The way scientists and policymakers measure heat as they seek to combat rising temperatures across the world’s cities requires more nuance than just looking at the daily outdoor temperature. There are a variety of tools that can help show a more complete picture.
Some measurements, like land surface temperature, can help compare different parts of a city. Others, like air temperature, heat index or wet bulb globe temperature describe the regional conditions across a neighborhood or urban area. Increasingly, experts look to more complex data, known as thermal comfort indices, to show the impacts on people where they live.
After 2024 was recorded as the hottest year on record researchers project a nearly 3 degree C (5.4 degree F) rise in temperature by 2100 if we don’t significantly reduce global greenhouse gas emissions. This increase would disproportionally impact cities — where the urban characteristics, such as miles of pavement and lack of green space, can add another 1 degree C (1.8 degrees F) warming — and the more than 4 billion people who live in them.
As part of the global weather system, extreme heat can span across broad geographic areas, but individuals experience it at very local scales. Choosing the best metric to measure a desired outcome appropriately will be critical for cities worldwide to effectively address the challenge of urban extreme heat as temperatures continue to rise.
Traditional Ways of Measuring Heat
When we talk about heat exposure, we are referring to a series of different heat sources that make someone hot. Suppose a person stands outside during a heatwave: direct radiation from the sun, reflected radiation from surfaces like roads or buildings, ambient air and moisture in the air, all contribute to how hot that person can feel.
Here are a few of the traditional heat metrics, their uses and limitations:
Land Surface Temperature
Land surface temperature (LST) is calculated from satellite data and measures the temperature of surfaces, including roofs, treetops and roads. LST doesn’t mean much to the everyday person, however, because it’s easy to calculate anywhere on Earth, researchers use it for scientific heat mapping and, for many years, have considered it the go-to for urban heat analysis. LST is useful in comparing different parts of cities and evaluating which surfaces are likely to absorb heat and slowly re-radiate it throughout the day. However, LST isn’t helpful for understanding people’s heat exposure because it doesn’t account for most of the ways humans experience heat: Even if a person is standing on cool grass, they will feel hot if they’re in the sun on a warm, humid day.
Air Temperature
Air temperature measures the temperature of the air about two meters above the ground. Because it is easily modeled and understood, air temperature is the measurement local news organizations and other outlets use in their reporting. It’s measured with a simple thermometer and gives a clear picture of a location’s ambient weather condition, minus any other factors (like humidity or sunlight) that might make a person feel hot. Because air temperature looks at the background conditions, air temperature is an ideal metric for analyzing and comparing temperatures across time.
Across a neighborhood or city, though, air temperature can be difficult to measure. Consistent measurements over a wide area rely on weather stations, which can be geographically sparse and expensive to maintain, or on low-cost sensors, which can be faulty and prone to breaking. Scientists then have to interpolate the data between observed points and model the temperature distribution across an area.
However, air temperature doesn’t capture other atmospheric or environmental factors, like the number of trees that provide shade,, that can affect how hot a person might feel in a particular area or neighborhood. Plus, it would take something really big, like a body of water or an entire neighborhood of cool roofs to significantly affect the air temperature of an area. So, if cities want to address the impacts of local heat exposure, air temperature would not provide enough specificity to inform human-scale interventions designed to protect people’s health.
Heat Index and Wet Bulb Globe Temperature
Meteorologists calculate what the temperature “feels like” using the heat index, a metric that adjusts air temperature based on humidity. For example, when humidity and temperature are high, the body has a harder time regulating its temperature through perspiration. In contrast, low humidity at a high temperature — often called “dry heat” — would feel cooler because low moisture in the air leads to the faster evaporation of sweat, which cools down the body. Heat index can therefore predict how the body will react to the weather on a given day, making it a helpful tool for determining if it’s safe to work outdoors or play sports.
Wet bulb globe temperature (WBGT) is a similar metric, but it considers direct sunlight and wind speed in addition to air temperature and humidity. This metric can provide additional insight into heat exposure by determining how much hotter a person feels in the sun and with limited breeze.
Heat index and WBGT are measured with special thermometers or weather stations and interpolated like air temperature. Given that both these measurements account for more variable heat sources, heat index and WBGT are better indications of heat load— the cumulative impact of heat from the environment on a person —than temperature alone. But, just like air temperature, they don’t vary much across space and, therefore, cannot always account for changes across the local environment.
These traditional methods often used to measure heat sources do not describe all the dimensions of how someone feels heat or its impact on their health, which can vary widely from person to person. Environment, for example, plays a significant role and can change the impact even within a single city. Residents of downtown Los Angeles, with its crowded city blocks and lack of green space, will experience heat differently than those on the west side of the city who might benefit from the cooling effects of the Pacific Ocean.
Each heat measurement has utility and can inform different interventions — from cool roofs to early-warning heat alert systems. However, these traditional methods of measuring heat don’t account for the full scope of how heat affects people in their neighborhoods. For city officials to ensure their interventions adequately address the growing impacts of extreme urban heat, cities should also use thermal comfort modeling.
Measuring Heat with Thermal Comfort Modeling
When someone stands outside on a hot day, direct radiation from the sun, reflected radiation from surfaces, ambient air temperature, wind level and humidity all simultaneously affect them. Thermal comfort metrics calculate the cumulative effects of these collective heat sources, providing our most detailed assessments of human heat exposure.
If, for example, a person walks down an asphalt street with partial shade coverage from trees at noon on a hot, humid day, thermal comfort metrics provide a sense of their heat exposure, combining factors such as sunshine filtering through the tree canopy, the hot asphalt below their feet and the weather around them. If the person walks back along the same street at 6 p.m., a thermal comfort metric can predict that they’ll feel cooler because of the longer shadows and the lower air temperature, even if the heat from the asphalt may have increased.
Thermal comfort is measured using a set of instruments, including specialized thermometers and a wind meter. But, more frequently, it is calculated using three-dimensional models that consider meteorology, direct and reflected radiation, tree canopy, buildings, land use, and shade. Scientists increasingly use metrics like the Universal Thermal Comfort Index, Mean Radiant Temperature, or Physiological Equivalent Temperature to calculate thermal comfort. In Singapore, for instance, planners model thermal comfort to pilot urban design interventions that reduce heat exposure, including wind corridors, green buildings and shade that can cool the spaces where people feel the hottest.
Thermal comfort metrics are calculated at very local scales — in radiuses between one and five meters — which capture variations in shade, vegetation, surfaces and other relevant aspects of a city’s design. As a result, thermal comfort indices help scientists not only understand individuals’ heat exposure but also heat-resilient infrastructure that could potentially mitigate that exposure.
Modeled metrics are also useful in prioritizing and locating interventions. Because thermal comfort metrics use modeling, they can show the local effects of constructing a new park, planting trees, or orienting buildings to provide shade to pedestrians. Modeled air temperature can similarly inform the impacts of scaling interventions, like cool roofs, across a whole city.
In recent years, improved high-resolution data availability around the world has made it possible to calculate thermal comfort modeling more accurately and cheaply, making these metrics feasible to use alongside traditional methods. Now, scientists and policymakers have more options when assessing extreme heat: They can choose the right indicator, physical scale and time-period to look at how the city affects how hot people get and how hot they feel.
Matching the Right Metrics to Local Heat Goals
As temperatures continue to rise in cities worldwide, interventions to mitigate extreme heat will become increasingly important. Using the right metric to evaluate the impact of these interventions is critical in considering how, where and when the changes will be effective.
Different stakeholders have varied goals related to heat. As they plan for more heat-resilient cities with better infrastructure and co-benefits for cooling across sectors, they should select appropriate metrics for measuring and informing those goals. Some policies and projects will focus on protecting human health during heat events, while others might prioritize reducing the formation of ground-level ozone or lowering burdens on energy grids. Each goal depends on a different kind of heat metric and data to help plan and prioritize its implementation.
| Heat Related Goal | Metrics to Inform Implementation |
|---|---|
| Lower citywide temperatures | Air temperature |
| Protect pedestrians outdoors | Thermal comfort indices |
| Set safety limits on outdoor activities | Heat index; wet bulb globe temperature |
| Keep people cool in transit stops or in public spots | Thermal comfort indices |
| Reduce indoor heat exposure and energy use | Air temperature; land surface temperature of roofs |
| Reduce the formation of ground-level ozone | Air temperature |
Entire cities experience heat, but so do individuals, so city officials should consider interventions that target heat at a citywide scale to move the needle on ambient outdoor temperatures while also prioritizing local- to neighborhood-scale change to impact people’s day-to-day heat exposure.
Just as different metrics are appropriate for different goals, the same is true of metrics for measuring the effectiveness of different interventions. For example, cool roofs reduce heat across a neighborhood, so their effect is best measured by looking at regional air temperature. Shade protects individuals by lowering their heat exposure, and so it can be best understood through thermal comfort metrics.
As the risks of extreme heat continue to intensify in cities, leaders and policymakers need a diverse set of tools to understand and respond to risks. Simply knowing that it’s hot in a city is not enough: Choosing the right metric for each dimension of the heat problem can point policymakers and residents toward deeper knowledge of their local challenges and potential solutions.
