Can You Calculate Wet Bulb Temperature Using Relative Humidity

An expert tool for calculating heat stress using air temperature and relative humidity.

What is Wet Bulb Temperature?

Wet Bulb Temperature (WBT) is the lowest temperature to which air can be cooled by the evaporation of water into it at a constant pressure. It’s measured with a thermometer that has its bulb wrapped in a wet cloth. As water evaporates from the cloth, it cools the bulb, and the resulting temperature reading is the WBT. This metric is a crucial indicator of heat stress on the human body because it combines the effects of both heat and humidity. When humidity is high, sweat evaporates more slowly, hindering the body’s natural cooling mechanism. Therefore, a high wet bulb temperature is more dangerous than a high dry-bulb temperature alone. This wet bulb temperature calculator helps you assess these conditions accurately.

Anyone working or exercising outdoors, from athletes and military personnel to construction workers and farmers, should use a wet bulb temperature calculator to assess risk. It is also a critical metric for public health officials during heatwaves to issue warnings and prevent heat-related illnesses like heat exhaustion and heatstroke. A common misconception is that heat index and WBT are the same; while related, WBT is a more direct measure of evaporative cooling potential.

Wet Bulb Temperature Formula and Mathematical Explanation

While the true thermodynamic calculation is complex, a widely used and accurate approximation was developed by Roland Stull (2011). This is the formula our wet bulb temperature calculator uses. It provides a direct calculation from standard weather measurements without needing psychrometric charts. The formula requires temperature in Celsius.

The step-by-step derivation involves a complex empirical fit to data, but the final formula is as follows:

Tw = 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

Variables in the Stull Formula

Variable	Meaning	Unit	Typical Range
Tw	Wet Bulb Temperature	°C	-20 to 50
T	Dry Bulb Air Temperature	°C	-20 to 50
RH	Relative Humidity	%	5 to 99
atan	Arctangent function	Radians	N/A

Practical Examples (Real-World Use Cases)

Example 1: A Hot and Humid Day in a City

Imagine a summer day in a coastal city. The local weather reports an air temperature of 35°C (95°F) with a relative humidity of 70%. Using the wet bulb temperature calculator:

• Inputs: T = 35°C, RH = 70%
• Output (WBT): Approximately 29.8°C (85.6°F)
• Interpretation: This WBT is in the “Extreme Danger” zone. The combination of high heat and high humidity severely limits the body’s ability to cool itself through sweating. Outdoor labor should be stopped, and people are at high risk of heatstroke without access to cooling.

Example 2: A Hot and Dry Day in a Desert

Consider an afternoon in a desert environment with an air temperature of 42°C (107.6°F) but a very low relative humidity of 15%. Using the wet bulb temperature calculator:

• Inputs: T = 42°C, RH = 15%
• Output (WBT): Approximately 23.5°C (74.3°F)
• Interpretation: Despite the scorching air temperature, the WBT is much lower. Because the air is so dry, sweat evaporates very effectively, allowing for efficient cooling. While dehydration is a major risk, the immediate danger of heatstroke from failed evaporative cooling is lower than in the humid example. Check out our dew point calculator for more insights.

How to Use This Wet Bulb Temperature Calculator

Using this tool is straightforward and provides instant results for assessing heat safety.

1. Enter Air Temperature: Input the current dry-bulb air temperature into the first field.
2. Select Units: Choose whether you are entering the temperature in Celsius or Fahrenheit. The calculator will handle the conversion.
3. Enter Relative Humidity: Input the current relative humidity as a percentage (e.g., 65 for 65%).
4. Read the Results: The calculator instantly updates. The main result is the Wet Bulb Temperature (WBT). Intermediate values from the formula are also shown to provide more insight into the calculation.
5. Analyze the Chart: The dynamic chart visualizes how WBT changes with humidity at the current temperature, offering a broader understanding of the heat stress landscape. For more on atmospheric moisture, see our article on what is relative humidity.

Key Factors That Affect Wet Bulb Temperature Results

• Air Temperature: This is the primary driver. Higher air temperature generally leads to a higher WBT.
• Relative Humidity: The most critical secondary factor. As humidity rises, the air is more saturated with moisture, reducing evaporative cooling potential. This causes the WBT to rise and approach the dry-bulb temperature. At 100% humidity, WBT equals the dry-bulb temperature.
• Atmospheric Pressure (Altitude): While our wet bulb temperature calculator uses a formula standardized for sea-level pressure, lower pressure at higher altitudes can slightly increase the rate of evaporation, marginally lowering the WBT.
• Solar Radiation: Direct sunlight heats surfaces (including you) and can increase the perceived heat stress. The Wet Bulb Globe Temperature (WBGT) is a more advanced metric that includes this factor.
• Wind Speed: Wind accelerates evaporation. Increased airflow can help cool the body more effectively, which is why a windy day feels less oppressive. A psychrometric chart calculator can visualize these interactions.
• Cloud Cover: Clouds block solar radiation, reducing the overall heat load on a person and leading to a lower perceived and actual heat stress level compared to a clear, sunny day.

Frequently Asked Questions (FAQ)

1. What is a dangerous wet bulb temperature?

WBT values above 31°C (88°F) are considered dangerous even for healthy, acclimatized individuals at rest. A WBT of 35°C (95°F) is considered the theoretical limit of human survivability, as the body can no longer cool itself at all.

2. How is this different from the Heat Index?

The Heat Index calculates how hot it “feels” by combining air temperature and humidity to estimate a perceived equivalent temperature in the shade. The Wet Bulb Temperature is a direct physical measurement of the cooling potential of the air. WBT is generally considered a more accurate measure of heat stress, especially for those in direct sun. Our wet bulb temperature calculator provides this direct physical metric.

3. Can I use this wet bulb temperature calculator for any location?

Yes. The formula is based on fundamental physical properties (temperature and humidity) and can be used for any location, as long as you have accurate input data.

4. Why does the wet bulb temperature matter for outdoor workers?

For outdoor workers, high WBT indicates conditions where the body cannot cool itself effectively through sweat. This dramatically increases the risk of heat exhaustion and life-threatening heatstroke. Using a wet bulb temperature calculator is a critical safety practice. You can find more information in our guide to workplace heat safety.

5. At what humidity is wet bulb temperature equal to air temperature?

At 100% relative humidity, no more water can evaporate. Therefore, there is no evaporative cooling, and the wet bulb temperature will be identical to the dry bulb (air) temperature.

6. What is Wet Bulb Globe Temperature (WBGT)?

WBGT is a more complex heat stress index that incorporates air temperature, humidity (using WBT), wind speed, and solar radiation (measured with a special “black globe” thermometer). It is considered the gold standard for occupational and athletic heat safety.

7. How does wind affect wet bulb temperature?

The standard WBT calculation assumes some airflow. Increased wind speed enhances evaporation, which would lower the actual WBT on a person’s skin compared to the value in still air. However, the Stull formula used in this wet bulb temperature calculator provides a standardized value that doesn’t directly input wind speed.

8. Can I calculate WBT without a calculator?

Without a tool like this wet bulb temperature calculator, you would need to use complex psychrometric charts or solve the Stull formula manually, which is impractical. The most traditional method is direct measurement with a sling psychrometer.

Related Tools and Internal Resources

• Dew Point Calculator: Understand the temperature at which air becomes saturated and dew forms.
• Understanding the Heat Index: A detailed article explaining how the “feels like” temperature is calculated.
• Workplace Heat Safety Guide: Essential guidelines for keeping workers safe in high-heat conditions.
• Psychrometric Chart Calculator: An advanced tool for visualizing the properties of moist air.
• What is Relative Humidity?: A foundational article explaining the science behind air moisture.
• Comfort Index Calculator: Explore different metrics for thermal comfort beyond just heat.