What is an Urban Heat Island?

A typical urban heat island profile.
A typical urban heat island profile.

Urban heat islands occur, as the name suggests, in metropolitan areas. It is defined as the presence of significantly higher temperatures in urban areas compared to the temperatures in surrounding rural zones. Urban heat islands are more apparent in the winter and summer months, especially during nighttime hours and when wind levels are low. City centers are often several degrees warmer than the surrounding, less developed areas. During the day, this temperature difference can be between 1.8° and 5.4° Fahrenheit. In the evening, this difference can increase by as much as 20° Fahrenheit.

What Causes An Urban Heat Island?

The higher temperatures of urban heat islands can be attributed to human activity, particularly to changes in land surfaces. Urban development requires the use of significant amounts of cement and asphalt for roofing purposes and to pave sidewalks and roads. These materials have thermal bulk properties that absorb more solar radiation than the surfaces found in rural areas. Additionally, these materials have different surface radiative properties, which means they emit energy as thermal radiation or heat.

Many urban areas also lack green coverage, such as vegetation. Plants help create evapotranspiration, an important stage of the water cycle. During evapotranspiration, two exchanges happen: evaporation and transpiration. In evaporation, water evaporates from the soil, treetops, and bodies of water into the surrounding air. In transpiration, plant water content is lost as vapor through the stomata of plant leaves. The evapotranspiration process helps to cool the surrounding air. Not only do cities experience lower levels of evapotranspiration than rural areas, but they also have less areas of cooling shade and reduced removal of carbon dioxide (a greenhouse gas that contributes to increased temperatures).

Researchers have determined that urban heat islands are also caused by the urban canyon effect. This takes place due to tall buildings in close proximity, which provides multiple surfaces for reflecting and absorbing sunlight. Additionally, these buildings work to block wind from flowing through the city. Where wind cannot flow freely, convection cooling does not occur and air pollution is able to stagnate and increase. The increase in sunlight absorption and reflection, the buildup of pollution, and the lack of convection cooling all work together to increase surrounding temperatures as well. As urban temperatures rise due to this combination of events, ozone levels within the city also increase.

The Impact Of Urban Heat Islands On Weather Conditions

In addition to increasing temperatures, urban heat islands also influence local weather conditions. Some of these changes to weather conditions include: precipitation levels, cloud coverage, fog presence, humidity levels, and wind patterns. Increased precipitation and thunderstorm activity occurs because the hotter temperatures result in an upward movement of surrounding air. This upward movement has its greatest impact between 20 and 40 miles downwind of the city center, where precipitation levels can be 28% greater than upwind of the city center.

During the day, urban heat islands experience low pressure, which allows moist air from nearby rural areas to collect. This moist air counteracts with the hot, urban air and creates the ideal condition for cloud coverage and increased humidity.

City climates are, however, less likely to experience weak tornado activity because of turbulent mixing within the urban heat island. Another potential benefit of urban heat islands is that they work to increase plant growing seasons in nearby locations. For example, some studies have found that growing areas located around 6.2 miles outside of city limits are extended by an average of 15 days.

The Impact Of Urban Heat Islands On Animals

In some cases, urban heat islands attract more animal life. This is particularly true of colonizing species, like ants, common house geckos, and grey-headed flying fox. The grey-headed flying fox, for example, has recently been attracted to Melbourne, Australia due to the increased heat index within city limits. This increased temperature has made the city’s winter climate similar to the species’ common habitat in the northern regions.

In other cases, urban heat islands negatively affect food and water supplies within city limits. For example, the surface temperature of city buildings is significantly hotter than the surrounding air. Rain then runs off of these much hotter buildings, warming up in the process. This warmer runoff makes its way into nearby streams, rivers, lakes and other bodies of water. When the heated runoff enters these bodies of water, it goes on to increase their temperature as well. This is referred to as thermal pollution and it can increase water temperatures by as much as 20° to 30° Fahrenheit. Thermal pollution has a negative impact on local fish populations, sending them into thermal shock.

Because the plant growing season is often extended in urban heat islands, the food supply for other animal species is also affected. Animals that rely on certain plants as an integral part of their diet begin to adapt their natural breeding seasons to match the plant growing season. This, in turn, has a chain reaction on other plant and animal species in the area.

The Impact Of Urban Heat Islands On Human Health

Urban heat islands not only affect the health of the environment and local wildlife, but also the health of humans. Increased city temperatures can be fatal during summer heatwaves, particularly for senior citizens. In the US alone, an average of 1,000 people die every year due to extreme heat, which is exponentially increased by urban heat islands. Urban inhabitants also suffer during heat waves because the urban heat island prevents nighttime temperatures from decreasing as they do in rural areas. Research has found that urban heat islands not only increase temperatures during heat waves but also prolong their duration.

Extreme temperatures lead to heat cramps, heatstroke, and heat exhaustion. Heatstroke may cause respiratory distress syndrome, impaired mobility, or decreased awareness. Individuals with already sensitive health conditions, such as diagnoses of Parkinson’s disease or dementia, are particularly susceptible to significantly impaired cognitive functioning during heat waves. Additionally, people with diabetes, obesity, asthma, sleep deprivation, or cardiovascular disorders are advised to heat exposure. In these cases, heat exposure may aggravate and intensify symptoms. If these resulting heat-related conditions are severe enough, permanent organ damage could occur. If an individual survives extreme heatwaves with permanent organ damage, they have an increased risk of early death.


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