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Why should we care about the wet bulb temperature?
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Media > All articles > Global Warming > Why should we care about the wet bulb temperature?

Why should we care about the wet bulb temperature?

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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.
Ecology
2025-07-11T00:00:00.000Z
en-gb
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 cover:
  • What wet bulb temperature is – and why it matters
  • Why heat and humidity together are so dangerous
  • The thresholds where wet bulb temperatures become life-threatening
  • Who’s most at risk – and which regions are already affected
  • How climate change is making extreme heat events more likely
  • What can be done to reduce the risks and protect vulnerable populations

What is wet bulb temperature? An overview

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.

Wet bulb temperature 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 dry bulb temperature and wet bulb temperature is known as the wet bulb depression. It helps indicate how humid the air is:

  • Larger depression → drier air, faster evaporation, more cooling
  • Smaller depression → humid air, slower evaporation, less cooling

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:

  • In low humidity, sweat evaporates easily – helping to cool the body
  • In high humidity, sweat evaporates slowly or not at all – trapping heat

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:

  • Heat stress
  • Organ failure
  • Death – even in shade, with water and rest

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:

  • More moisture in the air
  • Higher wet bulb temperatures
  • Less efficient body cooling

Combined with wet bulb and dry bulb readings, dew point and humidity help us assess the full heat risk to humans.

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Why is wet bulb temperature important?

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.
🌡️
Why wet bulb temperature matters

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.

⚠️
The danger threshold: 35°C

At a wet bulb temperature of 35°C (95°F), sweat can no longer evaporate – no matter how much the body produces.

  • Internal body heat builds up
  • Core temperature rises dangerously
  • Even healthy people at rest can suffer heat stroke, organ failure, or death within hours
🧪
It doesn't take extreme heat

35°C wet bulb temperature can be reached even without record-breaking heat.

  • 39°C with 75% humidity
  • 45°C with 50% humidity

That’s why this is a widespread risk, not just a desert or equator problem.

🌍
Millions are at risk

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:

  • People without access to cooling are especially vulnerable
  • Densely populated urban areas and tropical/coastal regions are most at risk
  • Millions could be affected as extreme wet bulb events become more common
man wiping his sweat with a towel

Why is humidity and heat a dangerous combination?

🔄
Why evaporation fails in high humidity

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.

⚠️
What is hyperthermia – and why it’s so dangerous

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.

😰
Warning signs of heat-related illness

It often starts subtly – sweating, dizziness, rapid pulse. Without action, it can lead to:

  • Nausea and fainting
  • Seizures
  • Organ failure or death
🚨
Who is most at risk?

Anyone can be affected, but risks are higher for:

  • 👵 The elderly
  • 👶 Children
  • 🏥 People with chronic conditions
  • 🌆 Residents in dense or poorly ventilated cities
🌡️ 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, heart rate spike, temp > 40°C, confusion, fainting
🌡️ 35°C and above
Life-threatening heat stroke
Seizures, unconsciousness, organ failure, possible death within hours
someone in a hospital bed connected to an IV drip

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?

👵

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.

❗️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.
elderly man socialising

What regions are most vulnerable to high wet bulb temperatures?

🌍
Regions already at risk

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.

📊
What the data shows

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.

Short events — for now

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.

🚨
What’s at stake

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.

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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.

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.

Measuring heat stress

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
🌡️ Wet bulb temperature
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
Calculates the lowest air temperature possible through evaporation under ideal conditions.

Use: Meteorology, engineering, and scientific modelling.
⚙️ Isobaric wet bulb
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.

thermometer

What rising wet bulb temperatures mean for our 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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