Wet-bulb temperature
Based on Wikipedia: Wet-bulb temperature
There is a temperature at which your body stops being able to cool itself. Cross that threshold on a humid day, and no amount of sweating will save you. Your core temperature will rise inexorably, and within hours, even a young, healthy person sitting in the shade with unlimited water will die.
This isn't speculation. It's physics. And the measurement that captures this lethal boundary is called the wet-bulb temperature.
The Thermometer Wrapped in a Wet Sock
The name sounds almost comical, like something from a children's science experiment. And in a way, it is exactly that simple. Take an ordinary thermometer. Wrap the bulb in a small piece of cloth. Soak that cloth in water. Now swing the thermometer through the air, or set up a fan to blow across it, and watch what happens.
The temperature drops.
How much it drops depends entirely on how dry the surrounding air is. On a parched desert afternoon with humidity near zero, the wet cloth will evaporate rapidly, and the thermometer might read twenty or even thirty degrees cooler than the actual air temperature. On a muggy tropical evening where the air is already saturated with moisture, the wet thermometer will read almost exactly the same as a dry one.
This is the wet-bulb temperature: the lowest point you can reach by evaporating water under current atmospheric conditions. It represents a fundamental physical limit, the floor beneath which evaporative cooling cannot push.
Why Evaporation Steals Heat
To understand why wet-bulb temperature matters, you need to understand why evaporation cools things down in the first place.
Water molecules in a liquid are constantly jostling against each other, moving at different speeds. Some move slowly. Others move fast. The fastest molecules have enough energy to break free from the surface and escape into the air as vapor. But here's the crucial part: when those energetic molecules leave, they take their energy with them.
The remaining liquid is now composed only of the slower-moving molecules. Slower movement means lower temperature. The liquid has cooled.
This is why stepping out of a swimming pool on a breezy day makes you shiver. The water on your skin is evaporating, and each molecule that escapes carries away a tiny packet of heat. This is why dogs pant, why we sweat, why hospitals use evaporating alcohol to cool feverish patients.
But evaporation can only happen if the surrounding air has room for more water vapor. Air can only hold so much moisture, and how much it can hold depends on temperature. Warm air can carry more water than cold air, which is why summer days can feel so oppressively humid while winter air tends to be dry.
The Concept of Relative Humidity
Relative humidity measures how full the air currently is, expressed as a percentage of its maximum capacity. At fifty percent relative humidity, the air is holding half as much water vapor as it could possibly contain at that temperature. At one hundred percent, the air is completely saturated. It cannot absorb another molecule of water.
When relative humidity reaches one hundred percent, evaporation stops.
This is where wet-bulb temperature becomes critical. A wet-bulb thermometer reads the same as a regular thermometer only when relative humidity is at one hundred percent. Below that, the wet bulb always reads lower, because evaporation is still possible and is actively cooling the wet cloth.
The gap between the dry-bulb temperature (what a normal thermometer reads) and the wet-bulb temperature tells you something profound about how much evaporative cooling is available in the current atmosphere.
Three Temperatures, One Story
Meteorologists actually track three different temperature measurements, and understanding how they relate to each other reveals the full story of atmospheric moisture.
The dry-bulb temperature is simply the air temperature, what you see on a normal thermometer and what weather reports typically give you.
The wet-bulb temperature, as we've discussed, is the lowest temperature achievable through evaporation alone.
The dew point is different still. It's the temperature at which the air would become saturated if you cooled it without adding any moisture. Cool air below its dew point, and water vapor starts condensing out as dew, fog, or clouds.
In dry conditions, these three numbers spread far apart. On a desert afternoon with a dry-bulb temperature of one hundred degrees Fahrenheit and very low humidity, the wet-bulb might be only sixty-five degrees, and the dew point might be down around forty. But as humidity rises, the three temperatures converge. When relative humidity hits one hundred percent, all three numbers become identical.
Your Body's Cooling System
Human beings are tropical animals with a peculiar superpower: we can shed heat through sweating more efficiently than almost any other creature on Earth. This ability let our ancestors hunt prey to exhaustion on the African savanna, running for hours under the midday sun while other animals collapsed from overheating.
But this cooling system depends entirely on evaporation. We secrete water onto our skin, that water evaporates, and the evaporation carries heat away. As long as the air can absorb our sweat, we can survive in remarkably hot conditions. Desert temperatures that would kill an un-air-conditioned car engine pose no threat to a well-hydrated human being in dry air.
Humidity changes everything.
When the air is already laden with moisture, sweat evaporates slowly or not at all. Instead of cooling you, it simply drips off your body, accomplishing nothing. Your core temperature begins to rise. At a certain point, no amount of shade, water, or rest can save you.
This is why wet-bulb temperature matters more than regular temperature for human survival.
The Lethal Threshold
Physiologists have established that a sustained wet-bulb temperature of around thirty-five degrees Celsius, which is ninety-five degrees Fahrenheit, represents the absolute limit of human tolerance. At this point, even a perfectly healthy person doing nothing more strenuous than lying naked in front of a fan will eventually die of heat stroke.
To be clear about what this means: thirty-five degrees Celsius sounds mild. As a regular air temperature, it's a warm summer day. But as a wet-bulb temperature, it represents a combination of heat and humidity so extreme that the human body cannot shed heat fast enough to survive.
You could reach a wet-bulb temperature of thirty-five degrees with many different combinations. An air temperature of thirty-five degrees Celsius at one hundred percent humidity would do it, since dry-bulb and wet-bulb converge at saturation. But so would an air temperature of forty-five degrees Celsius at around fifty percent humidity, or fifty degrees at around thirty percent.
The mathematics of lethality depends on both numbers together.
How We Measure It
The traditional instrument for measuring wet-bulb temperature is called a sling psychrometer, and it looks like something from a nineteenth-century laboratory because it essentially is. Two glass thermometers with mercury bulbs are mounted side by side on a handle that swivels. One thermometer is left bare. The other has its bulb wrapped in a small cloth sleeve, the "sock," which is moistened with distilled water.
The operator then whirls the device through the air like a rattle, spinning it rapidly for a minute or so. This forces air past both thermometers, allowing the wet one to reach equilibrium with its environment. The difference between the two readings tells you everything you need to know about atmospheric moisture.
Modern weather stations use electronic sensors instead, but the principle remains identical. You need to know how much cooling potential the atmosphere offers, and the only way to find out is to measure how much a wet surface cools.
A Fortunate Coincidence
Scientists distinguish between what they call the "thermodynamic wet-bulb temperature" and what an actual wet-bulb thermometer reads. The thermodynamic version is a precise theoretical value representing perfect adiabatic cooling, meaning cooling that happens without any heat exchange with the outside environment. A real thermometer, with its imperfect cloth and imperfect airflow, cannot quite achieve this ideal.
But for the specific system of water evaporating into air at normal Earth temperatures and pressures, a remarkable coincidence saves us. The ratio between heat transfer and mass transfer, something engineers call the psychrometric ratio, happens to be almost exactly one. This means that a simple wet-bulb thermometer gives readings that closely approximate the true thermodynamic value.
This would not be true for other liquids evaporating into other gases. The wet-bulb temperature of ethanol evaporating into nitrogen, for instance, would require much more sophisticated measurement. But for the combination that matters most to life on Earth, water and air, the simple sock-wrapped thermometer works beautifully.
Practical Applications Beyond Survival
Outside the context of lethal heat, wet-bulb temperature matters enormously for engineering and economics.
Cooling towers at power plants and industrial facilities rely on evaporation to shed waste heat. These massive structures work by letting water trickle down through the tower while air flows upward, evaporating some of the water and cooling the rest. The theoretical limit of how cold they can make the water is the wet-bulb temperature of the incoming air. On humid days, cooling towers become less effective, which is why power plants often struggle most on exactly the days when air conditioning demand is highest.
Evaporative coolers, sometimes called swamp coolers, use the same principle for climate control in buildings. These devices are cheap to operate and can dramatically cool a house in arid climates like the American Southwest. But they become useless as humidity rises. A swamp cooler in New Orleans would accomplish nothing except making the indoor air damper.
Air conditioning engineers use a tool called a psychrometric chart, which maps all the relationships between dry-bulb temperature, wet-bulb temperature, dew point, and relative humidity onto a single diagram. Any two of these values determines the other two, and the chart lets engineers quickly find what they need to design systems for heating, cooling, humidifying, or dehumidifying buildings.
The Difference from Dew Point
People sometimes confuse wet-bulb temperature with dew point, since both relate to atmospheric moisture. But they measure different things.
Wet-bulb temperature tells you how much you can cool something by evaporating water. It represents an active process of adding water vapor to the air.
Dew point tells you how much you must cool the air before water starts condensing out. It represents a passive threshold where vapor becomes liquid.
At any given moment, the dew point is always lower than or equal to the wet-bulb temperature, which is always lower than or equal to the dry-bulb temperature. On an absolutely saturated day at one hundred percent humidity, all three numbers collapse into one.
Climate Change and Rising Danger
As the planet warms, wet-bulb temperatures are climbing. This happens for two reasons. First, warmer air can hold more water vapor, so absolute humidity is increasing. Second, the baseline temperatures are rising, pushing the whole system upward.
The combination is particularly dangerous because it affects places where humans have lived for millennia without air conditioning. Regions around the Persian Gulf, the Indian subcontinent, and Southeast Asia have always been hot and humid, but they stayed below the survivability threshold. Some of these regions are now approaching or briefly exceeding wet-bulb temperatures that were once thought impossible outside extreme microclimates.
In 2015, a heat wave in Pakistan and India killed over three thousand people, with wet-bulb temperatures spiking near human tolerance limits. Similar events have struck the Persian Gulf region, where extreme humidity combines with summer heat to create conditions that challenge even modern infrastructure.
The uncomfortable truth is that air conditioning creates its own feedback loop. Running millions of air conditioners to survive extreme heat pumps more heat into the outdoor environment and consumes electricity that, in most places, comes from burning fossil fuels. The very act of surviving today's heat makes tomorrow's heat worse.
A Simple Measurement with Profound Implications
The wet-bulb temperature is, at heart, a humble measurement. Wrap a thermometer in a wet cloth and see how much it cools. Children can do this experiment in an afternoon.
But what that simple number reveals is nothing less than the boundary between life and death for warm-blooded creatures. It tells us how much cooling the atmosphere can provide through the oldest technology on Earth: evaporation.
For most of human history, the wet-bulb temperature was a curiosity for meteorologists and a calculation for engineers designing cooling systems. Now it has become a metric of existential importance, a measure of how close we are to conditions under which human beings simply cannot survive outdoors, no matter how carefully they try.
The thermometer wrapped in a wet sock turns out to be measuring something far more important than the weather. It is measuring the habitability of our planet.