S.G. Kubrak

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Wet Bulb Globe Temperature (WBGT) is similar to the Heat Index in that it is an indicator of the stress on the body from the heat.

Wet-bulb Temperature: It’s Not “Just Humidity”

Posted on January 8, 2024January 31, 2024 by sgkubrak

Introduction

When we talk about temperature, most of us think about the readings we see on our weather apps or thermometers – a straightforward number indicating how hot or cold it is. However, there’s another measure of temperature, less commonly understood but equally crucial, especially in our warming world: the wet-bulb temperature.

Wet-bulb temperature isn’t just a different way to measure how hot it feels; it’s a critical indicator of how heat and humidity combine to affect the human body’s ability to cool itself. In a world where climate change is making extreme heat events more frequent and severe, understanding and monitoring wet-bulb temperatures is becoming increasingly important for public health and safety.

This blog post dives into the science behind wet-bulb temperature. I will explain it in simple terms and explore why it’s a vital metric in the context of climate change. We’ll look at how it affects human health, its implications in different parts of the world, and what rising wet-bulb temperatures mean for our future. We’ll also discuss strategies for adaptation and mitigation, emphasizing the urgency of addressing this aspect of our changing climate.

Basic Explanation of Wet-Bulb Temperature

To truly grasp the concept of wet-bulb temperature, we first need to understand what it measures and how it differs from the regular, or “dry-bulb,” temperature readings we’re used to.

What is Wet-Bulb Temperature?

At its core, the wet-bulb temperature is a measure of the lowest temperature that can be achieved through evaporative cooling. It’s essentially how cool a surface can get when it’s wet and exposed to moving air. This is different from the dry-bulb temperature, which is what we normally refer to as the air temperature, measured by a regular thermometer not influenced by moisture.

How Is It Measured?

To measure wet-bulb temperature, meteorologists use a specialized device called a sling psychrometer. I know, it sounds scary, but its really fun to use, and really inexpensive. This instrument has two thermometers: one with a dry bulb and one with a bulb wrapped in a wet cloth (the wet-bulb). When the psychrometer is swung through the air, evaporation occurs from the wet cloth, cooling the wet-bulb thermometer. Calculating wet-bulb temperature is the difference in temperature readings between the dry bulb and the wet bulb gives an indication of humidity: the smaller the difference, the higher the humidity.

Web-bulb temperature is measured using a psychrometer. Pictured here.
A psychrometer, ready to do its job.

Why is Wet-Bulb Temperature Important?

Understanding wet-bulb temperature is crucial because it gives us a more accurate representation of how the human body experiences heat. Our bodies cool down through the evaporation of sweat. In high humidity, this evaporation process is less efficient, and the body feels hotter than the actual air temperature. Wet-bulb temperature reflects this combination of heat and humidity. This provides a more realistic indicator of potential heat stress, especially in outdoor settings or non-air-conditioned environments.

The Science Behind Wet-Bulb Temperature

Delving deeper into the science of wet-bulb temperature helps us understand its significance in relation to human comfort and survival. This section explores the relationship between humidity, body cooling, and the critical thresholds of wet-bulb temperature.

Humidity’s Role in Wet-Bulb Temperature

Humidity plays a central role in determining wet-bulb temperature. It’s a measure of the amount of water vapor in the air. The more water vapor present, the higher the humidity. When air is humid, it’s already saturated with water vapor. This limits the amount of moisture that can evaporate from surfaces, including human skin. This is why high humidity makes it feel much hotter than the actual temperature.

Human Body Cooling and Wet-Bulb Temperature

The human body relies on the evaporation of sweat to cool down. In conditions where the wet-bulb temperature is high, the air is so saturated with moisture that sweat doesn’t evaporate efficiently, impeding the body’s ability to cool itself. This can lead to a dangerous rise in body temperature, causing heat stress or even heatstroke in severe cases.

Critical Thresholds of Wet-Bulb Temperature

There are critical thresholds for wet-bulb temperatures beyond which human survival is at risk. Researchers have identified a wet-bulb temperature of 95°F (35°C) as a critical limit. At this point, a healthy person sitting in the shade without any physical exertion may not be able to cool down. However, even lower wet-bulb temperatures can pose significant health risks. Often this is particular to vulnerable populations such as older people, children, and those with pre-existing health conditions.

I see this discussion often on the internet, and typically people who live in environments where it is normally hot and humid. “Sounds like August in Mobile. Its no big deal.” This statement is minimizing the effects of this extreme condition. As they push though it, their body temperature continues to rise, potentially hitting 106°F.

Implications of Rising Wet-Bulb Temperatures

The increasing instances of high wet-bulb temperatures have significant implications, particularly in the context of global climate change. This section will explore how rising wet-bulb temperatures impact human health and the environment.

Impact on Human Health

As wet-bulb temperatures rise, the risk of heat-related illnesses escalates. This is particularly concerning in regions that experience high levels of humidity. When the wet-bulb temperature approaches human body temperature, the body struggles to cool itself, leading to a range of health issues from mild heat exhaustion to severe heatstroke, which can be fatal. Again, vulnerable populations, including outdoor workers, older people, and those without access to air conditioning, are at a higher risk.

Signs of Heat Stroke

It’s important to note that heat stroke requires immediate medical attention. If you suspect someone is experiencing heat stroke, call emergency services right away. While waiting for medical help, move the person to a cooler place, remove excess clothing, and try to cool them down with whatever means available, such as cool water or fans.

  1. High Body Temperature: A core body temperature of 104°F (40°C) or higher, which is the most telling symptom of heat stroke.
  2. Altered Mental State or Behavior: Confusion, agitation, slurred speech, irritability, delirium, seizures, and coma can all be symptoms of heat stroke.
  3. Alteration in Sweating: In heat stroke caused by hot weather, your skin will feel hot and dry to the touch. However, in heat stroke brought on by strenuous exercise, the skin may feel dry or slightly moist.
  4. Nausea and Vomiting: A person may feel sick to their stomach or vomit.
  5. Flushed Skin: The skin may turn red as the body temperature increases.
  6. Rapid Breathing: Breathing may become rapid and shallow.
  7. Racing Heart Rate: The heart rate may increase significantly because heat stress places a tremendous burden on the heart to help cool the body.
  8. Headache: A throbbing headache is common.
  9. Dizziness or Fainting: This might occur due to extreme temperatures or dehydration.

Recent Instances and Health Risks

In recent years, there have been several instances of dangerously high wet-bulb temperatures recorded around the world, particularly in South Asia and the Middle East. These conditions have led to spikes in heat-related illnesses and deaths, highlighting the need for urgent action to adapt to and mitigate these extreme heat events.

Global Impact and Future Predictions

Wet-bulb temperatures are not just a localized issue but a global concern. We will look at how different regions are affected and what future climate models predict regarding these trends.

Diverse Impact Across the Globe

Different regions experience the effects of rising wet-bulb temperatures in varying degrees. Tropical and subtropical areas, where high humidity is common, are particularly vulnerable. Countries in South Asia, Southeast Asia, and parts of the Middle East have already encountered wet-bulb temperatures nearing human tolerance limits. In contrast, dry but hot areas like the southwestern United States face their own set of challenges, as even moderate increases in humidity can significantly raise the wet-bulb temperature.

Future Trends According to Climate Models

Climate models project an alarming trend: as global temperatures rise, episodes of extreme wet-bulb temperatures will become more frequent and intense. This could lead to large areas of the planet becoming uninhabitable or dangerous for human life during heatwaves, especially if greenhouse gas emissions continue at their current rate.

The Increasing Frequency of Extreme Heat Events

The increase in extreme heat events is a harbinger of what’s to come. Cities, often heat islands due to their dense infrastructure, might face more frequent and severe heatwaves, exacerbating the urban-rural divide in terms of climate vulnerability. The future could see a rise in climate migration, as people move away from areas that are no longer safe or comfortable to live in.

Adaptation and Mitigation Strategies

In the face of rising wet-bulb temperatures, adaptation and mitigation strategies are crucial. We will explore the ways communities and individuals can respond to the increasing risks and the broader strategies needed to combat climate change.

Community Adaptation Strategies

Communities, especially in high-risk areas, can adopt various strategies to cope with rising wet-bulb temperatures. These include developing early warning systems for heatwaves, establishing public cooling centers, and increasing green spaces to reduce urban heat islands. Infrastructure can be adapted to be more heat-resilient, with buildings designed for better ventilation and reduced heat absorption.

Individual Protective Measures

On an individual level, awareness and preparedness are key. Staying hydrated, avoiding outdoor activities during peak heat hours, and having access to air-conditioned environments can significantly reduce the risk of heat-related illnesses. Wearing light, breathable clothing and using cooling technologies like misting fans are practical measures that can help.

Mitigating Climate Change

Ultimately, addressing the root cause of rising wet-bulb temperatures means mitigating climate change. This involves reducing greenhouse gas emissions through cleaner energy sources, promoting sustainable practices, and protecting natural ecosystems that act as carbon sinks. International cooperation and policy-making play a vital role in achieving these goals.

Innovation and Technology

Advancements in technology also offer potential solutions. Innovations in climate-resilient architecture, urban planning, and emergency response can help societies adapt to changing climate conditions. Research into new materials and technologies that aid in cooling could also play a part in mitigating the effects of extreme heat.

Conclusion

Understanding wet-bulb temperature is more than a scientific endeavor; it’s about recognizing the risks that come with our warming planet and the collective responsibility we have in addressing them. From community-level adaptation strategies to global efforts in mitigating climate change, every action counts. Innovation, technology, and policy-making are key in shaping a future where the dangers of extreme heat are effectively managed.

As we continue to witness the impacts of climate change, let’s remember that knowledge is power. Armed with an understanding of wet-bulb temperature and its implications, we can make informed decisions, advocate for change, and contribute to a world that is resilient in the face of environmental challenges. The journey towards a cooler, safer planet starts with each of us.

Resources and Further Reading

  • What is Wet Bulb Globe Temperature (WBGT)? Nicholas Institute for Energy, Environment & Sustainability. Duke University
  • Interactive Map: Daily Maximum Wet-Bulb Temperature. Columbia University
  • Wet Bulb Globe Temperature: How and when to use it. NOAA. (Fun Fact: I wrote this page when I worked there)

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