The Roof is on Fire, part 3: Looking for a Breath of Cool Air
Summers in Baltimore, MD can be brutally hot. While I was working in Baltimore during one Summer in the late 1980s, I lived on the top floor of a three story row house, with a black tar roof and brick façade. The row house baked in the heat during the day and at night retained that heat, making the top floor feel like an oven. My first few nights were spent without an air conditioner, and being young and overoptimistic, I attempted the use of a simple box fan to try and keep me cool. That was an utter failure!
Then I decided after a particularly scorching day and night that I would wrap my head in a cold towel that would cool my head off and allow me to fall asleep. Alarmingly in the middle of the night my head suddenly felt like it was being boiled in water after the moisture in the towel achieved the ambient or "wet bulb temperature". I finally splurged and spent money on an air conditioner, and, ahh.... my sleeping at night returned to a deep slumber.
In 2021 Baltimore experienced 50 days with temperatures above 90 deg. F, and "by 2080 scientists predict that the region's climate could warm by 9 deg. F, making it feel closer to the climate of Mississippi than Maryland" (Popkin, Gabriel, Rebooting our Warming Cities, Johns Hopkins Magazine Summer 2023).
While we can expect it to take decades to bring down global temperatures, that is, if we act decisively with commitment now, we must take action now to avert further catastrophic effects from global warming.
Urban heat island profile. Lawrence Berkeley National Laboratory, Heat Island Group.
Urban Heat Islands
Most cities in the North America consist of buildings and infrastructure that retain and add to the heat, creating Urban Heat Islands (UHIs). UHIs tend to have more pavement, fewer trees, and higher building densities, all of which absorb and retain heat more than natural surfaces. Some common design factors that contribute to the UHI effects:
Concrete and Asphalt Canyons: Cities are often built with extensive impervious surfaces such as asphalt roads, concrete sidewalks, and buildings with large amounts of concrete, steel, and asphalt. These surfaces absorb and store heat from the sun during the day, releasing it slowly at night, leading to elevated temperatures in urban areas. In addition, the layout of buildings, streets, and infrastructure, while helpful for earlier urban development and commerce, often create "urban canyons" that trap heat. Tall buildings block airflow, hindering natural ventilation and reducing the dissipation of heat; lower Manhattan is generally cooler than midtown for this reason. This stagnant air and limited circulation contribute to higher temperatures.
Lack of Vegetation: Urban environments typically have fewer trees, parks, and green spaces compared to rural areas. The removal of vegetation reduces shade and the cooling effect of evapotranspiration, where plants release moisture into the air through their leaves. Without sufficient vegetation, urban areas experience higher temperatures.
Energy Consumption and Heat Emission: The extensive use of air conditioning, industrial processes, and infrastructure have high energy demands in urban areas and also lead to the release of waste heat from these sources. Added to this is heavy vehicle traffic, which emits waste heat from engines and exhaust systems, and the cumulative effect of numerous vehicles significantly increases local temperatures.
As a result, UHIs can have temperatures that are up to 10 deg. F higher than surrounding rural areas. That 10 deg. can make a big difference when dealing with normal heat during the Summer, but can significantly threaten a community's health and well-being with increased risks from reduced air quality and increased heat-related illnesses and deaths. In addition, UHIs can lead to increased stress on infrastructure and reduced property values in the long term.
Urban Heat Islands: So What, Now What?
According to the World Bank 56% of the world's population live in cities, and this trend is expected to continue, with an expected doubling by 2050 with nearly 70% of the world's population living in urban environments. With much of that urban population will be in Asia, in India and China, Latin America, and Africa. All of which will put increasing stress on city systems and environments.
Source: Santa Monica Mirror: https://smmirror.com/2017/08/board-supervisors-takes-first-step-toward-energy-saving-cool-roofs-built/
However, there are a number of strategies that can be used to mitigate the effects of UHIs. Some of these strategies include:
Increased Tree and Vegetative Cover: Trees and vegetation provide shade and help to cool the air through a process called evapotranspiration. Evapotranspiration is the process by which water evaporates from plants and the ground, and then rises into the atmosphere. As water evaporates, it takes heat with it, which helps to cool the air.
Cool Roofs: Cool roofs are designed to reflect more sunlight and absorb less heat than traditional roofs. This can help to reduce the amount of heat absorbed by buildings, which can in turn help to reduce the overall temperature of the urban area. Just the simple practice of using a white roof which can reflect the suns rays can reduce the heat in a building. Just look at the Caribbean, where white rooves are the standard on many islands.
Cool Pavements: Cool pavements are designed to reflect more sunlight and absorb less heat than traditional pavements. This can help to reduce the amount of heat absorbed by roads and other paved surfaces, which can in turn help to reduce the overall temperature of the urban area.
Smart growth practices: Smart growth practices are designed to promote compact, walkable development that minimizes the use of cars. This includes planning for compact, walkable communities, designed to be easy to get around on foot or by bicycle. My wife grew up in Columbia, MD, which was a planned community by the Rouse Corp, and originally designed with plenty of trees, parks, and foot and bike paths, and so that residents could easily walk or bike to local shopping centers, schools, and other community centers. In fact, they are now planning the new Columbia downtown with the same concepts of walkability and reduced use of cars. These types of development practices can help to reduce the heat island effect by reducing the need for cars and by increasing the amount of shade and vegetation. This can help to reduce air pollution and heat island effects.
While the ultimate goal may be to reduce heat, these strategies have additional benefits that include:
Cleaner Air: Trees and vegetation help to absorb pollutants from the air, which can improve air quality. Particularly important with the combination of wildfire smoke and smog.
Reduced Energy Costs: Cool roofs and cool pavements can help to reduce the amount of energy needed to cool buildings and streets, which can save money on energy bills and the use of fossil fuels.
Lower Greenhouse Gas Emissions: Reducing energy use can help to reduce greenhouse gas emissions, which will help contribute to meeting our collective targets to reduce global warming and accelerating climate change.
The benefits of mitigating UHIs are clear, and we need to take action now. By implementing these strategies, we can make our cities cooler, healthier, and more sustainable in the long term.