Wet Bulb Calculator

Determine the wet-bulb temperature and psychrometric depression based on air temperature and relative humidity.

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Calculation Examples

Calculation Case Result
Summer Day: 30°C at 50% Humidity Wet-Bulb Temp: ~22.6°C (Moderate Risk)
Extreme Heat: 35°C at 80% Humidity Wet-Bulb Temp: ~31.9°C (Danger Zone)
Dry Climate: 35°C at 10% Humidity Wet-Bulb Temp: ~16.1°C (High Cooling Potential)

How to Use the Wet Bulb Temperature Calculator?

The wet-bulb temperature reflects the lowest temperature a surface can reach through evaporative cooling. To use this calculator, you need two primary inputs: the Dry-Bulb Temperature ($T_d$), which is the standard air temperature measured by a regular thermometer, and the Relative Humidity ($RH$).

After entering these values and selecting your preferred unit (Celsius or Fahrenheit), the tool computes the Wet-Bulb Temperature ($T_w$) and the Wet-Bulb Depression. The depression is the difference ($T_d - T_w$), which serves as a critical indicator of atmospheric dryness. In professional meteorology and occupational health, these figures are vital for calculating the Heat Index and evaluating safety for outdoor activities. This digital psychrometer is especially useful for HVAC technicians, athletes, and agricultural experts who need to monitor thermodynamic limits in real-time without manual charts.

Wet-Bulb Temperature Formula

The most widely accepted method for estimating this value without a physical sling psychrometer is the Stull empirical formula, published by Roland Stull in 2011. This formula is highly regarded for its accuracy in standard terrestrial conditions.

The calculation utilizes complex arctangent functions to model the non-linear relationship between moisture and temperature:
$T_w = T \cdot \arctan(0.151977 \cdot (RH + 8.313659)^{0.5}) + \arctan(T + RH) - \arctan(RH - 1.676331) + 0.00391838 \cdot (RH)^{1.5} \cdot \arctan(0.023101 \cdot RH) - 4.686035$
Where $T$ is the air temperature in °C and $RH$ is the relative humidity in %.

This equation is valid for a wide range of conditions, specifically for temperatures between $-20°C$ and $50°C$ and humidity levels from $5\%$ to $99\%$. It accounts for the latent heat of vaporization — the energy required for water to evaporate from the "wet bulb" sleeve, which naturally lowers the temperature until it reaches a state of adiabatic saturation.

Web Bulb temperature explained

Useful Tips 💡

  • For high-altitude calculations, remember that lower air pressure can slightly affect evaporative cooling rates.
  • When measuring dry-bulb temperature, ensure the sensor is shaded and well-ventilated for maximum accuracy.
  • If the relative humidity is $100\%$, the wet-bulb temperature will be exactly equal to the dry-bulb temperature.

📋Steps to Calculate

  1. Enter the ambient (dry-bulb) air temperature and select your unit.

  2. Provide the current relative humidity as a percentage (e.g., 65).

  3. Click "Calculate" to determine the wet-bulb temperature and the depression value.

Mistakes to Avoid ⚠️

  1. Confusing Wet-Bulb Temperature with Dew Point; they are distinct physical properties.
  2. Entering the humidity as a decimal (0.6) instead of a whole percentage (60).
  3. Failing to account for the "Wet Bulb 35°C" threshold, which is a life-threatening limit regardless of fitness levels.
  4. Using the formula for temperatures far outside the $-20°C$ to $50°C$ validation range.

Strategic Applications of Wet-Bulb Monitoring📊

  1. Assessing heat stress risks for workers and military personnel (WBGT related metrics).

  2. Optimizing the performance of cooling towers and evaporative (swamp) coolers.

  3. Monitoring "Wet Bulb 35°C" – the critical survivability limit for humans in extreme humidity.

  4. Predicting snow-making conditions at ski resorts and frost protection in agriculture.

Questions and Answers

What is wet-bulb temperature and why is it important?

Wet-bulb temperature ($T_w$) is the lowest temperature that can be achieved through the adiabatic evaporation of water into the air at constant pressure. Unlike standard air temperature, it accounts for the cooling effect of evaporation. This metric is fundamentally important because it represents the "real" cooling limit for any object or organism, including the human body, that relies on evaporation to regulate heat. In meteorology and industrial safety, $T_w$ is used to determine if environmental conditions allow for safe physical exertion, as it more accurately reflects heat stress than dry-bulb temperature alone.

How does wet-bulb temperature differ from dry-bulb temperature?

Dry-bulb temperature is the ambient air temperature measured by a standard thermometer shielded from radiation and moisture. In contrast, wet-bulb temperature is measured by a thermometer covered in a water-soaked cloth over which air is passed. The evaporation of water from the cloth absorbs latent heat, lowering the temperature. The difference between the two, known as the psychrometric depression, indicates how dry the air is: the larger the gap, the lower the humidity. If the air is $100\%$ saturated, the wet-bulb and dry-bulb temperatures will be identical because no evaporation can occur.

Why is wet-bulb temperature critical for heat stress assessment?

It is the ultimate indicator of the body's ability to cool itself via sweat. When wet-bulb temperatures rise, the air's capacity to absorb moisture decreases, making sweating less effective. Scientific consensus, including reports from the IPCC, identifies a sustained wet-bulb temperature of $35°C$ ($95°F$) as the theoretical limit of human survivability. At this point, even healthy individuals in the shade with unlimited water cannot shed enough metabolic heat, leading to hyperthermia. Monitoring this value is essential for occupational health, military training, and high-performance athletics to prevent heat stroke.

Can the wet-bulb calculator be used for indoor environments?

Yes, calculating indoor wet-bulb temperature is vital for managing climate-controlled spaces like factories, greenhouses, and data centers. In industrial settings, it helps evaluate whether ventilation systems are sufficient to protect workers from heat exhaustion. In agriculture, it is used to manage the "Vapor Pressure Deficit" (VPD), which dictates plant transpiration and nutrient uptake. For HVAC professionals, the wet-bulb value is a key parameter in psychrometric charts used to design efficient cooling systems and humidity control strategies for large buildings.

What is the relationship between wet-bulb temperature and WBGT?

Wet-bulb temperature ($T_w$) is a primary component of the Wet-Bulb Globe Temperature (WBGT) index, but they are not the same. WBGT is a composite index used by OSHA and ISO to estimate the effect of temperature, humidity, wind speed, and solar radiation on humans. Typically, the "natural" wet-bulb temperature accounts for $70\%$ of the WBGT value, while the globe temperature (radiant heat) accounts for $20\%$ and the dry-bulb temperature for $10\%$. This emphasizes that humidity and evaporative potential (represented by $T_w$) are the most significant factors in environmental heat risk.

How accurate are online wet-bulb temperature calculations?

Digital calculations are highly reliable estimates when based on verified empirical data. Our calculator utilizes the Stull formula, which is the industry standard for terrestrial conditions. While it provides precision within $0.1°C$ for most altitudes and standard pressures, direct measurements with a calibrated sling psychrometer remain the gold standard for laboratory or regulatory research. Factors like extreme altitude (very low pressure) can introduce slight variances, but for $99\%$ of weather monitoring and safety applications, this online tool provides sufficient accuracy for informed decision-making.

What formula does the wet-bulb temperature calculator use?

This tool applies the Stull empirical formula (2011), specifically developed for accurate estimation across a wide range of climates: $T_w = T \cdot \arctan(0.151977 \cdot (RH + 8.313659)^{0.5}) + \arctan(T + RH) - \arctan(RH - 1.676331) + 0.00391838 \cdot (RH)^{1.5} \cdot \arctan(0.023101 \cdot RH) - 4.686035$. Developed by Professor Roland Stull at the University of British Columbia, this equation has been rigorously validated against observational data. It accounts for the non-linear relationship between dry-bulb temperature ($T$) and relative humidity ($RH$), ensuring that users receive a scientifically backed result for professional use at CalcMate.

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Disclaimer: This calculator is designed to provide helpful estimates for informational purposes. While we strive for accuracy, financial (or medical) results can vary based on local laws and individual circumstances. We recommend consulting with a professional advisor for critical decisions.