"Towards Heat Where Sweat Doesn't Evaporate" — The Limits Imposed by Wet-Bulb Temperature

"Towards Heat Where Sweat Doesn't Evaporate" — The Limits Imposed by Wet-Bulb Temperature

2026年01月10日 00:35

The conversation often ends with "It's hot again this year," as the summer continues. However, what truly makes the body scream is not just the "temperature itself" but the heat combined with humidity —commonly known as "muggy heat." When sweat doesn't evaporate easily, the body can't release heat. Some people can escape to air-conditioned indoors, while others lose refuge due to outdoor work, commuting, or power outages. In other words, muggy heat poses risks not only to health but also to the economy, infrastructure, and inequality.


A representative indicator that scientifically addresses this "muggy heat" is the **Wet-bulb temperature**. Unlike a dry thermometer (dry bulb), the wet-bulb temperature reflects both air temperature and water vapor content. In the context of human physiology, the higher the wet-bulb temperature, the less effective sweat evaporation cooling becomes. Traditionally, a **wet-bulb temperature of 35°C is considered the human limit (a threshold that cannot be endured for long even under ideal conditions)**, and recent studies have pointed out the increase in extreme phenomena and the potential underestimation of localized extremes by reanalysis data.


"Humidity Records": Where and how often do they occur?

However, there is one problem here. While research on extreme temperature phenomena (heatwaves) is progressing worldwide, the extent of "record-breaking humid heat" on a global scale has surprisingly not been comprehensively organized.


Colin Raymond and his team attempted to fill this gap. They used reanalysis data (especially ERA5) as a "catalog close to observation" and combined it with multiple climate model large ensemble simulations (a method of running numerous simulations with different initial conditions under the same external conditions) to evaluate **"record-level wet-bulb temperatures" in 216 regions worldwide**.


According to an article (Eos content reprinted by Phys.org), when viewed against regional climate averages, extreme humid heat is particularly significant in the Middle East and North Africa , followed by the tropics . In these regions, the wet-bulb temperature can rise 4 to 5 standard deviations above the "warm season average" for that region.


Furthermore, in the Middle East and North Africa, extreme humid heat can potentially last for more than 20 days , becoming a "long-term event."


What's important here is that it doesn't end with "hot regions = dangerous." The abstract of the paper suggests that the eastern United States is "currently under-recorded," but model projections indicate that days exceeding current records are likely, making it vulnerable to record-breaking events. Additionally, Australia and eastern China also show "unexpected weaknesses" as more than 40% of the ensemble simulations produce cases exceeding the ERA5-based upper limits.


A single "outlier day" can account for one-fifth of future predictions

The challenge in studying muggy heat is that extreme phenomena are dominated by a few "abnormal days." The reprinted article states, "In many places, excluding a single outlier (extremely hot and humid day) can lead to statistical models estimating future 'hot and humid days' as one-fifth of what they would otherwise be."


In other words, how record-breaking days are observed and included in statistics can dramatically change risk assessments.


The paper also emphasizes that in half of the regions, record days can shift the recurrence period (an estimate of rarity) by more than 2.5 times. This indicates that it's not a simple matter of "models being right or wrong." Instead, using **"observations that don't miss extremes" and "model groups that can produce extremes" together** is closer to realistic crisis management.


Why does the tropics "concentrate" muggy heat: The shadow of El Niño

Another complexity of muggy heat is its "continuity." The reprinted article explains that in the tropics, three-quarters of the days with wet-bulb temperatures in the top 5% were concentrated in one-quarter of the years in the analysis period. As a background factor, El Niño is suggested to simultaneously push up atmospheric temperature and water vapor content, potentially leading to record-breaking days being "concentrated in El Niño years."


Muggy heat is more about "periods with no escape" than "occasional scorching days." When nights don't cool down, sweat doesn't dry, and sleep is disrupted, these "gradually wearing days" can eventually lead to emergency transport, overwork, or fatal injuries during power outages.


2023 was a "banner year" for "humid heat"

Researchers state that 2023 was a "banner year" for humid heat , with 23 regions breaking records. They even suggest that "without climate change, these record-breaking events wouldn't have occurred."


To clarify a common misunderstanding, it is not simply a natural phenomenon that "records are broken somewhere every year." Instead, the view is that "record-breaking events including humidity" are statistically more influenced by climate change.


Reactions on Social Media: More about "experience" and "fear" than "temperature"

This theme is often discussed on social media as an experience rather than in numbers.


1) LinkedIn: Spreading as a topic directly linked to infrastructure, economy, and health

On LinkedIn, posts quoting the article's introductory summary (how high humidity can strain power grids, economies, and health, while the global reality of record humidity events has not been sufficiently studied) are shared.

The movement to translate "climate talk" into "business continuity and labor safety talk" resonates well on corporate and governmental timelines.


2) Reddit: The basic question of "What is humidity?" gains traction

On bulletin board-style social media, explanations of "relative humidity and absolute humidity" are popular topics. For example, the explanation that "weather forecast humidity is relative humidity, and as temperature drops, the 'maximum capacity of air' decreases, raising relative humidity" hits a point where many people stumble.

Since the wet-bulb temperature discussed in this article is essentially about "temperature × water vapor," the more this basic understanding spreads, the deeper the discussion becomes.


3) Reddit: Articulating experiences—"It's not just when it's hot"

In another thread, explanations like "humidity affects heat transfer and perception. Not only does sweat dry slowly when it's hot, but when it's cold, moisture can draw heat away, making it feel colder" are also supported.

The sense that "humidity isn't just a summer issue" connects not only to heatstroke but also to home insulation and ventilation, mold, and hypothermia in winter.


So what should we focus on? From "temperature" to "wet-bulb temperature"

From a consumer perspective, there are three key points.

  1. Don't judge by "degrees" alone
    The danger level varies with humidity even at the same temperature. The Heat Index, WBGT, and in research contexts, the wet-bulb temperature quantify these differences.

  2. Consider "how many days it lasts"
    Continuity, which disrupts sleep and recovery, is more impactful than a single day's peak. The possibility of lasting more than 20 days in the Middle East and North Africa is significant from this perspective.

  3. Be aware of the "weaknesses" in observation and prediction
    If a single outlier day can significantly alter future estimates, addressing gaps in observation networks and handling urban and coastal locality become crucial.


Conclusion: The next heatwave will come with "humidity records"

Discussions about climate change tend to become abstract, focusing on "how many degrees the average temperature will rise." However, what truly shakes society is not the average but extremes , and the damage escalates when these extremes are combined with humidity.


The observation that 23 regions broke records for "humid heat" in 2023 poses the next question.
Can your region truly withstand the heat based only on "experience"?


And if a few abnormal days are so crucial that they can rewrite future predictions, then observing and preparing for those days without missing them will become the core of future "heat countermeasures."



References

Temperatures are rising, but what about humidity?
Source: https://eos.org/research-spotlights/temperatures-are-rising-but-what-about-humidity

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