The Urban Heat Island

It is not a secret that Arizona is hot! While its geographic location – the Sonoran Desert – plays a major role in shaping our regional climate, temperatures are also influenced locally by the way we develop and design cities. The man-made materials we use to build city infrastructure (from roads to parking lots to buildings) cause temperatures in the city to rise and be higher compared to the surrounding desert at night. Engineered materials such as concrete and asphalt store heat during the day when the direct sunlight hits the surface; the stored heat is then slowly released at night, contributing to the Urban Heat Island (UHI).  Average nighttime temperatures in Phoenix have increased more than 10 degrees F since 1980. This temperature change increases building energy use at night and has major implications for the health of our communities. The good news: while urban development contributes to urban heat, it can also help mitigate heat and cool our communities with the right design choices. Let’s first understand how heat can be defined before we touch on urban design solutions to mitigate heat. 

 Different Types of Temperature

Urban heat is reported in various ways. The most common way to talk about urban heat is air temperature, which simply refers to how hot or cold the air is. The weather forecast usually reports air temperature. In some cases, the surface temperature may be more relevant, for example, when it comes to urban surfaces that people touch or walk on barefooted, such as the pool deck or playground equipment. A less known but important temperature for thermal comfort and heat stress is mean radiant temperature, short MRT. MRT quantifies the heat load on the human body and is the sum of the radiation that hits the human body from all direction. This includes the longwave radiation emitted from hot surfaces, such as an asphalt parking lot in the summer, and the direct shortwave radiation from the sun in places with no shade. MRT roughly equals air temperature in the shade but can easily be 30 degrees F higher in the sun, making a person feel much less comfortable when it is hot. Since humidity is usually low in Arizona, MRT is the type of temperature that best describes how people experience heat. It can be measured using MaRTy, a mobile weather station with three net radiometers that measure the shortwave and longwave radiation in 6 directions. 

Heat Mitigation Solutions

Outdoor heat can be lowered using three main urban design strategies:

• Green infrastructure. Vegetation cools the environment through evapotranspiration, and trees provide shade, which reduces the radiant heat load on the human body and the surface temperatures under the canopy.
• Switching impervious materials such as concrete surfaces with pervious materials such as gravel and grass helps reduce the heat storage in cities but may not always be practical. Alternatively, the color (~albedo, reflectivity) of materials can be changed to increase the reflectivity and therefore reduce the amount of energy that is stored in the urban fabric.
• Urban form and design. Building design impacts wind flow, air temperature, and shade patterns. Shade is a major driver of thermal comfort and heat stress, so this mitigation strategy is highly effective but can mostly be implemented in the developer/design stage of a project.

There is no one-size-fits-all solution to heat mitigation, and all of the strategies listed above come with tradeoffs. It is important to recognize though that heat mitigation in our communities starts at the neighborhood scale and extends down to individual buildings (microscale). Developers, urban planners, and architects have the “design power” to significantly impact how residents experience outdoor heat in their communities through decisions that prioritize shade, walkability, and climate-sensitive design. In the upcoming posts we will focus on design strategies to lower outdoor heat and improve thermal comfort.