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Why should we care about the wet bulb temperature?
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You might have heard the term wet bulb temperature being mentioned in the news in recent years - and there’s a reason for that. As the climate heats up, the risk of surpassing this dangerous temperature threshold is rising, and in some regions, it’s already happening. What makes this so concerning is that high wet bulb temperatures don’t just signal intense heat, they represent conditions that are deadly for humans.
The reality is that when heat and humidity reach certain levels, our bodies can’t cool down effectively, leading to severe health risks and, in some cases, death.
👉 In this article, we’ll break down what wet bulb temperature is, why it’s becoming a growing threat, and what can be done to lessen its impact.
What is wet bulb temperature?
When we typically talk about temperature, we are referring to dry bulb temperatures - the standard measure of ambient air temperature using a thermometer that is exposed to air but shielded from moisture. This measurement doesn't account for humidity, which plays a crucial role in how heat is perceived by the human body.
Wet bulb temperature, on the other hand, is different because it factors in both ambient temperature and humidity. It’s measured by wrapping the bulb of a thermometer in a wet cloth and allowing water to evaporate (referred to as a wet bulb thermometer). As the water vapor evaporates, it cools the thermometer down, and the lower temperature recorded is the wet bulb temperature. The difference between the dry bulb temperature and the wet bulb temperature is known as the wet bulb depression. The size of the wet bulb depression helps to indicate the level of humidity - larger depressions suggest drier air, while smaller depressions indicate more humid conditions. The rate of evaporation, and thus the cooling effect, depends on the humidity in the air.
Relative humidity - the amount of moisture in the air compared to the maximum amount the air can hold at a given temperature - plays a key role in this process. In environments with low humidity, evaporation occurs quickly, leading to a significantly lower wet bulb temperature. However, in high-humidity conditions, evaporation slows down or stops altogether, meaning the wet bulb temperature remains close to the dry bulb temperature.
This is important because wet bulb temperature reflects how well our bodies can cool down through sweating. Normally, sweat evaporates from the skin, helping to regulate body temperature. But when the air is already saturated with moisture (high relative humidity), sweat evaporates much more slowly, making it harder for the body to cool down. In extreme heat and humidity, wet bulb temperature can reach levels that pose serious health risks, as the body’s natural cooling mechanism becomes less effective.
Finally, it's worth noting the role of dew point temperature, which indicates the point at which air becomes fully saturated and condensation begins. A higher dew point indicates more moisture in the air, which can contribute to higher wet bulb temperatures and reduced cooling potential for the body. Together, wet bulb temperature, dry bulb temperature, dew point temperature, and relative humidity provide a comprehensive picture of how heat affects us.
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Why is wet bulb temperature important?
Wet bulb temperature is critical because it highlights the point at which the combination of heat and humidity becomes unmanageable for the human body. Normally, we rely on sweating to regulate our body temperature, as the evaporation of sweat cools us down. However, when the wet bulb temperature reaches a certain threshold, and especially in cases of constant wet bulb temperatures, this natural cooling process stops working effectively.
At the critical wet bulb temperature of 35°C (95°F) or higher, sweat can no longer evaporate, no matter how much the body produces. This means that heat generated by the body can no longer be released, causing internal temperatures to rise dangerously. In these conditions, even a healthy person at rest can experience heat stress, organ failure, or death within a matter of hours.
What makes this measurement especially alarming is that it shows how even moderate heat, when combined with high humidity, can become lethal. For example, a wet bulb temperature of 35°C isn’t just the result of extremely hot weather. It can occur in conditions such as 39°C with 75% humidity or 45°C with 50% humidity, making it a widespread threat in a warming world.
Beyond individual risk, wet bulb temperatures are significant because they signal conditions where large populations could face severe danger, particularly in areas with inadequate access to cooling or medical care. As climate change accelerates, the frequency of extreme heat events combined with high humidity is expected to rise, putting millions at risk, especially in tropical regions, coastal cities, and densely populated urban areas.
Why is humidity and heat a dangerous combination?
The combination of heat and humidity poses a significant threat because it disrupts the body’s natural cooling mechanisms. Normally, living organisms rely on evaporative cooling (such as sweating in humans, panting in dogs, or saliva evaporation in other mammals) to regulate internal body temperature. However, the ability to cool down through evaporation is highly dependent on the surrounding environment. As humidity rises, the air becomes saturated with moisture, limiting the evaporation of sweat from the skin.
When high temperatures and high humidity combine, the body can no longer effectively release heat. The result is hyperthermia (different from hypothermia), a condition where the body absorbs more heat than it can expel. This isn't just a discomfort issue - it’s life-threatening. Hyperthermia can develop rapidly in hot and humid environments, even when the air temperature alone might not seem extreme. The risk is especially pronounced in places with high humidity levels, where sweating becomes ineffective.
What makes this combination particularly dangerous is that the body’s core temperature begins to rise uncontrollably. This leads to heat-related illnesses, the most severe being heat stroke. The initial symptoms are often subtle - heavy sweating, rapid breathing, and a fast pulse - but can quickly escalate. Without intervention, the body’s internal systems start to fail. Blood pressure drops, leading to dizziness, nausea, and fainting, followed by seizures, unconsciousness, and organ failure.
Heat and humidity combined also increase the risk of dehydration, which accelerates the onset of these symptoms. Dehydration further impairs the body’s ability to regulate temperature, creating a dangerous feedback loop that can lead to death if not addressed. Vulnerable populations, such as the elderly, children, and those with pre-existing health conditions, are particularly at risk.
The increasing frequency of extreme heat events globally is making this dangerous combination more common, particularly in densely populated cities and regions without adequate cooling infrastructure. As wet bulb temperatures rise in more regions, the combination of heat and humidity will continue to be one of the most critical climate-related threats to human health.
| Wet Bulb Temperature (°C) | Health Risk | Symptoms |
|---|---|---|
| 28°C - 30°C | Heat Exhaustion | Heavy sweating, fatigue, dizziness, nausea, headaches, muscle cramps. |
| 30°C - 32°C | Severe Heat Stress | Profuse sweating, rapid pulse, shortness of breath, confusion, weakness. |
| 32°C - 35°C | Heat Stroke (Onset) | Lack of sweating, dry skin, rapid heart rate, high body temperature (>40°C), confusion, dizziness, fainting. |
| 35°C and above | Life-threatening Heat Stroke | Seizures, unconsciousness, organ failure, extremely high body temperature, potential death within hours. |
Is there a new lower threshold for dangerous wet bulb temperature?
While the traditional threshold for a fatal wet bulb temperature has been set at around 35°C, recent research suggests that the critical threshold may actually be lower. A 2022 study published in the Journal of Applied Physiology challenges this long-held belief, finding that severe heat stress occurs at wet bulb temperatures as low as 31.5°C, even in healthy young adults under minimal physical activity. This is significantly lower than the widely cited 35°C limit and indicates that dangerous conditions can arise sooner than previously thought.
Moreover, the US National Weather Service considers wet bulb temperatures of 31°C or higher to be in the “extreme danger” category, which supports the idea that the human body's ability to cope with heat stress is compromised at lower thresholds than once believed. This revised understanding is crucial, especially as regions across the globe are increasingly experiencing extreme heat events due to climate change.
Who is most at risk from a high wet bulb temperature?
Certain groups are particularly vulnerable to the dangers posed by high wet bulb temperatures. These include the elderly, outdoor workers, and individuals with underlying health conditions.
The elderly are especially at risk because, as we age, our ability to regulate internal body temperature diminishes. This makes it harder for older adults to cool themselves through sweating. Additionally, many elderly individuals have pre-existing health conditions or take medications that can further impair their ability to respond to heat stress, increasing the risk of heat-related illnesses.
Outdoor workers also face heightened risks due to their direct and prolonged exposure to high temperatures and humidity. While hyperthermia can occur even without physical exertion, the added strain of physical labor accelerates the body’s overheating process, making these workers more susceptible to heat stress and heat stroke.
Individuals with underlying health conditions - especially those affecting the cardiovascular, respiratory, or renal systems - are also more prone to hyperthermia. When temperatures and humidity rise, these systems are put under immense stress, and any pre-existing damage or limitations in their function can lead to serious complications.
“ ❗️It's important to note that even healthy young individuals are not immune to the dangers of high wet bulb temperatures. Hyperthermia can still occur even if you're in light clothing, sitting in the shade, with access to water and a fan. In extreme conditions, the body’s cooling mechanisms can be overwhelmed regardless of precautions. ”
What regions are most vulnerable to high wet bulb temperatures?
Certain regions of the world are particularly prone to dangerously high wet bulb temperatures. Studies have shown that parts of South Asia (especially India and Pakistan), Southwestern North America, and areas around the Persian Gulf have already experienced conditions that exceed the critical threshold for human survivability. These areas are at the forefront of extreme heat stress due to the combination of high temperatures and humidity.
👉 Recent research revealed approximately 1,000 instances where wet bulb temperatures exceeded 31°C, and around a dozen cases where they briefly surpassed the 35°C threshold in countries like Pakistan, India, Saudi Arabia, Mexico, and Australia. These occurrences surpass earlier climate models, which predicted such extremes wouldn't happen until later in the century.
Though occurrences of wet bulb temperatures breaching 35°C have so far lasted only a few hours, scientists are warning that both the frequency and duration of these events are likely to increase as global warming accelerates.
If these trends continue, entire regions could eventually face conditions that become uninhabitable, forcing mass migration and creating significant challenges for public health, infrastructure, and resource management. Areas particularly at risk include coastal and tropical regions, where the convergence of high temperatures and humidity is becoming more frequent.
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Why has climate change made reaching the wet bulb temperature threshold more likely?
There is a direct link between rising wet bulb temperatures and climate change. As global temperatures rise due to greenhouse gas emissions, heatwaves - one of the most immediate consequences of climate change - are becoming both more frequent and more intense. Scientists estimate that heatwaves are now at least 100 times more likely to occur than in a world without man-made carbon emissions. Additionally, the intensity of these heatwaves is increasing, with global temperatures projected to rise by as much as 3.5°C under high-emission scenarios compared to pre-industrial levels.
What's more is that as global temperatures rise, the atmosphere can hold more moisture, leading to higher humidity levels, which is a key factor in wet bulb temperatures. This combination of increasing heat and humidity has created conditions where extreme wet bulb temperatures are occurring more frequently and in regions not previously considered at risk.
What’s particularly troubling is the acceleration of these changes. Research shows that wet bulb temperatures in tropical regions increase by approximately 1°C for every 1°C rise in global temperatures. This means that as climate change continues to push global temperatures upward, regions already prone to heat stress - especially those in tropical and coastal areas - will face increasingly unbearable conditions. 40% of the global population resides in these regions, which places billions of people at risk.
Additionally, it’s important to consider the cumulative effect of rising temperatures. As the planet continues to warm, even modest increases in wet bulb temperatures could push certain regions beyond the limits of human adaptability. With the rise of global temperatures, systems we rely on for survival, like agriculture, energy, and water, will face increasing strain, compounding the impacts on both human health and infrastructure.
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What is being done to mitigate the impacts of high wet bulb temperatures?
Efforts to mitigate the impacts of high wet bulb temperatures require both immediate action to protect individuals and long-term strategies to address the root causes of global warming. While individual responses can reduce immediate heat stress, large-scale interventions, and systemic changes are essential to prevent the worsening effects of dangerous heat and humidity.
Individual Action
On an individual level, taking steps to avoid heat-related illnesses is critical. Actions such as staying in shaded areas, hydrating frequently, limiting physical exertion, and wearing light, breathable clothing can help reduce the risk of hyperthermia in extreme conditions. However, in regions where wet bulb temperatures breach the critical threshold for extended periods, these measures may not be enough. Access to air conditioning or cooling centers becomes essential to lowering body temperature and preventing heat stroke. Public health advisories often recommend seeking refuge in air-conditioned spaces or using fans to increase air circulation, especially for vulnerable populations like the elderly and those with pre-existing conditions.
More innovative cooling solutions are also being explored, including the development of personal cooling devices and wearable technologies that can help individuals cope with extreme heat. However, such solutions are not universally accessible, especially in developing regions where resources and infrastructure are limited.
Early Detection and Monitoring
Accurate detection and monitoring of extreme wet bulb temperatures are key to managing the risk and implementing preventive measures. Unfortunately, the areas most vulnerable to dangerously high wet bulb temperatures - such as subtropical regions and developing countries - often lack the necessary infrastructure, including reliable weather stations and forecasting systems. This limits the ability to predict, understand, and respond to extreme heat events in these regions.
To address this gap advanced satellite technologies have been developed. Agencies like NASA have launched satellites, including the Atmospheric Infrared Sounder (AIRS) and the ECOsystem Spaceborne Thermal Radiometer Experiment (ECOSTRESS), to provide valuable data on heat stress. These satellites can detect areas where heat and humidity are at dangerous levels, helping scientists create more accurate climate models and develop better strategies for forecasting extreme heat events. These tools are especially useful for tracking conditions in remote or under-monitored regions, enabling earlier warnings and more effective public health responses.
Climate Adaptation and Infrastructure
Governments and organisations are increasingly recognising the need for infrastructure changes to cope with rising wet bulb temperatures. In many vulnerable regions, access to air conditioning is limited, and even in developed countries, power grids can be overwhelmed during extreme heat events. Investment in heat-resilient infrastructure - such as public cooling centers, better-insulated buildings, and more reliable energy grids - will be critical in the coming decades. Urban planning strategies that include the expansion of green spaces, tree canopy coverage, and cool roofs can also help mitigate the urban heat island effect, lowering temperatures in densely populated areas.
In parallel, early warning systems that alert communities about impending extreme heat events are being deployed in cities around the world. These systems, combined with public health campaigns, can help at-risk populations take preventive measures before conditions become life-threatening.
Reducing Carbon Emissions
At the global level, reducing greenhouse gas emissions is the most crucial step in preventing further increases in wet bulb temperatures. The long-term solution lies in transitioning to renewable energy, eliminating carbon emissions, and working to halt climate change. As the planet continues to warm, heatwaves will become more frequent, severe, and prolonged, directly increasing the risk of deadly wet bulb temperatures.
Countries must meet their emission reduction targets under international agreements like the Paris Agreement to avoid the worst-case scenarios of climate change. Without aggressive action to curb emissions, we risk pushing entire regions of the world into uninhabitable conditions due to the lethal combination of extreme heat and humidity. Mitigation efforts, such as carbon capture technologies and reforestation, can also play a role in stabilising the climate and slowing the pace of global warming.
While individual actions and local strategies are necessary, ultimately, preventing further global heating is the only way to ensure that rising wet bulb temperatures do not become an increasingly deadly reality.
Measuring heat stress
In high-heat environments, simply measuring air temperature is not enough to fully assess heat stress risks. Wet bulb temperature provides a useful measure by combining humidity and air temperature, but other variations of wet bulb temperature, such as thermodynamic wet bulb temperature and isobaric wet bulb temperature, offer additional precision in specific applications.
The thermodynamic wet bulb temperature is calculated based on principles of thermodynamics and reflects the lowest temperature air can reach through evaporation, which is valuable in more technical fields like engineering. The isobaric wet bulb temperature, on the other hand, represents the temperature measured at constant pressure, offering further insight into heat stress under controlled atmospheric conditions.
For an even more comprehensive heat stress assessment, Wet Bulb Globe Temperature (WBGT) incorporates additional factors such as wind speed, solar radiation, and the angle of the sun. WBGT is widely used in outdoor environments where sunlight and heat from surfaces significantly impact human heat stress.
WBGT is particularly critical in settings such as outdoor sports, military training, and construction work. It helps establish safe exposure limits, providing guidelines for hydration, rest breaks, and activity levels in hot weather. By integrating WBGT into environmental monitoring, organizations can more effectively protect individuals from heat-related health risks.
Looking to the future
As climate change heats up our world, wet bulb temperatures are a growing issue. With heatwaves now 100 times more likely, we can expect to see increasing incidences where the dangerous wet bulb temperature threshold is breached - and in fact, we're already seeing evidence of this acceleration.
Heat stress is already a leading cause of weather-related deaths across the globe, and breaching these dangerous wet bulb temperature thresholds means that these figures are set to rise. The only way to prevent these harmful levels of heat and humidity is to prevent further global warming - this is why it's so important that we work towards eliminating carbon emissions and reducing our environmental footprint.
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