How to Cook a Pedestrian?
Have you ever felt like a fried egg on a city sidewalk? Welcome to the world of urban heat islands, where poor design choices turn bustling metropolises into scorching ovens. Urban heat islands (UHIs) cause urban areas to experience significantly higher temperatures than their rural counterparts. According to the U.S. Environmental Protection Agency (EPA), daytime temperatures in urban areas are about 1–7°F (0.6–3.9°C) higher than temperatures in outlying areas, and nighttime temperatures are about 2-5°F (1.1–2.8°C) higher. [https://www.epa.gov/heatislands ]
In this post, we’ll explore the design missteps that make cities uncomfortably hot and we will offer solutions to create cooler, more pedestrian-friendly environments.
Understanding Urban Heat Islands
Urban heat islands (UHIs) occur when cities replace natural land cover with dense concentrations of pavement, buildings, and other surfaces that absorb and retain heat. This phenomenon results in urban areas experiencing temperatures that are significantly higher than those in surrounding rural areas. The increased heat leads to discomfort, higher air conditioning costs, and health risks such as heatstroke.
Heat islands increase the demand for air conditioning as buildings require more cooling. In case studies from multiple countries, electricity demand for air conditioning rose by approximately 1–9% for every 1.1°C (2°F) increase in temperature. [https://www.sciencedirect.com/science/article/abs/pii/S0378778822005813 ]
Countries with widespread air conditioning usage experienced the highest increase in electricity demand. They also contribute to elevated emissions of air pollutants and greenhouse gases. [https://www.epa.gov/heatislands/heat-island-impacts#_ftn1 ]
Health impacts are significant, too. Extreme heat can lead to increased mortality, with urban heat islands exacerbating these effects by trapping heat and reducing nighttime cooling. Globally, heatwaves are becoming more frequent and intense due to climate change, posing severe health risks, especially in urban areas. According to a study published in The Lancet, an estimated 356,000 deaths were attributed to heat exposure globally between 1991 and 2018. [https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(21)01860-2/fulltext ] Heat-related illnesses, such as heat exhaustion and heatstroke, are also rising worldwide, with vulnerable populations in urban areas particularly affected
Design Flaws that Contribute to the Heat
Urban areas are often designed with materials and structures that exacerbate the heat island effect. Let’s take a closer look at some of these design flaws:
Materials Matter
Urban environments are dominated by materials like asphalt, concrete, and metal, which absorb and retain heat. Unlike natural landscapes, these materials store heat during the day and release it at night, contributing to the elevated temperatures seen in cities. In Mesa, Arizona, during September, a study observed the following temperatures at the same time of the day:
Air Temperature: 105°F (40.6°C)
Asphalt Temperature: 155°F (68.3°C)
Concrete in Sun: 140°F (60°C)
Concrete in Shade: 93°F (33.9°C)
These temperatures illustrate how much hotter surfaces can get compared to the air temperature, highlighting the significant impact of urban materials on the heat island effect
Lack of Greenery
Green spaces act as natural air conditioners. Trees and parks provide shade, release moisture into the air, and help cool down urban environments by evapotranspiration. However, many cities lack sufficient greenery, leading to hotter and less comfortable conditions. Shaded surfaces, for example, may be 20–45°F (11–25°C) cooler than the peak temperatures of unshaded materials. Evapotranspiration, alone or in combination with shading, can help reduce peak summer temperatures by 2–9°F (1–5°C). [https://19january2017snapshot.epa.gov/heat-islands/using-trees-and-vegetation-reduce-heat-islands_.html ]
Poor Urban Planning
Urban planning often exacerbates the heat island effect due to poor design choices in urban geometry, which refers to the dimensions and spacing of buildings. Dense urban layouts, with buildings constructed against prevailing winds, reduce wind flow and natural cooling, trapping heat between structures. The lack of sufficient space between buildings further limits airflow, while the "urban canopy effect"—where heat is trapped by taller buildings—prevents it from dissipating into the atmosphere. This "canyon effect" increases urban temperatures and creates stifling conditions for pedestrians, leading to higher energy consumption for cooling.
Case Studies
To illustrate the severity of the urban heat island effect, let’s examine some real-world examples:
Los Angeles, California, USA
Los Angeles experiences significant urban heat island effects due to extensive urban development and heat-absorbing materials like asphalt and concrete. A study found that temperature variations in the city are primarily influenced by factors such as the distance from the ocean, green vegetation and urban density. Specifically, the study showed that green vegetation and urban density account for 58% and 27% of temperature variations, respectively, after adjusting for the effects of distance from the ocean and elevation. The research underscores the importance of increasing greenery and thoughtful urban planning to reduce urban heat.
Bangkok, Thailand
Bangkok has experienced a significant urban heat island effect due to rapid urbanization and expansion of urban areas. Between 1991 and 2016, the total urban area of Bangkok increased from 30% to 55% of the city's total area. During the same study period, the UHI intensity of Bangkok increased from 11.91°C to 16.21°C. Analysis results stated that 8.4% of increased urban areas in Bangkok can create a 3.15 °C increase in land surface temperature within an 8-years period. The study highlights the impact of urbanization on temperature rise and the importance of addressing UHI effects in Bangkok's urban planning. [https://www.sciencedirect.com/science/article/abs/pii/S2212095523003061# ]
Paris, France
Paris experiences a pronounced urban heat island effect, particularly during the night. During a study period in summer 2006, the UHI intensity in Paris reached a maximum of 6.1°C at night, about 6 km downwind of the city center. This intensity is primarily due to the city's thermal inertia and the lack of vegetation, which causes heat to be stored during the day and released at night. The UHI effect in Paris is most significant under conditions of clear skies and low wind speeds, which allow stored heat to remain in the urban canopy layer. The study highlights that the heat retained in urban materials, along with reduced vertical adiabatic cooling, can intensify nocturnal temperatures and maintain high UHI levels throughout the night. [https://www.researchgate.net/publication/226805722_The_Urban_Heat_Island_Intensity_of_Paris_A_Case_Study_Based_on_a_Simple_Urban_Surface_Parametrization ]
Solutions
Green roofs and walls are among the most effective solutions, involving the installation of vegetation on building surfaces. This greenery cools the air through evapotranspiration and provides insulation, reducing the need for air conditioning. Cities like Singapore and Toronto have extensively adopted green roofs, with Toronto requiring green roofs for new developments under its Green Roof Bylaw. These initiatives contribute significantly to reducing the UHI effect by lowering building temperatures and improving air quality.
Urban greening and tree planting are other crucial strategies. By increasing the number of trees and green spaces in cities, urban areas can be cooled naturally. New York City’s MillionTreesNYC initiative aims to plant one million trees, reducing surface temperatures and improving the environment. Similarly, Melbourne's Urban Forest Strategy plans to increase canopy cover to 40% by 2040, significantly mitigating the UHI effect while enhancing biodiversity and residents' quality of life.
Cool roofs are designed with reflective materials that absorb less heat, thus reducing the amount of heat transferred into buildings and lowering surrounding air temperatures. Los Angeles has mandated cool roofs for new residential buildings, while Ahmedabad in India has implemented a cool roofs program in slum areas, effectively reducing indoor temperatures and improving living conditions.
Reflective and cool pavements are also used to combat UHI. These pavements are designed to reflect more sunlight and absorb less heat than traditional materials. Phoenix, Arizona, is testing cool pavement technology in several neighborhoods, with early results showing significant reductions in surface temperatures. Tokyo has extensively implemented this technology, particularly ahead of the 2020 Olympics, leading to cooler streets in the city.
Urban planning and zoning play a critical role in integrating UHI mitigation strategies into city development. Copenhagen’s urban planning includes increasing green spaces and utilizing waterways to manage temperatures and enhance resilience against climate change. Paris has also implemented a comprehensive plan to combat UHI through green spaces, tree planting, and cool surfaces.
Water features and bodies of water in urban areas can provide a cooling effect through evaporation. Projects like Madrid Río in Spain, which transformed the banks of the Manzanares River into a green space, and Chicago’s Riverwalk, which includes water elements, demonstrate the effectiveness of water in cooling urban environments.
Finally, albedo enhancement involves increasing the reflectivity of urban surfaces, such as roads and rooftops, to reflect more solar radiation and reduce heat absorption. Cities like Tel Aviv and Athens have implemented high-albedo materials in public spaces and buildings to mitigate the UHI effect.
Conclusion
The urban heat island effect poses significant challenges for cities around the world, affecting everything from energy consumption to public health. As cities grow and temperatures rise, understanding and addressing the design flaws contributing to urban heat islands become increasingly important. By implementing solutions like green roofs, reflective materials, and urban forestry, we can create cooler, more comfortable urban environments. It’s time to prioritize sustainable urban planning and advocate for greener, more pedestrian-friendly cities.
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