by Daniel Brouse
June 24, 2025
With today's forecast of 101°F and a dew point of 72°F, Philadelphia is expected to reach a wet-bulb temperature of approximately 72.2°F. This may seem like just another hot day in summer, but it marks a dangerous threshold--one where the body's ability to cool itself through sweating begins to break down, especially for vulnerable populations such as the elderly, young children, and those with preexisting health conditions.
What Is Wet-Bulb Temperature and Why Does It Matter?
Wet-bulb temperature is not the same as the air temperature. It represents the lowest temperature that can be reached through evaporation. It’s measured by wrapping a wet cloth around a thermometer and simulating the effect of evaporation cooling the surface--just like how sweat cools the human body. When both temperature and humidity are high, evaporation slows, and the wet-bulb temperature rises.
When wet-bulb temperatures exceed 70°F, prolonged exposure becomes dangerous. At 35°C (95°F) at 100% humidity--or 115°F at 50% humidity--the body can no longer cool itself at all. This "wet-bulb temperature threshold" is the upper physiological limit of human survivability in the shade, even with unlimited access to water and rest. Beyond this point, death from heat stress can occur in just a few hours, regardless of age or fitness level.
Philadelphia's 72.2°F Wet-Bulb: A Warning Sign
Today's conditions in Philadelphia aren't just uncomfortable--they're approaching a boundary that climatologists and health experts have long warned about. Wet-bulb temperatures above 31°C (87.8°F) have already been recorded in parts of the U.S., especially during recent extreme heat waves. Such readings were once thought to be limited to tropical regions and essentially impossible in the U.S. climate. But thanks to climate change, what was once improbable is now becoming increasingly likely.
The Physics of Hot, Moist Air
The greatest threat posed by climate change to those alive today is not sea level rise decades from now--it's the increasing frequency of hot, humid extremes. As the atmosphere warms, it can hold more moisture. According to the Clausius-Clapeyron equation, for every 1°C (1.8°F) rise in temperature, the air can hold about 7% more water vapor. This amplifies humidity and raises the heat index, further increasing wet-bulb temperatures.
This shift is not gradual. It’s exponential, and it creates conditions where even modest increases in average temperature can produce devastating extremes--such as the kind of heat that renders large regions of the planet uninhabitable for humans and livestock.
Health Risks and Human Limits
Once wet-bulb temperatures pass the survivability threshold, the human body can no longer cool itself through sweating. This leads to heat-related illnesses such as heat exhaustion and, more dangerously, heatstroke. Heatstroke occurs when the core body temperature rises above 104°F (40°C) and can result in seizures, delirium, organ failure, and death. Without immediate cooling and medical intervention, heatstroke is often fatal.
Symptoms can begin with dizziness, fatigue, nausea, rapid heartbeat, and confusion. In many cases, these symptoms escalate rapidly in high humidity because the body is trapped in a feedback loop: the hotter it gets, the more you sweat, but if sweat can't evaporate, cooling never happens.
The Broader Climate Implications
This isn't just a health issue--it's a planetary warning. As more areas approach or exceed wet-bulb limits, we face cascading consequences:
Mass migration from now-uninhabitable regions
Agricultural collapse due to heat stress on crops and livestock
Infrastructure failures as power grids buckle under cooling demand
Social and political instability from resource scarcity and displacement
Wet-bulb temperatures are more than a scientific curiosity--they're the frontline metric for human habitability in a warming world. Today's conditions in Philadelphia are a stark reminder that climate breakdown is not a future problem--it's a present emergency.
Conclusion
If wet-bulb thresholds are regularly exceeded--even in places like the northeastern U.S.--it will change where and how people can live. As climate change accelerates, adaptation and mitigation are no longer optional; they are urgent imperatives. Cities must prepare with heat response plans, cooling infrastructure, and public health initiatives. But more importantly, we must address the root causes of climate change to avoid locking in a future where heat becomes not just oppressive--but unsurvivable.
Our climate model — incorporating complex social-ecological feedback loops within a dynamic, nonlinear system — projects that global temperatures could rise by up to 9°C (16.2°F) within this century. This far exceeds earlier projections, which estimated a 4°C rise over the next thousand years, and signals a dramatic acceleration of planetary warming.