In a state known for its intense summer heat, it’s helpful to understand how to stay comfortable in hot temperatures while living or visiting Arizona. However, when referring to ‘being comfortable,’ there tends to be a spectrum of what people consider comfortable. People who have spent their whole lives in Arizona have an excellent tolerance for heat but typically get cold easily, and vice versa for those who grew up in much colder states. This spectrum is defined as thermal comfort.

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According to ASHRAE (the American Society of Heating, Refrigerating, and Air-Conditioning Engineers), thermal comfort is defined in their Standard 55 as “the condition of mind that expresses satisfaction with the thermal environment, assessed by subjective evaluation.” The key part of this definition is “subjective evaluation,” emphasizing that thermal comfort varies for everyone.

Environmental Factors and Personal Factors

Environmental conditions comprise about 50% of a person’s comfort. These measurable factors include:

• Air temperature
• Mean radiant temperature (the heat radiated from surrounding surfaces)
• Wind speed
• Relative humidity

However, these conditions alone don’t fully explain thermal comfort. Personal factors, which account for about 50%, play a significant role. These include:

• Metabolic rate: The energy a person expends during physical activity.
• Clothing insulation: How much thermal insulation does the clothing provide?
• Age, gender, and fitness level can all impact how a person perceives temperature.

Psychological and Behavioral Factors

Beyond the physical and physiological factors, psychological factors also affect thermal comfort. For example, if a thermal environment meets a person’s expectations, they are less likely to feel uncomfortable. The individual’s adaptation to certain temperatures — whether they are used to heat or cold — and how long they are exposed to these conditions also affect their comfort level.

How Do We Measure Thermal Comfort?

Objective environmental factors can be easily measured to assess thermal comfort using specialized instruments such as temperature and humidity probes, wind sensors, and devices like the globe thermometer. However, measuring the subjective personal factors typically involves conducting surveys. In these surveys, respondents use a scale from 1 to 9 to rate their thermal sensation, with one indicating “very cold,” 5 being neutral, and nine representing “very hot.” Additional questions on clothing, thermal history, and personal characteristics help researchers understand how these factors correlate with environmental conditions.

The Importance of Shade in Thermal Comfort

A study conducted in Phoenix, Arizona, found that shade is the most significant environmental factor contributing to improved thermal comfort in hot, dry climates. In summer, providing shade raised thermal comfort by one point on the thermal sensation scale. The study also concluded that during hot, dry, sunny weather, mean radiant temperature becomes more important for thermal comfort than air temperature or humidity. This is because, under these conditions, heat from the sun’s radiation is a major driver of discomfort.

A follow-up study examined the impact of shady areas in urban settings. The findings showed that shade significantly improves thermal comfort, especially in hot weather. This insight is valuable for cities and urban planners, as it offers solutions for managing shade in areas where tree planting might not be feasible due to infrastructure or space limitations.

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Thermal comfort is a complex interaction of environmental, personal, and psychological factors that must be considered when designing spaces, especially in hot climates. The key takeaway? Providing shade is one of the most effective ways to improve comfort in outdoor areas. Whether natural or artificial, shade is crucial for creating spaces where people can stay comfortable.

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