Wet Bulb Temperature Calculator

A crucial tool for assessing heat stress. Answering the question: can you calculate wet bulb temperature using relative humidity?

Calculated Wet Bulb Temperature

Calculation based on your inputs.

This calculation uses the Stull formula, a common approximation for wet bulb temperature based on air temperature and relative humidity at standard pressure.

Comparison of Dry Bulb vs. Wet Bulb Temperature

Understanding the Wet Bulb Temperature Calculation

A) What is Wet Bulb Temperature?

So, can you calculate wet bulb temperature using relative humidity and air temperature? The answer is yes, and it’s a critically important metric. The wet bulb temperature (WBT) is the lowest temperature to which air can be cooled by the evaporation of water into the air at a constant pressure. It represents the combined effect of heat (dry bulb temperature) and humidity. Unlike the standard “dry bulb” temperature you see on a weather report, WBT indicates how efficiently our bodies can cool off through sweating. When the WBT is high, sweat evaporates less effectively, making it feel much hotter and increasing the risk of heat-related illnesses.

This measure is crucial for public health officials, athletes, outdoor workers, and military personnel to assess the actual heat stress on the human body. A high WBT, especially above 31°C (88°F), can be dangerous or even lethal, as the body loses its ability to cool itself. Understanding this concept is more important than ever in a warming climate. This guide to understanding heat stress provides further context.

B) The Wet Bulb Temperature Formula and Explanation

There is no simple, direct analytical formula to solve for wet bulb temperature. However, several accurate approximations exist. This calculator uses a widely-cited version of the Stull formula, which provides a reliable estimate based on dry bulb temperature (T) and relative humidity (RH).

The formula is:

Tw = T * atan[0.151977 * (RH + 8.313659)1/2] + atan(T + RH) - atan(RH - 1.676331) + 0.00391838 * RH3/2 * atan(0.023101 * RH) - 4.686035

Where T must be in Celsius and RH is a percentage value.

Formula Variables

Variable	Meaning	Unit	Typical Range
Tw	Wet Bulb Temperature (The Result)	°C / °F	-20 to 40 °C (-4 to 104 °F)
T	Dry Bulb Temperature (Input)	°C / °F	-20 to 50 °C (-4 to 122 °F)
RH	Relative Humidity (Input)	Percentage (%)	0 to 100

For more advanced analysis, you might want to consult our guide on Psychrometric Chart Explained.

C) Practical Examples

To see how much humidity impacts heat stress, let’s consider two scenarios.

Example 1: A Hot, Humid Day

• Inputs: Dry Bulb Temperature = 35°C (95°F), Relative Humidity = 70%
• Calculation: The combination of high heat and high humidity severely limits evaporation.
• Result: The calculated wet bulb temperature is approximately 29.8°C (85.6°F). This is a level considered extremely dangerous for strenuous activity.

Example 2: A Hot, Dry “Desert” Day

• Inputs: Dry Bulb Temperature = 35°C (95°F), Relative Humidity = 15%
• Calculation: While the air temperature is the same as the first example, the low humidity allows for efficient evaporation.
• Result: The calculated wet bulb temperature is approximately 19.5°C (67.1°F). While still hot, the heat stress is significantly lower and more manageable. For more on humidity, see our page on understanding relative humidity.

D) How to Use This Wet Bulb Temperature Calculator

Using this tool is straightforward. Follow these steps to accurately calculate wet bulb temperature:

1. Select Your Units: Begin by choosing between Metric (°C) and Imperial (°F) using the dropdown menu. The calculator will automatically adjust.
2. Enter Dry Bulb Temperature: Input the current air temperature into the first field.
3. Enter Relative Humidity: Input the current relative humidity as a percentage (e.g., 55 for 55%).
4. Review the Results: The calculator instantly updates, showing the final Wet Bulb Temperature in the large display. You’ll also see a bar chart comparing the dry and wet bulb temperatures visually.
5. Copy or Reset: Use the “Copy Results” button to save the outcome, or “Reset” to clear the inputs to their default state.

E) Key Factors That Affect Wet Bulb Temperature

While our calculator focuses on temperature and humidity, other factors influence WBT in the real world.

• Dry Bulb Temperature: The starting point for the calculation. Higher air temperature generally leads to a higher WBT.
• Relative Humidity: The most critical secondary factor. The higher the humidity, the closer the WBT will be to the dry bulb temperature, indicating less evaporative cooling potential.
• Air Pressure / Altitude: The Stull formula assumes standard sea-level pressure. At higher altitudes (lower pressure), water evaporates more easily, which can slightly lower the WBT compared to its sea-level equivalent.
• Solar Radiation: Direct sunlight heats surfaces and can increase the *perceived* heat stress, which is measured by a related metric, the Wet Bulb Globe Temperature (WBGT), but doesn’t directly alter the meteorological WBT.
• Wind Speed: Wind accelerates evaporation. A breezy day will feel more comfortable because the wind helps sweat evaporate, effectively lowering the temperature at the skin’s surface.
• Dew Point: The dew point is intrinsically linked to this. WBT will always be between the dew point and the dry bulb temperature. When relative humidity is 100%, dry bulb, wet bulb, and dew point temperatures are all equal. For more information, try our Dew Point Calculator.

F) Frequently Asked Questions (FAQ)

1. Why is wet bulb temperature so important for human safety?

It’s the best single indicator of heat stress on the body. High WBT means your sweat can’t evaporate to cool you down, leading to a rapid rise in core body temperature, heat exhaustion, or fatal heatstroke.

2. What is considered a dangerous wet bulb temperature?

WBTs above 28°C (82°F) are considered very high risk for strenuous activity. A WBT of 35°C (95°F) is considered the theoretical survivability limit for a healthy human at rest, as the body can no longer shed heat to the environment. Recent studies suggest the practical limit might be even lower, around 31°C (88°F).

3. How is this different from the Heat Index?

The Heat Index is a simpler model that also combines temperature and humidity to represent perceived heat. WBT is a more direct physical measurement of the cooling potential of the air. While related, they are calculated differently. Our Heat Index Calculator can provide a comparison.

4. Can you calculate wet bulb temperature using relative humidity alone?

No, you cannot. You need both the dry bulb (air) temperature and the relative humidity to perform the calculation. The humidity’s effect is relative to the temperature.

5. Why is my calculated WBT lower than the air temperature?

The WBT will always be equal to or lower than the dry bulb temperature. The only time they are equal is at 100% relative humidity. The difference between the two indicates the “evaporative cooling” potential of the air.

6. Does this calculator work for any altitude?

This calculator uses an approximation that is most accurate at standard sea-level pressure. At very high altitudes, the actual WBT may be slightly lower than the value calculated here, but for most populated areas, this provides a very reliable estimate.

7. What is the difference between Wet Bulb and Dew Point?

Dew Point is the temperature at which air must be cooled to become saturated with water vapor (100% humidity). Wet Bulb Temperature is the temperature a surface reaches through evaporative cooling. WBT is always at or above the Dew Point temperature.

8. Can I use this for industrial processes like cooling towers?

Yes. The wet bulb temperature is the theoretical limit for how cool the water in an evaporative cooling tower can get. It is a fundamental parameter for designing and assessing the efficiency of such systems.