Ep 4: Urban Heat Island Effect

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Introduction

The Urban Heat Island (UHI) effect is commonly referred to as a “heat accumulation phenomenon” that occurs in metropolitan areas that are at significantly higher temperatures than their surrounding and/or rural areas (Yang et al., 2016; Center for Science Education). The main factors that contribute to the UHI effect include urban heat release, urban surface temperatures, vegetation coverage, population density, and weather conditions (Yang et al., 2016).

Heat Island Characteristics

While there are many contributing factors to the UHI effect one of the main causes is related to urban infrastructure and surface temperatures. Infrastructure includes various things such as roads, buildings, parking lots, roofs, and sidewalks (EPA). In the UHI effect, infrastructure is more likely to absorb and re-emit the sun’s heat and because there is increased density and less vegetation in urban cities, the city effectively turns into an “island” that traps the heat and leads to overall higher temperatures (EPA) Often there may even be differences throughout an urban city which is known as Intra-Urban Heat Islands (EPA). As an environmental justice concern, intra-UHI will be discussed in greater detail later.

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Measuring the Heat Island Effect

The UHI effect is the easiest way to see how humans have impacted and changed our planet (NASA). From the development and expansion of urban cities, the amount of infrastructure and reduction in natural landscapes has drastically changed. Rather than measuring an increase in a city’s temperature over time, those studying the UHI effect will measure the relative difference in temperatures between cities and surrounding areas (EPA). Per the Environmental Protection Agency (EPA), temperatures could be as much as 10 degrees Fahrenheit warmer (Center for Science Education). Typically, the highest temperatures are recorded three to five hours after sunset as infrastructure begins to release the heat it has absorbed throughout the day (Center for Science Education). Favorable conditions for UHI include clear skies, low wind speeds, higher temperatures, and higher humidity (Heaviside et al., 2017). Additionally, we can measure the UHI effect through ground observations of temperature differences in multiple geographic locations, satellite imaging to detect differences in thermal radiation, and modeling through meteorological models (Heaviside et al., 2017).

Environmental Impact

As it may be assumed, the UHI effect has a significantly negative impact on our environment and is exacerbated by increased urbanization (Yang et al., 2016). Since the 1960s we continue to see an increase in urban temperatures on a global scale (Yang et al., 2016). The primary causes of UHI include:

Reduced Natural Landscapes in Urban Areas

Natural landscapes are more likely to naturally cool the air because of shade, transpiration from plants, and evaporating surface water (EPA). However, in urban cities with built infrastructure, there is a drastic reduction in natural landscapes. These reductions lead to less shade and moisture, meaning there is an increase in temperatures (EPA). Discussed more in the following section, man-made infrastructure materials are impermeable to water absorption, which would typically be a natural cooling mechanism for vegetation (Center for Science Education). This reduction in vegetation and increase in infrastructure causes an increase in temperature and decrease in evapotranspiration as a natural cooling strategy (Center for Science Education). Transpiration occurs when plants take up and store water from the ground through their roots and then evaporates into the environment as it gradually turns into water vapor (NASA).

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Urban Infrastructure Materials

Because of the types of materials that are used, man-made surfaces are less likely to be able to reflect the sun’s light, meaning there is an increase in absorption and re-emitting of heat (EPA). Throughout the day these surfaces accumulate heat and then as the sun sets slowly emit heat back into the atmosphere (EPA). In addition to their absorption properties, man-made materials are also more impermeable to water which leads to a lack of evapotranspiration creating a phenomenon known as urban deserts (Center for Science Education).

Urban Geometry

Urban geometry primarily focuses on the density of buildings. Cities with higher density buildings cause a decrease in wind flow and overall ventilation of heat (EPA). For example, in New York City, tall buildings and narrow streets create the perfect combination for blocking natural wind flow and creating urban canyons (EPA; Center for Science Education). Because there is less air flow, the heat that is released from the infrastructure (buildings, roads, sidewalks, vehicles) throughout the day is unable to be removed and becomes stagnant increasing ambient temperatures.

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Anthropogenic Heat

Anthropogenic heat, or heat generated from human activities is a significant contributor to the UHI effect. This form of heat generates waste heat from two main sources humans via outdoor activities and driving, and tools via air conditioning units, industrial facilities and vehicles (EPA; National Geographic Society, 2012). In urban cities with an increase population means there is a greater amount of waste heat causing an increase in temperatures and the UHI effect (National Geographic Society, 2012).

Weather and Geography

As previously mentioned, calm and clear days are the ideal condition for the UHI effect and certain geographic locations increase the likelihood of the UHI effect. For example, because Las Vegas, Nevada is situated in a valley surrounded by mountains they are less likely to receive wind and rain needed to reduce the UHI effect (EPA). Additionally, cities located in humid regions are more likely to experience higher temperatures and therefore the UHI effect (EPA).

Photo Source: NASA

Air Pollution

Urban haze, or air pollution, in large cities creates a large-scale greenhouse effect because it stops thermal radiation (heat) from escaping urban areas (Center for Science Education).

Changes in Weather Patterns and Climate

When considering the effects of the UHI effect, many are quick to think that the UHI effect contributes to climate change however there are some major differences between the two concepts. Climate change is measured as the rise in temperature of the Earth’s surface temperature, whereas heat island is measured as the difference between urban and rural areas (Center for Science Education). Because of these differences it is evident that urban heat isn't primarily to blame for climate change yet is rather an effect of climate change (Center for Science Education). While there are many environmental effects of the UHI effect, evidence shows that the UHI effect also impacts weather patterns such as (Heaviside et al., 2017):

Rainfall Patterns
Worsening Air Pollution
Increased Flood Risk
Decreased Water Quality

Human Health Impact

While the UHI effect has a major impact on our overall environment, its greatest impact is on human health in two main ways –human behaviors and physiological health. While all individuals are susceptible to the negative effects of the UHI effect there are many who are at an increased risk, including:

Young Children (Shahmohamadi et al., 2011)
Seniors (Shahmohamadi et al., 2011)
Chronically Ill (Shahmohamadi et al., 2011)
Those with Compromised Health Status (Shahmohamadi et al., 2011)
Those Living with Poor Air Quality (Shahmohamadi et al., 2011)
Those without Access to Air Conditioning (Shahmohamadi et al., 2011)
Those on Certain Medications (Cardiovascular and Respiratory Related) (Shahmohamadi et al., 2011)

And as many are impacted, this number will only increase. By 2050, 70% of the population is expected to be urban dwellers (Center for Science Education). This means that as more people move into urban areas, the UHI effect will continue to increase and increased generation of air pollution due to waste and anthropogenic heat (Shahmohamadi et al., 2011).

Human Behaviors

In general, the main change to human behavior due to the UHI effect is an increase in anthropogenic heat, which is heat generated by human activities. In simple terms, higher heat means humans will be using more electricity due to fans and air conditioning units (NASA). This increased concentration of electricity being used is going to cause an increase in power outages which only exacerbates the negative physiological effects of heat stress on the body (NASA; Anderson & McMinn, 2019).

In addition, these changes in temperature can negatively affect humans due to an increase in:

Foodborne Illnesses (Shahmohamadi et al., 2011)
Vector Borne Infectious Diseases (Shahmohamadi et al., 2011)

Bacteria or viruses causing foodborne illnesses, and those that cause vector borne infectious disease are going to thrive on warm and humid environments which are created by the UHI effect.

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Physiological Health

Overall, the UHI effect will have a number of physiological health concerns for humans, primarily due to the added stress of heat on the body. For example, heat stress may lead to:

Elevated Body Temperature (Shahmohamadi et al., 2011)
Dehydration (Shahmohamadi et al., 2011)
Heat Related Illness (heat stroke, fainting, cramps, heat rash) (Shahmohamadi et al., 2011)
Exacerbated Existing Medical Conditions (asthma and allergies) (Shahmohamadi et al., 2011)
Physical and Mental Stress (Shahmohamadi et al., 2011)
Respiratory and Cardiovascular Disease (Shahmohamadi et al., 2011)
Digestive System Disease (decrease appetite and indigestion) (Yang et al., 2016)
Nervous System Disease (insomnia, irritability, depression, memory decline) (Yang et al., 2016)

When considering the physiological effects of the UHI effect on human health, scientists often reference the Thermal and Humidity Index (THI). The THI is used to determine how different temperatures will impact the human body (Yang et al., 2016). For example, a range of 15 to 20 degrees Celsius is the most comfortable for humans, where as anything below or above those temperatures would be considered uncomfortable and negatively impact a number of things such as quality of life, general discomfort, and work productivity (Yang et al., 2016).

Effects of a Temperature Spike

When temperatures spike in lower income cities, there is an increased number of emergency calls (Anderson & McMinn, 2019). Overall, data shows an increased number of calls in the following areas:

20% increase for respiratory distress (Anderson & McMinn, 2019)
- Due to triggers from high heat and humidity
80% increase for cardiac arrest (Anderson & McMinn, 2019)
- Due to the body being unable to circulate blood properly and cool the body
50% increase in high blood pressure (Anderson & McMinn, 2019)
General increase in (Anderson & McMinn, 2019):
- Psychiatric disorders
- Substance use
- Dehydration
Increased number of visits among Medicaid patients due to (Anderson & McMinn, 2019):
- Asthma
- COPD (70% increase)
- Heart Disease
Increased number of calls for fatal heat stroke (Anderson & McMinn, 2019)

Historical Overview

1833

UHI effect first recorded in London (Heaviside et al., 2017) by Luke Howard (Yang et al., 2016)

1920 to 1940

Field of local climatology beginning to emerge through the world (Urban heat island 2022)

1929

1929 – Albert Peppler coined a phrase that is most similar to UHI in German (Urban heat island 2022)

1930s

1930s – Guy Stewart Callendar studied temperature records in Europe and North America determining that the earth had warmed. Originally thought to be due to Greenhouse effect, was actually more closely related to the idea of UHI effect (2022).

2007

2007 – Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) – UHI has less of an effect on overall global warming that once through (2022).

2007

2007 – Globally more people were living in urban than rural areas (Heaviside et al., 2017)

Overall, the UHI effect has been a highly debated topic throughout the 20^th and 21^st centuries and is constantly evolving making it difficult to list a specific overview of this phenomenon.

Environmental Justice Perspective

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Not only do UHI impact human and environmental health, but there is also an environmental justice component as well. Cities that are experiencing the strongest correlations between heat and income include (What is the urban heat island effect? 2022):

Oakland, CA
North Las Vegas, NV
Anchorage, AL
Las Vegas, NV
Lexington, KY
Los Angeles, CA
Minneapolis, MI
Chesapeake, VA
Toledo, OH

That’s just to name a few; out of 14,000 United States neighborhoods, communities of color are more likely to be located near smaller asphalt-filled parks, and parks are five times as likely to be crowded compared to parks in predominantly white communities (What is the urban heat island effect? 2022). In general, these lower income areas are going to have less green cover and be at greater risk for the UHI effect (What is the urban heat island effect? 2022).

As a specific example, a neighborhood in Baltimore known as Franklin Square is two-thirds (2/3) hotter than other neighborhoods, proving to be 6 degrees hotter than the coolest neighborhood (Anderson & McMinn, 2019). Franklin Square is among the city’s poorest neighborhood with one-third (1/3) of residents living in poverty (Anderson & McMinn, 2019).

Outside of the poorest neighborhoods in the country being impacted by the UHI effect, many of these areas are also located near undesirable lands such as highways and industrial areas, which create more heat (What is the urban heat island effect? 2022). Because of redlining, this probably only continues to increase and while redlining has been banned, there aren’t many differences in the separation of neighborhoods today compared to 1937 (What is the urban heat island effect? 2022). Redlining formally began in the 1930s and was banned by the Fair Housing Act in 1968. Prior to becoming banned, areas were marked as high risk for mortgage lenders based on the racial makeup of the neighborhood (What is the urban heat island effect? 2022).

Residents who are in these hotter communities are at a great risk for the human health impacts associated with the UHI effect and at a great risk for experiencing and having UHI be exacerbated by the environmental impacts. These individuals already have a decreased access to health care so they are less likely to get the medical care that is needed when experiencing some of these human health effects (What is the urban heat island effect? 2022).

Looking Towards the Future

Looking towards the future, the best strategy to reduce and alleviate the effects of UHI is reduction. Efforts include:

Lightning Streets

Streets are primarily made up of dark asphalt which is able to absorb more heat causing the surface to become very hot. Streets can be lightened by resurfacing using a reflective gray coating. This coating will help reduce the absorption of heat and ultimately the UHI effect (NASA).

Photo Source: Arch Daily

Permeable Building Materials

Most buildings are meant to keep water out and such past and current building materials are impervious and do not allow for water to flow through. Without utilizing permeable building materials, the material itself is not able to effectively cool (NASA).

Photo Source: OSU

Lightening Buildings

First, we could use lighter colored building materials. By having building materials that are able to increase the reflection of sunlight, we are able to reduce surface temperatures (National Geographic Society, 2012).

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Reflective Rooftops

Reflective roof tops could be installed to effectively reflect all the sun's light. By completely reflecting the light, we can mitigate a great deal of heat from being absorbed (Anderson & McMinn, 2019).

Photo Source: Green Building Advisor

Green Rooftops

Another building strategy is to install green rooftops. Green rooftops are roofs that are covered in greenery and vegetation to reduce absorption of heat by allowing transpiration. Additionally, green rooftops have the capability of absorbing CO2, reducing pollution, a factor that exacerbates the UHI effect (National Geographic Society, 2012).

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Increased Green Spaces

This item is especially important in lower income communities and neighborhoods. By increasing green spaces we are able to effectively reduce the environmental justice impact of the UHI effect. Not only that, in many communities individuals are able to reallocate vacant lots into community green spaces. By 2037, the city of Baltimore aims to increase tree canopy by 40% in low-income neighborhoods (Anderson & McMinn, 2019).

Photo Source: Scientific American

Improve Transportation

One of the main factors impacting the UHI effect in cities, is the pollution created by automobiles. By investing in public transportation, cities would be able to reduce the amount of pollution they are generating and reduce the UHI effect (Anderson & McMinn, 2019; Shahmohamadi et al., 2011).

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References

Anderson, M., & McMinn, S. (2019, September 3). As Rising heat bakes U.S. cities, the poor often feel it most. NPR. Retrieved March 19, 2022, from https://www.npr.org/2019/09/03/754044732/as-rising-heat-bakes-u-s-cities-the-poor-often-feel-it-most

Center for Science Education. Urban Heat Islands | Center for Science Education. (n.d.). Retrieved March 19, 2022, from https://scied.ucar.edu/learning-zone/climate-change-impacts/urban-heat-islands

Encyclopedia.com. (2022, March 20)." Climate change: In context. encyclopedia.com. 1 Mar. 2022. Encyclopedia.com. Retrieved March 19, 2022, from https://www.encyclopedia.com/environment/energy-government-and-defense-magazines/urban-heat-islands

EPA. (n.d.). Learn About Heat Islands. EPA. Retrieved March 19, 2022, from https://www.epa.gov/heatislands/learn-about-heat-islands

NASA. (n.d.). What is an urban heat island? NASA. Retrieved March 19, 2022, from https://climatekids.nasa.gov/heat-islands/

National Geographic Society. (2012, October 9). Urban heat island. National Geographic Society. Retrieved March 19, 2022, from https://www.nationalgeographic.org/encyclopedia/urban-heat-island/

Shahmohamadi, P., Che-Ani, A. I., Etessam, I., Maulud, K. N. A., & Tawil, N. M. (2011). Healthy environment: The need to mitigate urban heat island effects on human health. Procedia Engineering, 20, 61–70. https://doi.org/10.1016/j.proeng.2011.11.139

What is the urban heat island effect? American Forests. (2022, February 26). Retrieved March 19, 2022, from https://www.americanforests.org/article/what-is-the-urban-heat-island-effect/

Wikimedia Foundation. (2022, January 27). Urban heat island. Wikipedia. Retrieved March 19, 2022, from https://en.wikipedia.org/wiki/Urban_heat_island#History

Yang, L., Qian, F., Song, D.-X., & Zheng, K.-J. (2016). Research on urban heat-island effect. Procedia Engineering, 169, 11–18. https://doi.org/10.1016/j.proeng.2016.10.002

Published March 2022

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