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The Urban Heat Island Effect

Though summer is waning, regional temperatures in Southeast Pennsylvania are still reaching highs in the nineties this week. It is not likely these hot, dry days are about to disappear either, even with the approach of autumn. This intense heat is even worse in cities and urban spaces thanks to the urban heat island effect (UHI). You have likely experienced this phenomenon passing a building or crossing concrete into a bubble of hot air. In urban settings, the surrounding infrastructure absorbs and re-radiates heat into the local environment, thus raising ambient daytime temperatures. Due to the nature of a built-up environment, similar amounts of solar energy will be experienced differently in an urban setting versus a non-urban setting.

How surfaces temperatures are experienced in different environments. Photo from the EPA:

Urban spaces are conducive to heat islands for a few reasons. As urban settings have less vegetation (in comparison to continuous forests and non-urban environments), less canopy cover is available to provide shade and cooler air temperatures. Plants do not absorb as much as solar energy as synthetic surfaces do. Some building materials are more heat-conductive, such as asphalt pavements and rooftops. A high density of buildings can reduce airflow; therefore, hot air collects rather than dissipates via the wind. Human activities tend to exacerbate heat islands as well. Exhaust or air pollution from cars, which also absorbs heat, and the release of heat into the environment, a byproduct of high energy demand in cities, all contribute to the UHI.

The UHI is measured at the surface (surface UHI) or above the ground to the top of buildings and tree lines (known as canopy UHI). Air temperatures measured at the canopy UHI level can be measured directly by sensors (like radiosondes) and weather stations, but these methods provide more localized data. Landsat satellite imagery and infrared thermography can measure and map city-wide patterns of surface heat distribution. Combining these methods and climatic patterns helps scientists understand the heat footprint of a city and surrounding areas.

A heat map of Philadelphia from Arizona State University (2015).

Because building materials used in urban areas slowly release heat, relatively higher temperatures can persist for a long time and are not likely to greatly fluctuate. Even after the sun sets, cities remain. Surfaces that retain heat are problematic for the environment and regional ecology. Warmed surfaces can heat stormwater runoff and raise the temperatures of local streams. Aquatic species intolerant to heat stress and altered metabolism rates suffer as a result.

Cities experience high amounts of particulate matter and pollution; this is responsible for hazy smog often seen around urban settings. Rising hot air during the daytime can carry smog particulates high above cities. A bubble of smog can trap heat, so as the smog particulates disperse over the city atmosphere, warm air wrapping around the city prevents cooler air and nighttime temperatures from cooling the urban environment.

Heat islands are problematic for human health. Regions all over the world have been experiencing record high temperatures this summer, and this trend is likely to continue with climate change. In the event of a heatwave, which can be exacerbated in urban environments, some people are at greater risk of heat-related illnesses than others. Lower-income communities that lack AC or live in smaller facilities are more likely to suffer from the heat. Older adults, young children, and those with health issues are also susceptible to the heat. Those working outside or those without access to reliable shade are vulnerable as well. As heat disproportionately harms certain members of society more than others, it is important to reduce contributions to UHIs.

As climate change increases regional air temperatures, local UHIs will continue to be an issue and may worsen in time. Luckily, urban settings can be designed to combat intense heat. Creating green infrastructure, like parks and ponds, can help reduce heat islands. Using "cool infrastructure," such as using reflective colors in building materials, may mitigate UHI. Increasing canopy cover by planting street trees can increase shade and help cool ambient air temperatures. Promoting permeable surfaces will reduce the amount of heated storm runoff impairing stream quality. Planting gardens and increasing vegetative cover on surfaces in cities will reduce the amount of heat-absorbing materials present in urban areas. By increasing green spaces and relief from the heat, cities will better support the health of the local community and environment - a win for everyone.

Cool infrastructure, such as rooftop gardens and street tree canopies, can mitigate the adverse effects of UHI.

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