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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.
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. But this measurement doesn't account for humidity, which plays a crucial role in how heat is perceived by the human body.
The difference between dry bulb temperature and wet bulb temperature is known as the wet bulb depression. It helps indicate how humid the air is:
The more humid the air, the less evaporation occurs – meaning the body can't cool down as effectively.
Relative humidity is the amount of moisture in the air compared to how much it can hold. It directly affects how well sweat can evaporate:
This is why humid heat feels so oppressive – your body can't release excess heat efficiently.
Wet bulb temperature reflects how well the body can cool through sweating. If sweat can’t evaporate, the body overheats. In extreme cases, this can lead to:
It’s not just heat that’s dangerous – humidity is what makes heat deadly.
The dew point is the temperature at which air becomes fully saturated with moisture. A higher dew point means:
Combined with wet bulb and dry bulb readings, dew point and humidity help us assess the full heat risk to humans.
Wet bulb temperature shows when the combination of heat and humidity becomes too much for the human body.
Normally, sweat cools us down as it evaporates – but when humidity is high, this process breaks down. Wet bulb temperature reveals the point at which this natural cooling system stops working.
At a wet bulb temperature of 35°C (95°F), sweat can no longer evaporate – no matter how much the body produces.
35°C wet bulb temperature can be reached even without record-breaking heat.
That’s why this is a widespread risk, not just a desert or equator problem.
This isn’t just an individual concern – it’s a public health crisis in the making.
As climate change drives more frequent, high-humidity heatwaves:
Our bodies cool down by evaporating sweat. But when the air is already full of moisture, sweat can't evaporate – and the body can't release heat, even with heavy sweating.
Hyperthermia occurs when the body absorbs more heat than it can expel. It can develop rapidly in hot and humid conditions, even if the air temperature isn’t extreme.
It often starts subtly – sweating, dizziness, rapid pulse. Without action, it can lead to:
Anyone can be affected, but risks are higher for:
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.
The elderly
As we age, the body’s ability to regulate temperature weakens. Older adults sweat less efficiently and may have health conditions or medications that make heat stress more dangerous.
Outdoor workers
Working in direct sun and high humidity increases risk dramatically. Physical exertion adds strain, making it harder for the body to cool down — even in short periods.
People with health conditions
Those with cardiovascular, respiratory, or kidney conditions are more vulnerable. Heat and humidity place extra stress on already compromised systems.
Parts of South Asia (India, Pakistan), Southwestern North America, and the Persian Gulf have already recorded dangerously high wet bulb temperatures — exceeding what the human body can tolerate.
Recent research found over 1,000 instances of wet bulb temperatures above 31°C — and a dozen cases where they briefly exceeded the 35°C threshold in countries like Pakistan, India, Saudi Arabia, Mexico, and Australia.
These events occurred earlier than climate models had predicted.
So far, wet bulb temperatures over 35°C have lasted only a few hours. But as the climate warms, scientists expect both the frequency and duration of these events to increase.
If trends continue, large parts of the world could become uninhabitable — particularly tropical and coastal regions. This could trigger mass migration and put huge pressure on infrastructure, public health, and resource management.
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.
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.
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.
Staying cool in extreme heat starts with personal measures: stay in the shade, drink plenty of water, wear light, breathable clothing, and avoid physical exertion during peak heat hours.
But when wet bulb temperatures breach critical thresholds, these steps may no longer be enough. Access to air conditioning or public cooling centres becomes essential — especially for vulnerable populations like the elderly or those with health conditions.
Emerging solutions such as wearable cooling devices are promising but remain inaccessible in many developing regions.
Accurate tracking of wet bulb temperatures is vital to managing heat risk. Yet, the regions most at risk — like subtropical zones and low-income countries — often lack reliable weather stations and forecasting infrastructure.
To close this gap, NASA and others are deploying satellites like AIRS and ECOSTRESS, which detect dangerous heat-humidity conditions. These tools help forecast extreme heat events and support early public health responses, especially in under-monitored areas.
Resilient infrastructure will be key in coping with rising wet bulb temperatures. This includes better-insulated buildings, reliable energy grids, and equitable access to cooling technologies.
Urban planning also plays a role: expanding green spaces, planting trees, and installing cool roofs can help lower temperatures in cities. Many regions are also implementing early-warning systems and heatwave alerts to help communities act before conditions become dangerous.
The only long-term solution to rising wet bulb temperatures is stopping global warming at the source. That means cutting carbon emissions, transitioning to renewables, and scaling solutions like carbon capture and reforestation.
If countries fail to meet climate targets, entire regions could become uninhabitable. It’s essential we act now to prevent the worst-case scenarios and stabilise the climate for future generations.
In extreme heat, air temperature alone doesn't tell the full story. To understand how heat truly affects the body, we need to look at more advanced indicators, especially those that combine heat and humidity.
Wet bulb temperature is one such metric, but there are also more refined tools used in technical or high-risk environments. The table below compares the main measurements used to assess heat stress:
Measurement | What it includes & when it's used |
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🌡️ Wet bulb temperature
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Combines heat and humidity to estimate how hot it feels to the human body.
Use: General heat stress monitoring, public health alerts, climate risk. |
🔬 Thermodynamic wet bulb
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Calculates the lowest air temperature possible through evaporation under ideal conditions.
Use: Meteorology, engineering, and scientific modelling. |
⚙️ Isobaric wet bulb
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Measures wet bulb temperature at a constant pressure.
Use: Controlled lab environments and academic research. |
☀️ WBGT (Wet Bulb Globe Temp)
|
Combines temperature, humidity, wind, solar radiation, and surface heat for a full-body heat stress index.
Use: Military training, outdoor work, sports competitions. |
Why it matters: Tools like WBGT are especially important in real-world environments where sun, wind, and surface heat amplify risk. By using the right measurement for the context, we can more effectively prevent heat-related illness and protect vulnerable populations.
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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