The Hidden Science of How We Really Feel the Heat
A preliminary comparison of the Corrected Effective Temperature (CET) and Swedish Wet-Bulb Globe Temperature (SWBGT) indices
You step outside on a summer day. The thermometer reads 85°F (29°C), but in the direct sun, it feels like an oven, while under a tree, it's almost pleasant. Why the disconnect? The answer lies in the complex world of microclimates—the unique atmospheric conditions of a small, specific area. Understanding these microclimates is crucial for everything from planning a city park to preventing heatstroke in athletes. But how do we measure "feel"? Scientists have developed specialized indices, and two intriguing contenders are the Corrected Effective Temperature (CET) and the Swedish Wet-Bulb Globe Temperature (SWBGT).
This article dives into the preliminary clash of these two metrics, exploring how they compete to become the ultimate yardstick for human comfort and safety in our ever-warming world.
Traditional thermometers measure air temperature, but human thermal comfort depends on multiple environmental factors including humidity, wind, and solar radiation.
Before we pit them against each other, let's understand what these indices are actually measuring. Both go far beyond a simple temperature reading by factoring in how the human body exchanges heat with its environment.
Born from military and sports science, the SWBGT is a powerhouse for assessing heat stress during physical activity. It cleverly combines three distinct measurements:
Measures evaporative cooling potential, accounting for humidity and wind.
Measures radiant heat from sunlight and surrounding surfaces.
Standard shaded temperature reading.
Notice the heavy weighting (70%) on the wet-bulb, highlighting the critical role of sweat evaporation in cooling an active body .
CET has its roots in assessing thermal comfort for people at rest or doing light work. It's a more complex calculation that aims to describe the "feels-like" temperature by creating a physiological equivalent .
It starts with basic temperature and humidity and then applies corrections for:
The final CET value is expressed as the temperature of a still, saturated (100% humidity), shade environment that would produce the same thermal sensation as the complex environment you're actually in.
To compare these indices head-to-head, researchers designed a meticulous field experiment. The goal wasn't just to see which number was higher, but to see which one better correlated with actual human physiological strain.
The data revealed a fascinating story. On a cool, cloudy, and breezy day, the CET and SWBGT values were very similar. However, as the sun came out, they began to tell different tales.
| Location | Air Temp (Ta) | SWBGT | CET |
|---|---|---|---|
| Open Pavement (Full Sun) | 90°F (32°C) | 96°F (36°C) | 102°F (39°C) |
| Shaded Forest | 86°F (30°C) | 82°F (28°C) | 83°F (28°C) |
In the shade, both indices dropped significantly and were in close agreement. But in the full sun, the CET was substantially higher than the SWBGT. Why? The CET's correction for radiant heat was more aggressive. The black pavement was absorbing and re-radiating immense amounts of solar energy, which the CET index picked up on more sensitively. The participants' physiological data (higher heart rate, perceived exertion) in the paved area aligned more closely with the elevated CET reading, suggesting it might be a better indicator of the true "oppressive" heat felt in high-radiance environments.
| Index | Correlation Coefficient (r) with Heart Rate |
|---|---|
| Air Temperature (Ta) Alone | 0.65 |
| SWBGT | 0.78 |
| CET | 0.89 |
A perfect correlation is 1.0. This table shows that while SWBGT is a good predictor of physiological strain, the CET's more comprehensive model had a stronger correlation with the actual stress placed on the human body in this specific test of varied microclimates.
The experiment also highlighted how the indices handle wind.
| Condition | SWBGT | CET |
|---|---|---|
| Hot, Humid, and Still | 94°F (34°C) | 97°F (36°C) |
| Hot, Humid, and Breezy | 91°F (33°C) | 88°F (31°C) |
A breeze increases evaporative cooling, lowering both indices. However, the CET dropped more dramatically. This is because its algorithm gives significant weight to wind speed's cooling effect, arguably reflecting the "feels cooler" sensation we get on a windy day more accurately than SWBGT.
What does it take to run such an experiment? Here's a breakdown of the essential gear.
The gold standard for measuring dry (Ta) and natural wet-bulb (Tnw) temperature. It uses a fan to pull air past the thermometers at a constant speed for extreme accuracy.
A 6-inch diameter hollow copper sphere painted matte black, with a thermometer inserted in the center. It directly absorbs radiant heat, providing the crucial Globe Temperature (Tg).
A device with spinning cups or a sonic sensor that measures wind speed, a critical input for the CET calculation.
An electronic unit that automatically records measurements from all the sensors at set intervals, creating a clean, time-synced dataset for analysis.
So, which index is better? The preliminary answer is: it depends on the context.
Where the primary concern is preventing heat illness during intense activity, the SWBGT remains a robust, globally recognized standard. Its focus on evaporative cooling (via the wet-bulb) is perfectly suited for a body generating immense internal heat .
For urban planning, architectural design, and assessing general public comfort in complex microclimates, the CET shows promising potential. Its heightened sensitivity to radiant heat and wind may provide a more nuanced picture of how a city square, a park, or a building's exterior will feel to a person.
This scientific showdown isn't about crowning a single winner. It's about refining our tools to build safer, more comfortable, and more resilient environments for everyone. The humble thermometer tells us the temperature of the air, but it's indices like CET and SWBGT that finally give us a language for the temperature of experience.