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How Can Localities Use Zoning to Protect against Climate Change?

Impacts of form and design policies on urban microclimate: Assessment of zoning and design guideline choices in urban redevelopment projects
Mehdi P. Heris, Ariane Middel, Brian Muller
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Mitigating climate change and its exacerbation of extreme heat events is critical to preventing their negative effects on air quality and human health. This is particularly true in urban areas, which can be at an added risk for high heat because of high building density.

Urban planners can use zoning as a tool to regulate building components, such as height, setbacks, and required parking, to increase resiliency against extreme heat. This study explored how existing urban zoning regulations affect the microclimate of densely built redeveloped areas.

The authors examined several policy scenarios in which zoning requirements differed by building height, street direction, and tree density to simulate different kinds of urban developments in Denver, Colorado. Modeling after two Denver shopping malls—the Belmar (which used form-based code that allows for a mixture of building types and structures) and Twenty Ninth Street (traditional zoning that prescribes land based on its use)—the authors used a microclimate simulation model to measure mean radiant temperature, wind circulation, and air temperature for each scenario.

Key findings
  • In the simulation, building height, street pattern, and tree density didn’t appear to significantly affect average air temperature, either independently or combined.
  • Mean radiant temperature, an indicator of physiological human comfort, mainly correlated to shade and tree cover and was lower for areas with relatively higher tree densities.
  • Taller buildings correlated with a decrease in mean radiant temperature by 59 degrees Fahrenheit, implying greater comfort on the ground level, though building heights did not have a significant effect on air temperature in the surrounding area.
  • The street pattern shapes air circulation in sites. For example, grid patterns can facilitate air movement and natural ventilation.
  • Open parking lots increase the near-surface air temperature.
  • Wind direction and speed represent how air moves in a site. If air crosses a parking lot before getting to a public space, it will transfer warmer air. The landscape up-wind will affect the temperature of down-wind areas.
Policy implications
  • Grid patterns for urban streets can facilitate airflow and help mitigate warm temperatures near the surface.
  • Building taller buildings can provide better shade and lower mean radiant temperatures by creating more shade.
  • Creating taller parking structures can help reduce the number of asphalt lots in an urban area, leading to reduced heat on the ground and more comfort for pedestrians.
  • Form-based zoning code provides a more flexible framework to regulate building types and structures that could mitigate urban heat.
  • Tree shade can increase comfort for those on the street level, especially pedestrians. Increasing tree cover in urban areas can also mitigate the heat island effect.
  • Zoning for urban development can have sizeable impacts on resident comfort and health.