), but the heat island effect shouldn’t be taken lightly. Cities are becoming increasingly vulnerable to the negative impacts of urban heat as Earth’s climate warms.

Increased Risk of Heat Illness

By elevating high temperatures during the day and discouraging atmospheric cooling at night, urban heat increases the risk of heat-related illnesses, such as dehydration, heat stroke, and even death. Heat is the leading cause of U.S. weather-related fatalities in the most recent 10- and 30-year periods.

Increased Energy Consumption

Energy demand is also higher within heat island cities, since residents depend more heavily on air conditioning and fans to keep cool during summer months. This, of course, means higher utility bills. It can also mean power outages if the demand for electricity becomes so high that it overloads the energy grid and triggers citywide brownouts or blackouts.

Air Pollution

As fossil fuel power plants keep pace with summer’s increased demand for electricity, they release more greenhouse gases into the atmosphere. Urban heat also directly contributes to air pollution by mixing with vehicle exhaust to form ground-level ozone (O₃). The sunnier and hotter the air becomes, the faster the rate at which ozone forms.

How Are Urban Communities Cooling Off?

Most efforts to cool urban communities rely on reintroducing vegetation back into city settings so as to mimic the natural cooling, shading, and reflective techniques of Mother Nature herself. For example, some cities are adding more parks, green spaces, golf courses, tree-lined streets, and urban farms to their development projects. 

Communities are also increasingly adopting “green-” or eco-architecture, and are including features like green roofs, which lower indoor and outdoor temperatures, in building designs.

A few cities are also taking initiatives to reduce the impacts of heat islands by boosting the reflectivity of existing city surfaces. New York City, for instance, added rules on white roofs to its building codes as long ago as 2008. (White surfaces, such as fresh snow, reflect up to 90 percent of sunlight, compared to dark surfaces, such as asphalt, which reflect around five percent.) Similarly, Los Angeles, California, has introduced various “cool pavement” pilot projects during which the city painted traditional asphalt roadways shades of light gray and white.

Such seemingly simple actions can have significant effects. A study by the Victorian Centre for Climate Change Adaptation Research found that by increasing vegetation in Melbourne, Australia, by 10 percent, the city’s daytime air temperatures cooled by nearly 2 degrees F during extreme heat events.