Urban Heat Islands
Content on urban heat islands, heat impacts, and recruitment for the Urban Heat Island Mapping Campaign was developed for the City's Climate Protection Program through the UVA Climate Ambition Summit summer internship program in 2021. We’d like to acknowledge and thank Seth Evans, Grace George, Gavin Oxley, and Yulun Wu for their work.
Heat and its effects on environments
How do urban heat islands form?
During the day and night, rural and urban areas are subject to the same sun exposure, yet they have different temperatures that occur. This happens because the composition and geometry of these two areas are drastically different. An urban area is mostly made up of asphalt and buildings, amongst other opaque materials; in contrast, a rural area is going to have more green areas and typically fewer vertical structures. In an urban area there is the added factor of this phenomenon called an urban surface heat island as well as an urban atmosphere heat island. These two things are different as the surface and atmosphere can have differing temperatures for a variety of reasons. The surface heat islands are more dramatic and influential to humans during the day than the atmospheric ones are. These surface heat islands linger throughout the day and night and can have temperatures that are 1-7 degrees Fahrenheit warmer and 2-5 degrees warmer, respectively so. These urban heat islands form in large and small cities, as well as northern and southern ones, however, it does seem to be more dramatic in eastern cities.
Figure: Variations of Surface and Atmospheric Temperatures (source: U.S. EPA)
These heat islands form from a variety of reasons such as:
- Reduced natural landscapes: vegetation and water bodies provide shade, transpire water from plant leaves, and evaporate surface water which cools the air. Hard, dry surfaces found in urban areas, such as roofs, sidewalks, roads, buildings, and parking lots provide less shade and moisture, leading to higher temperatures and less cooling effects.
- Urban materials: pavement and roofing tend to reflect less solar energy and instead absorb and re-emit the sun’s heat more than natural landscapes. Due to the absorption during the day, heat islands can build throughout the day and become more severe after sunset as the heat slowly releases from urban materials. According to the EPA, downtown areas can absorb and store twice the amount of heat compared to their rural areas during the day.
- Urban geometry: dimensions and spacing of buildings within a city affect the urban material’s absorption of solar energy and the design of a city can block wind flow. This can combine with the issue nighttime cooling, in that the heat that is released by urban material gets trapped by the urban design.
- Human activities: anthropogenic waste heat, such as vehicles, air-conditioning units, and industrial facilities.
- Weather and geographic location affects heat islands as well, for example, clear weather conditions allow more solar energy to reach the urban area while strong winds and clouds may suppress heat. Being located near large bodies of water can dramatically help with temperature moderation or having nearby mountains can block wind from entering the city.
How Does High Heat Affect Charlottesville?
Impact on Human Health
- Heat is one of the most dangerous natural phenomena that affects people, according to sciencedirect.com “records around the world show almost 59,114 persons passed away during extreme heat events.” With the unfortunate fact that for next few centuries we will only be having hotter summers, it is crucial that we start to mitigate the rise of heat in local areas in whatever possible ways science has to offer. Urban areas are also subject to elevated levels of human activity which lessens air quality and combines to be an even greater threat for heat exhaustion.
- Elevated temperatures also are a threat to the local water supply for people. Aquatic lifeforms and ecosystems are especially vulnerable to heat fluctuations. They have adapted to relatively stable temperature environment, as changing the temperature of water requires a lot of energy, so with the increase in temperatures in atmosphere it also raises the temperature of standing water and groundwater. This will lead to mass death-offs which will contaminate the water that is available for people.
- For every 1°F increase in summertime temperatures there is an increase of 1.5-2% in urban electric demand. As it gets warmer, people demand more energy to the eventual reality that there will not be enough energy for everyone. Thus, there is the threat of rolling and controlled blackouts to avoid power outages.
Environmental Racial Justice
- The unfortunate truth about urban heat islands is that they do not exist at random across urban landscapes. In a research paper published by Climate, the paper reported that “94% of studied areas display consistent city-scale patterns of elevated land surface temperatures in formerly redlined areas relative to non-redlined neighbors by as much as 7 °C (44.6 °F)”. Evidence also displayed that the urban heat islands were more closely related to race than poverty level. As Charlottesville continues its reconciliation with racial discrimination, it is imperative that it addresses and begins to combat this issue as well. To learn more about redlining and intersections between race and development patterns in Charlottesville, please visit the independent Mapping Cville project website.
Climate Change Mitigation
- Climate change in the 21st century has nearly cemented the idea that every summer from here on out will be not only the warmest one yet, but also one of the coldest for the coming centuries. Increases in extreme heat events are one of the top climate hazards that Charlottesville is expected to face.
Map of Charlottesville by Land Cover Type
What are Ways to Combat Heat Islands?
The primary objective to combatting heat islands is to identify where they are and the specifics of the locations. The identification of heat islands begins with the pinpointing of them using satellite data. This way of getting data is limited by the distance and lack of focus from this instrument. The other issue is that the satellites can really only detect the atmospheric temperature of the area and not the detection of the surface heat islands. To find the actual heat of these surface heat islands there needs to be physical and direct observations of suspected areas.
- Green Roofs
In Chicago on the roof of its city hall, they installed 20000 various plants. The energy savings that accumulated form this have rounded out to $3,600 annually and saved more than 9,270 kWh. Charlottesville’s City Hall also has a green roof. It can be viewed from the NE stairwell of Market Street Parking Garage and has a signage about the roof project.
- Cool Roofs
In Tucson, they documented how a cool roof reduced temperatures inside and on the roof of the building. This was achieved by the installation of a white elastomeric coating over a 28000 sq ft. unshaded metal roof. After installation, energy savings were calculated at 50-65% of the building’s cooling energy.
In order to spur individual heat island reductions, incentives are sometimes available from governments, utilities, and other organizations. This can be in form of coupons for purchasing trees from nurseries (Baltimore), having grant programs for green and cool roofs (Chicago), partnering with local programs to install free shade trees (Sacramento), and many other examples.
Urban Forestry Programs
- These programs are called urban forestry or tree planting programs. They exist in most large cities and counties in the United States. They usually have support from diverse stakeholders and they are often short-term, one-time programs that have long-term community revitalization. These programs are effective for helping low-income communities.
- Enacted tree and landscape ordinances can help ensure public safety, protect trees or views, and provide shade. Most useful ordinances for heat island mitigation have been: tree protection, street trees, and parking lot shade.
This is the process of making the homes of qualifying residents, generally low-income families, more energy efficient at no cost to the residents. States use weatherization funds provided by the US Department of Energy (DOE) Weatherization Assistance Program (WAP) to help recipients cover heating bills and invest in energy efficiency actions that lower costs. In Philadelphia they used the DOE’s WAP to create the Cool Homes Program, which installed cool coatings on 550 homes and eliminated 90% of the heat gain through the ceiling. This lowered A/C loads by one third in a typical rowhouse.