This may seem like just another hot summer day, but it marks a dangerous threshold—one where the body’s ability to cool itself through sweating begins to degrade, especially for vulnerable populations including the elderly, young children, outdoor workers, and individuals with preexisting health conditions.
What Is Wet-Bulb Temperature and Why Does It Matter?
Wet-bulb temperature differs from air temperature. It represents the lowest temperature that can be reached through evaporative cooling. It is measured by wrapping a wet cloth around a thermometer to simulate the cooling effect of sweat evaporating from human skin.
When both temperature and humidity are high, evaporation slows. As evaporation declines, the wet-bulb temperature rises.
When wet-bulb temperatures exceed roughly 70°F (21°C), prolonged exposure becomes dangerous. At 35°C (95°F) at 100% humidity—or roughly 115°F at 50% humidity—the body can no longer cool itself through sweating at all. This is considered the upper physiological survivability limit for humans in the shade, even with unlimited drinking water and rest.
Beyond this threshold, death from heat stress can occur within hours.
Philadelphia’s 80+°F Wet-Bulb: A Warning Sign
Conditions approaching 72°F wet-bulb are not yet the absolute survivability limit, but they represent a clear escalation in humid heat exposure for the northeastern United States.
Wet-bulb temperatures above 31°C (87.8°F) have already been recorded in parts of the U.S. during extreme heat waves—levels once thought to be largely confined to tropical climates. Climate change is shifting these probabilities.
The Physics of Hot, Moist Air
As the atmosphere warms, it holds more water vapor. According to the Clausius–Clapeyron relationship, for every 1°C (1.8°F) rise in temperature, the atmosphere can hold approximately 7% more moisture.
More moisture means:
- Higher dew points
- Elevated heat index values
- Higher wet-bulb temperatures
- Increased nighttime heat retention
This amplification is nonlinear. Even modest increases in average temperature can generate disproportionately severe extremes.
Health Risks and Human Limits
Once wet-bulb temperatures approach survivability thresholds, the body’s thermoregulation system begins to fail.
Heat-related illnesses include:
- Heat exhaustion
- Heatstroke (core body temperature above 104°F / 40°C)
- Organ failure
- Cardiovascular collapse
Symptoms can escalate rapidly in humid conditions because sweat cannot evaporate efficiently. Without evaporation, cooling stops.
Broader Climate Implications
Extreme humid heat is not just a health concern—it is a habitability threshold.
- Increased mortality during heat waves
- Strain on power grids due to cooling demand
- Outdoor labor limitations
- Agricultural stress on crops and livestock
- Urban heat island amplification
Wet-bulb temperatures are a frontline indicator of how close a region is to physiological climate limits.
Conclusion
Adaptation measures—cooling centers, green infrastructure, reflective surfaces, public health alerts—are critical. But mitigation remains essential. Without rapid reductions in greenhouse gas emissions, the probability of crossing survivability thresholds increases.
This is not simply about hotter summers.
It is about the narrowing margin between discomfort and uninhabitability.