Wet bulb temperature: The crucial weather concept that actually tells us when heat becomes lethal

The most insidious threat posed by climate change may lie in how it exacerbates existing hazards — making all our problems, conflicts, and vulnerabilities harder to deal with. (Three recent examples, for instance: forest fires intensified by previous heat waves, aggravated immigration pressures, and increased risks from infectious diseases, including future pandemics.)  These multiple layers of cause and effect are complex, and make it harder to recognize the damage we are causing.

But sometimes the effects of human-caused global warming is not so subtle and indirect. Indeed, it is quickly becoming dreadfully clear that we can expect more frequent and intense heat waves. Heat waves are already among the deadliest “natural” disasters, and if one could invest in the growth of their destructive power, now would be a good time to do so. 

What is less well recognized is that when it comes to heat stress, and where and when things will get bad, not all heat is created equal. Record high temperatures make headlines because we are intimately familiar with these numbers. Yet record temperatures do not tell the full story. When it comes to the brave new climate world we are creating, we need to get familiar with something called “wet bulb temperature” — a much more meaningful concept to ascertaining when hot weather conditions become dangerous. Wet bulb temperature even tells us where and when our cities will become uninhabitable.

What is wet bulb temperature anyway?

The normal temperature of a healthy human is about 98 °F (37 °C). So why don’t we feel most comfortable when that’s the temperature of the air around us? Because our bodies are engines, burning food to do work, and engines need to lose heat to their surroundings, or they overheat and stop working. As in, die. 

Fortunately, even if our surroundings are warmer than our bodies, we still lose heat if our sweat evaporates, by the miracle of the energy required to convert water from liquid to vapor. In dry air, this evaporative cooling system works great, even at air temperatures that sound really hot, because evaporation makes our skin feel cooler. But as moisture in the air increases, evaporation and heat removal slows, as anyone who has worked outside on a hot humid day knows.

But when do things go from uncomfortable to dangerous — and when will climate change take us there? 

Scientists have come up with more than 120 ways to quantify heat stress. One of the most useful is wet bulb temperature — the temperature that a wet thermometer in the shade measures as water evaporates freely off it. This temperature will be lower than the temperature on a dry thermometer in the same place (which is known as dry bulb temperature), and the difference between the two is a measure of humidity. So to get wet bulb temperature we can either measure it directly with a wet thermometer, or calculate it from dry bulb temperature and humidity. 

The usefulness of wet bulb temperature is it makes it clear how close conditions are to lethal. The closer wet bulb temperature gets to our body temperature, the less heat is lost and the closer we are to heat death.

It has been known for more than a century that wet bulb temperatures higher than 88 °F (31 °C) make it impossible to do physical labor, and a wet bulb temperature of 95 °F (35 °C) kills healthy humans within a few hours. Interestingly, reconstructions of wet bulb temperatures in hothouse periods of Earth’s past have also been used to interpret possible geographic limits and body temperatures of ancient mammals. 

Wet bulb temperature also explains why sky-high temperatures in places like Arizona are typically not lethal. Even a warm 100 °F (38 °C) May day in Tucson with relative humidity of 20% is a relatively comfortable wet-bulb temperature of 70 °F (22 °C). But the same dry bulb temperature with the average relative humidity in Jacksonville (75%) leads to a wet bulb temperature of 94 °F (34 °C), close enough to human body temperature to cause severe and potentially fatal heat stress. And while extreme temperatures usually get the headlines, globally, humidity dominates wet bulb temperature. 

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Today, wet bulb temperatures do not often exceed 88 °F (31 °C). But even recent heat waves with wet bulb temperatures of 85-88 °F (29-31 °C) have killed tens of thousands of people, and they have reached 95 °F (35 °C) for short periods in some places. And what about the future? Extremely high temperature will certainly occur more frequently over large parts of the planet. Trends in humidity are more complex, but are generally predicted to increase as well.

It is unfortunate that wet bulb temperature is not an everyday concept, as it is much more meaningful for understanding human heat stress both from climate change as well as everyday dangers on hot summer days. Indeed, while it’s often assumed that regions most vulnerable to climate change are those with the highest (dry bulb) temperatures, like the desert southwest, models project future lethal wet bulb temperature events in areas that are already humid. In North America, this is most of the midwest and eastern United States. 

Likewise, in as little as 60 years many parts of the world with some of the densest populations may experience frequent wet bulb temperature events approaching or exceeding 95 °F (35 °C), causing mass mortality of humans and livestock, enormous strain on power grids, and socioeconomic instability.  

It is important to understand that wet bulb temperatures of 95 °F (35 °C) are not conditions we can just get used to. Human bodies have fundamental physiological limits; our planet’s perturbed, angry climate doesn’t care about them. Air conditioning may save some, but increased demand and likelihood of outages in already strained power grids makes this a risky bet at best.  

Responsible action can minimize chances of worst-case scenarios. But like it or not, dangerous moist heat waves with lethal wet bulb temperatures are coming, and will raise dreadful new awareness of global warming, and cause devastating heat stress. As is common with climate change, the effects will be felt disproportionately by the most vulnerable.