A wet bulb thermometer is a standard thermometer with its sensing tip wrapped in a damp cloth or wick. As water evaporates from the wick, it cools the thermometer, producing a reading lower than the actual air temperature. The difference between this cooled reading and the regular (dry bulb) air temperature tells you how much moisture is in the air. It’s one of the oldest and most reliable ways to measure humidity, and it remains widely used in weather forecasting, industrial cooling, food processing, and climate science.
How It Works
The principle is straightforward: evaporation absorbs heat. When water on the wick turns to vapor, it pulls energy away from the thermometer bulb, dropping the temperature reading. How much it drops depends entirely on how dry the surrounding air is. In dry air, water evaporates quickly and the wet bulb reading falls well below the actual air temperature. In humid air, evaporation slows because the air is already holding a lot of moisture, so the wet bulb reading stays closer to the dry bulb.
At 100% relative humidity, no evaporation happens at all. The wet bulb and dry bulb readings become identical because the air is fully saturated and can’t accept any more water vapor. The gap between the two readings, called the “wet bulb depression,” is what makes the whole system useful. A large depression means dry air. A small one means humid air. Zero depression means the air is completely saturated, and fog, dew, or rain is likely.
Parts of a Wet Bulb Thermometer
You can build a functional wet bulb thermometer with just a few components:
- Temperature probe or thermometer: Any standard thermometer works, whether mercury, alcohol, or digital. The key is that the sensing tip must be fully covered by the wick.
- Wick: An absorbent material, ideally cotton, that wraps around the probe tip. A round cotton bootlace works well as a substitute. The wick should have a loose, absorbent inner layer to hold water and a tighter outer layer to keep the moisture against the sensor and shield it from direct air contact.
- Water reservoir: A small vessel that keeps the wick constantly wet. A narrow opening helps reduce unnecessary evaporation from the container itself. The reservoir needs to be large enough to last through the measurement period without running dry, or you need to refill it.
- Tape or fastener: Something to secure the wick snugly to the probe so it doesn’t shift or fall off.
A sling psychrometer pairs two thermometers side by side on a handle: one dry, one with a wet wick. You spin the device through the air to ensure consistent airflow over the wick, then compare the two readings.
Measuring Humidity
To calculate relative humidity, you take both a dry bulb and wet bulb reading simultaneously. The dry bulb gives you the actual air temperature. The wet bulb gives you the evaporatively cooled temperature. You then look up both values on a psychrometric chart or table, which translates the depression between the two into a relative humidity percentage. Digital psychrometers do this math automatically, but the physics behind them is the same.
For example, if the dry bulb reads 30°C and the wet bulb reads 22°C, you have an 8-degree depression, corresponding to roughly 50% relative humidity. If the wet bulb instead reads 28°C (only a 2-degree depression), the air is much more humid, around 85%. These relationships are well-established and form the backbone of meteorological humidity measurement worldwide.
Getting Accurate Readings
Wet bulb thermometers are simple devices, but small errors in setup can throw off your readings significantly. The National Park Service recommends always using distilled or deionized water to wet the wick. Tap water contains salts and minerals that can coat the wick fibers over time, creating an uneven surface that disrupts evaporation and skews the reading.
Before you start, check that both thermometers read the same temperature when both are dry. If they don’t agree, one of them is off and your humidity calculation will be wrong from the start. Avoid touching the wick with bare hands, since skin oils and dirt build up on the fabric and interfere with evaporation. New wicks sometimes contain a starchy sizing from manufacturing that resists wetting. Washing a new wick in distilled water before installing it removes that coating. Replace the wick whenever it looks dirty or discolored.
Industrial and HVAC Uses
Wet bulb temperature is a critical number in building cooling and industrial refrigeration. Cooling towers, which reject waste heat from air conditioning systems and power plants, can only cool water down to the outdoor wet bulb temperature. That’s their theoretical floor. The gap between the actual water temperature leaving the tower and the wet bulb temperature is called the “approach,” and it’s the primary measure of cooling tower performance.
This approach temperature shifts dramatically with outdoor conditions. At a wet bulb of 30°F, a cooling tower might have an approach of around 21°F, meaning it can’t cool water closer than 21 degrees above the wet bulb. At a wet bulb of 85°F, that same tower can achieve an approach of just 3.5°F. Engineers use these relationships to size equipment, predict energy costs, and optimize when to run chillers versus when the tower alone can handle the load. Getting the wet bulb wrong means getting the entire system’s capacity wrong.
Wet Bulb Temperature and Human Heat Tolerance
Wet bulb temperature has become central to understanding how heat waves threaten human health. Your body cools itself primarily through sweating: moisture on your skin evaporates, carrying heat away. But this only works if the air is dry enough to accept that moisture. As wet bulb temperature rises, your sweat evaporates more slowly, and your ability to shed heat decreases.
For years, climate scientists pointed to 35°C (95°F) wet bulb as the theoretical upper limit of human survival. The reasoning was simple: if the wet bulb temperature equals or exceeds normal skin temperature (about 35°C), heat can no longer flow from your body to the environment through either evaporation or convection. Core temperature would rise uncontrollably.
That theoretical limit turns out to be too generous. A 2022 study in the Journal of Applied Physiology tested young, healthy subjects under controlled conditions and found that no participant reached the 35°C threshold before their body began overheating uncontrollably. In hot, dry environments, critical wet bulb temperatures ranged from 25°C to 28°C. In warm, humid conditions, limits were slightly higher, around 30°C to 31°C. The researchers concluded that many parts of the world already experience wet bulb temperatures associated with dangerous, uncompensable heat stress more regularly than previously thought.
WBGT vs. Standard Wet Bulb
You’ll often see “wet bulb globe temperature” (WBGT) in sports safety guidelines, military training protocols, and workplace heat rules. This is not the same as a plain wet bulb reading. WBGT combines three measurements: the natural wet bulb temperature (weighted at 70% of the formula), a black globe temperature that captures radiant heat from sunshine (20%), and the dry bulb air temperature (10%). By folding in solar radiation and wind effects, WBGT gives a more complete picture of heat stress for people working or exercising outdoors in direct sun.
The distinction matters practically. The standard heat index, which most people check in weather apps, is calculated from temperature and humidity alone and assumes you’re in the shade. WBGT measures conditions in direct sunlight and accounts for wind and cloud cover. OSHA’s proposed heat safety rules for workplaces use both metrics: employers can track either heat index or WBGT to determine when to trigger protective measures. At a heat index of 80°F (or the equivalent WBGT threshold), employers would need to provide cool water, shaded break areas, and acclimatization plans. At 90°F heat index or its WBGT equivalent, mandatory 15-minute rest breaks every two hours kick in, along with active monitoring for signs of heat illness.
For outdoor workers and athletes, WBGT is the more informative number. For HVAC engineers, meteorologists, and anyone measuring humidity indoors, the standard wet bulb reading is what matters.