Large cities and urban sprawls affect the environment in a wide variety of ways, ranging from generating substantial quantities of pollution to contributing to deforestation. The activities that civilians who populate these areas partake in each day impact the climate the most through producing high levels of CO2 emissions, but even the very presence of metropolitan areas causes measurable harm on our planet. When densely populated urban areas yield enough anthropogenic activity and artificial landscape from materials such as cement and asphalt, they will become significantly warmer than surrounding areas. This report will discuss some key aspects of this phenomenon, which is commonly known as an Urban Heat Island.

Urban Heat Islands (UHIs) are metropolitan areas that are measurably warmer than surrounding areas. This is caused by human activities that result in increased air temperatures and the synthetic materials that make up the streets and buildings of a city. Because human activity peaks during the day, the temperature difference is often most noticeable before night.

However, these heat pockets that cities form can and still do occur during the night as well. The temperature increase in urban areas is mainly brought about when natural land surfaces such as grass are covered by artificial matter such as asphalt. Although natural turf mainly reflects heat emitted from the sun, roads and sidewalks absorb the heat due to their composition. This leads to greater temperatures for sustained periods until hours after sunset.

The emergence of UHIs is the direct result of replacing natural vegetation such as grass and trees through deforestation and placing cement, concrete, and asphalt over the land to construct buildings and roads. The roads and sidewalks that are made absorb great amounts of heat from the sun, remaining at elevated temperatures until they have time to cool down during the night. Buildings contribute to UHI since they are very good at storing heat. When houses or skyscrapers are built closer together, this insulative effect is magnified even further. Waste heat from human activity also raises urban temperatures when released into the air.

Heat islands produce a number of effects on the climate and human health. Raised temperatures stemming from urban regions can induce changes in meteorological patterns, and lead to greater thunderstorm activity by precipitating cloud and fog formation, along with increased humidity. Changes in weather phenomena onset by UHI temperature differences are mainly experienced only on a local scale, however. The more widespread effects of heat islands on the climate are the result of CO2 emissions, which is not an occurrence unique to cities, but rather a relationship caused by most human activity.

The raised city temperatures in UHIs can contribute to an elevated risk of heat stroke. As heat islands are fundamentally characterized by their raised temperatures, their formation increases the possibility and duration of heat waves within cities. During these heat waves, civilians may be in danger of overheating, which can raise the mortality rates of cities. Collectively, UHIs can then raise the mortality rates of their respective countries. In the United States, UHIs contribute to the 1,000 heat-related deaths annually through this increased risk.

Now that you know what an urban heat island (UHI) is, below are some techniques that you can use to reduce the presence and impact of these heat pockets:

1. Increase shade around houses

2. Install green roofs

3. Install cool roofs

4. Use energy-efficient appliances

5. Use air conditioning to reduce heat-related illness

New York City is one of the best examples of UHIs in the US (Credit: CC0 Public Domain)

References

American Meteorological Society

Environmental Protection Agency
NASA

National Geographic

UCAR Center for Science Education