Why Wet Bulb Temperature Matters
Many people are familiar with air temperature but less aware of the role humidity plays in how hot or cold the air feels. The wet bulb temperature combines both factors and reflects the lowest temperature to which air can be cooled through evaporation alone. In other words, it tells us how effective sweat or other evaporative processes will be in removing heat from our bodies. On a dry day with low humidity, evaporation happens quickly, so the wet bulb temperature can be many degrees lower than the actual air temperature. When humidity climbs near 100%, evaporation slows dramatically, causing the wet bulb temperature to approach the dry bulb temperature.
The concept originated with the psychrometer, a classic instrument containing both a dry thermometer and a second thermometer whose bulb is covered by a wet cloth. By rotating the device or exposing it to airflow, water evaporates from the cloth, cooling the wet thermometer. The temperature difference between the dry and wet bulbs reveals the moisture content of the air. Today, meteorologists still rely on this principle to estimate humidity and heat stress, but electronic sensors have largely replaced manual psychrometers.
Applications from Forecasting to Human Comfort
Understanding wet bulb temperature is crucial across many fields. Farmers monitor it to protect livestock from heat stress, since animals cool primarily through evaporation. In the energy industry, power plant operators track wet bulb values to gauge the efficiency of cooling towers that rely on evaporation. Firefighters and construction crews use it to determine safe work-rest cycles when wearing heavy gear. The same principle informs public health guidelines during heat waves, as dangerously high wet bulb temperatures can prevent the human body from shedding heat, leading to heat exhaustion or even heat stroke.
Wet bulb temperature also influences how comfortable a home or building will feel. Air conditioning systems remove both heat and moisture from indoor air. If humidity remains high, occupants may still feel clammy even when the thermostat reads a moderate temperature. By comparing wet bulb temperatures indoors and outdoors, engineers can design ventilation and dehumidification systems that optimize comfort while conserving energy.
Estimating Wet Bulb Temperature
Several formulas exist to approximate wet bulb temperature from the dry bulb temperature and relative humidity . One popular method for typical weather ranges is known as the Stull formula:
This empirical approximation is accurate to within about 1 °C for typical meteorological conditions. The formula may look intimidating, but a few multiplications, square roots, and trigonometric functions are all that’s required. Our calculator executes these steps instantly in your browser without sending data anywhere.
Sample Calculation
Imagine a summer day where the air temperature is 30 °C and the relative humidity stands at 70%. Plugging those numbers into the Stull formula yields a wet bulb temperature near 26 °C. That means even vigorous sweating can only cool the skin to around 26 °C under ideal conditions. If your body temperature rises above that point, heat will build up quickly, so outdoor activities should be paced carefully or rescheduled for a cooler time of day.
Limits of Simple Approximations
The Stull formula works well for everyday weather but becomes less accurate at extreme temperatures or humidity levels. More rigorous psychrometric equations account for air pressure and use iterative techniques to model saturation vapor pressure more precisely. Meteorological agencies rely on such detailed formulas, but their complexity would weigh down a simple browser-based tool. For professional-grade results, you may need to consult technical references or use advanced software packages. Nonetheless, this calculator offers a quick and reasonably accurate estimate for most planning and comfort calculations.
Table of Relative Humidity Effects
| RH (%) | Evaporation Rate | Feels Like |
|---|---|---|
| 20 | Very fast | Much cooler than air temp |
| 50 | Moderate | Slightly cooler |
| 80 | Slow | Close to air temp |
| 95 | Very slow | Almost same as air temp |
Using This Tool
To use the calculator, enter the current air temperature in degrees Celsius along with the relative humidity as a percent. When you press the button, the JavaScript retrieves both values and applies the Stull formula. The output appears below the form with one decimal place of precision. If you accidentally enter unrealistic numbers, the script displays an error message prompting you to check your inputs.
Practical Tips and Considerations
Remember that actual comfort depends on more than temperature and humidity. Wind speed influences evaporation, while solar radiation adds heat load beyond what a thermometer measures. Clothing and physical activity also change how quickly sweat evaporates. Wet bulb temperature therefore represents a best-case scenario for cooling. Real-world heat stress can be worse, particularly in direct sun or with heavy clothing. Local weather agencies often use the related Wet Bulb Globe Temperature, which adds adjustments for sun and wind. Nevertheless, wet bulb alone provides valuable insight when comparing different climate conditions or evaluating the effectiveness of evaporative coolers.
Where Wet Bulb is Critical
Across the globe, farmers monitor wet bulb forecasts to protect livestock and schedule irrigation. Industrial facilities measure it to fine-tune cooling towers and evaporation-based chillers. Military training camps use wet bulb thresholds to determine when recruits must take water breaks. Even the aviation industry watches wet bulb temperatures because humidity can alter aircraft performance and runway length requirements. Understanding this measurement can therefore benefit a surprising range of professions.
Historical Perspective
The link between humidity and heat stress has long been recognized. Early 20th-century researchers studying mine safety developed the psychrometric charts still seen in engineering textbooks. Over time, those charts evolved into formulas that were easier to program into computers. Today, meteorologists rely on automated weather stations with electronic sensors, but the concept remains the same: evaporation potential dictates how hot the human body will feel. Simple web-based calculators, like the one on this page, extend that knowledge to anyone with internet access.
Conclusion
Wet bulb temperature may sound like an obscure meteorological term, yet it has practical implications in daily life, agriculture, industry, and emergency planning. By entering just two numbers—air temperature and relative humidity—you can estimate how effectively evaporation will keep you cool. While advanced models exist for specialized applications, this simple calculator lets you explore the concept quickly, building intuition about the interplay between heat and humidity. Keep an eye on wet bulb values during hot, muggy weather, and you’ll be better prepared to stay safe and comfortable.