An extreme heat event becomes particularly threatening when the air holds a significant amount of moisture, a combination that is measured by the wet-bulb temperature (WBT). This metric is a more accurate indicator of heat stress risk than the standard air temperature alone, as it accounts for both heat and humidity. A “wet-bulb event” describes a period when this combined temperature reaches a level that severely compromises the human body’s ability to cool itself.
Defining the Wet-Bulb Temperature
The wet-bulb temperature (WBT) represents the lowest temperature air can achieve solely through the evaporation of water into it. It is a value that integrates both the ambient air temperature and the amount of moisture present in the air. This WBT differs significantly from the standard temperature, known as the “dry-bulb” temperature (DBT), which is measured by a regular thermometer.
Historically, WBT was measured by wrapping a thermometer’s bulb in a water-soaked cloth and exposing it to moving air. As water evaporates from the cloth, it draws heat away, causing the reading to drop. This lower temperature is the WBT.
If the air has low humidity, significant evaporation occurs, and the WBT will be much lower than the DBT. Conversely, if the air is completely saturated with moisture, evaporation cannot occur, and the WBT will equal the DBT. The WBT therefore serves as a practical measure of the air’s capacity to absorb more moisture.
The Physics of Evaporative Cooling
The wet-bulb temperature relies on evaporative cooling, a process where the phase change from liquid to gas removes heat from the environment. When water molecules transition into vapor, they require a significant amount of energy, known as the latent heat of vaporization.
This energy is extracted from the liquid water and the surrounding air, converting sensible heat into latent heat. As the highest-energy molecules escape as gas, the average kinetic energy of the remaining molecules decreases, causing a temperature drop. This transfer of heat energy cools the air.
High humidity limits this cooling mechanism because the air is already holding a large amount of water vapor. When the air is close to saturation, the rate of evaporation slows dramatically, meaning less heat is removed from the system.
Physiological Thresholds and Human Danger
The danger of a wet-bulb event stems from its impact on the human body’s primary cooling mechanism: sweating. Humans constantly generate internal heat through metabolic processes and must dissipate this heat to maintain a stable core temperature of around 98.6°F (37°C). Evaporation of sweat from the skin is the most effective way the body achieves this heat loss.
When the WBT is high, the air is too saturated for sweat to evaporate efficiently. If the WBT reaches a critical threshold, the body’s cooling system fails because internally generated heat cannot escape. This inability to shed heat causes the core body temperature to rise rapidly, leading quickly to heat exhaustion and heatstroke.
The theoretical limit of human survival for sustained exposure is a WBT of 35°C (95°F). This represents an environment where the body can no longer cool itself, even with shade and unlimited water, leading to organ failure and death within a few hours. However, serious health effects are observed in healthy individuals at WBTs as low as 31°C (88°F), and research suggests the functional limit for young adults may be 30.6°C.
Global Trends and Future Risk
Wet-bulb events are already occurring globally, primarily near large bodies of water that supply the necessary humidity. The Persian Gulf and parts of South Asia, particularly India and Pakistan, have recorded instances of WBTs briefly exceeding the 35°C theoretical survival limit. Cities like Jacobabad in Pakistan have crossed this deadly threshold multiple times.
Climate change is driving an increase in both the frequency and intensity of these extreme humid heat events. As global temperatures rise, the atmosphere holds more water vapor, directly increasing the wet-bulb temperature.
Scientists project that the frequency of present-day extreme WBT events could rise by a factor of 100 to 250 in tropical and mid-latitude regions by 2080. This warming trend puts areas currently just below the critical threshold, such as coastal Mexico, Southeast Asia, and Africa, at risk of crossing into unlivable conditions.