Can You Calculate Wet Bulb Temperature Using Relative Humidity?
Accurately determine the wet bulb temperature (WBT) based on dry bulb air temperature and relative humidity levels using the Stull empirical formula.
Enter the current ambient air temperature.
Enter the relative humidity percentage (0-100%).
21.8°C
(71.2°F)
WBT Sensitivity Chart
How Wet Bulb Temperature changes as Humidity increases (Fixed Temp)
Formula Note: This calculator uses the Stull empirical formula, which is accurate for standard atmospheric pressure and most common weather conditions.
What is Wet Bulb Temperature?
Wet bulb temperature is the lowest temperature that can be reached by evaporating water into the air at a constant pressure. It represents the limit of evaporative cooling—essentially, the temperature a wet surface (like human skin) can reach through sweating. When people ask, “can you calculate wet bulb temperature using relative humidity?” they are usually looking for a way to assess heat safety without specialized meteorological equipment.
Unlike the standard air temperature (dry bulb), wet bulb temperature accounts for the cooling effect of evaporation. This makes it a critical metric for athletes, outdoor workers, and meteorologists. If the relative humidity is 100%, the wet bulb temperature equals the dry bulb temperature because no further evaporation can occur.
How to Calculate Wet Bulb Temperature Using Relative Humidity: The Formula
The most widely accepted method to calculate wet bulb temperature using relative humidity without complex psychrometric charts is the Stull Formula. It is an empirical equation developed by Roland Stull that provides high accuracy for standard pressures.
The Stull Formula:
Twb = T * atan(0.151977 * (RH + 8.313659)0.5) + atan(T + RH) – atan(RH – 1.676331) + 0.00391838 * (RH)1.5 * atan(0.023101 * RH) – 4.686035
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| T | Dry Bulb Air Temperature | Celsius (°C) | -20 to 50°C |
| RH | Relative Humidity | Percentage (%) | 5% to 99% |
| Twb | Wet Bulb Temperature | Celsius (°C) | Variable |
Table 1: Variables used to calculate wet bulb temperature using relative humidity.
Practical Examples
Example 1: High Humidity Environment
Imagine a tropical afternoon with an air temperature of 32°C (89.6°F) and 80% relative humidity. Using the logic to calculate wet bulb temperature using relative humidity, the result is approximately 28.6°C. This is considered very high and suggests extreme caution for outdoor activities, as the body’s ability to cool itself is severely limited.
Example 2: Arid Environment
In a desert at 40°C (104°F) with only 10% relative humidity, the wet bulb temperature drops significantly to about 18.2°C. Even though the air is much hotter, the low humidity allows for efficient sweat evaporation, making the “felt” temperature lower if moisture is present.
How to Use This Calculator
- Select your Temperature Unit: Choose between Celsius (°C) and Fahrenheit (°F).
- Enter Air Temperature: Input the dry bulb temperature from your thermometer.
- Input Relative Humidity: Enter the percentage from your hygrometer.
- Review the Results: The tool will instantly calculate wet bulb temperature using relative humidity and display the heat stress level.
- Analyze the Chart: View the trend line to see how increasing humidity would impact the wet bulb reading.
Key Factors That Affect Wet Bulb Results
- Air Pressure: Standard formulas assume sea-level pressure. Higher altitudes change the evaporation rate.
- Air Velocity: While WBT itself is a thermodynamic property, wind speed affects how quickly you reach that temperature.
- Instrument Accuracy: To calculate wet bulb temperature using relative humidity accurately, your humidity sensor must be well-calibrated.
- Temperature Range: Most empirical formulas lose accuracy below freezing or above 50°C.
- Evaporation Rate: The chemical composition of the liquid (e.g., saltwater vs. fresh water) slightly alters results.
- Proximity to Water: Localized humidity near bodies of water can differ from regional weather station data.
Frequently Asked Questions
Why is it important to calculate wet bulb temperature using relative humidity?
It is vital for assessing human survivability and heat stress. A wet bulb temperature of 35°C (95°F) is often cited as the theoretical limit for human survival over extended periods, even for healthy individuals in the shade.
Is Wet Bulb Temperature the same as the Dew Point?
No. Dew point is the temperature where air becomes saturated. Wet bulb temperature is always between the dry bulb temperature and the dew point (unless at 100% humidity, where all three are equal).
Can I calculate wet bulb temperature using relative humidity for indoor air?
Yes, the physics remains the same. It is frequently used in HVAC design to determine cooling tower efficiency.
Does wind affect the wet bulb temperature?
No, the wet bulb temperature is a property of the air mass. However, wind increases the rate of evaporation, which helps a surface reach the wet bulb temperature faster.
What is a dangerous wet bulb reading?
Generally, readings above 27°C (80°F) start to become uncomfortable for physical labor, and above 31°C (88°F) are considered extremely dangerous.
How does elevation affect the calculation?
At higher elevations, lower atmospheric pressure increases the evaporation rate, which slightly lowers the wet bulb temperature compared to sea level for the same T and RH.
Is the Stull formula always accurate?
It is accurate within 0.3°C for standard conditions, but specialized psychrometric charts or software are better for industrial applications requiring extreme precision.
Can I calculate wet bulb temperature using relative humidity in Fahrenheit?
Most scientific formulas require Celsius. Our calculator handles the conversion automatically for your convenience.
Related Tools and Internal Resources
- Heat Index Calculator: Determine the “feels like” temperature for safety.
- Dew Point Guide: Deep dive into the temperature where condensation begins.
- Psychrometric Chart Basics: Understand the relationship between all air properties.
- Weather Safety Protocols: How to stay safe during extreme heat events.
- Humidity Basics: Learn the difference between absolute and relative humidity.
- Meteorology Tools: A collection of resources for professional weather tracking.