Wet bulb temperature is the temperature a wetted thermometer reaches after evaporation, and it depends on both air temperature and relative humidity. This guide explains the physics behind wet-bulb temperature and shows how to estimate it with a practical Wet Bulb Calculator.
You can use wet-bulb temperature to assess evaporative cooling, heat stress risk, and HVAC or industrial water-loss performance. The calculator in this page computes wet-bulb temperature using a standard thermodynamic approach with built-in unit handling.
What Is Wet Bulb Temperature?
Wet-bulb temperature (often written Twb) is measured by wrapping a thermometer bulb with a water-soaked wick and letting evaporation cool the thermometer. Evaporation removes heat, so the reading is typically lower than the dry-bulb air temperature (Tdb).
Because evaporation depends on how much water vapor the air can still hold, relative humidity strongly influences Twb:
- When air is dry (low RH), evaporation is strong and Twb drops further below Tdb.
- When air is humid (high RH), evaporation is weaker and Twb approaches Tdb.
Core Concepts and Variables
The calculator uses these common inputs:
- Dry-bulb temperature (Tdb): the current air temperature.
- Relative humidity (RH): the fraction of saturation water vapor pressure present in the air (0–100%).
- Pressure (optional): affects saturation vapor pressure and therefore the thermodynamic balance. Default is standard atmospheric pressure.
Key outputs:
- Wet-bulb temperature (Twb): the estimated wet-bulb temperature.
- Vapor pressure (e): the actual water vapor partial pressure in the air.
- Saturation vapor pressure (at Twb): used in the wet-bulb energy balance.
The Wet-Bulb Calculation (Simple Explanation)
Wet-bulb temperature is not a direct average of temperature and humidity. It is tied to the evaporation-driven energy balance between the air and the evaporating water on the wick.
A widely used method expresses the wet-bulb condition through psychrometric relations. In practice, the calculator estimates Twb by:
- Computing the actual vapor pressure from Tdb and RH.
- Using an iterative solution that finds the Twb where the wet-bulb energy balance is satisfied.
This approach is accurate enough for engineering estimates and field work, while still being fast for a web calculator.
Formulas Used in the Calculator
1) Actual vapor pressure from RH
The calculator uses saturation vapor pressure over water, then multiplies by RH:
| Quantity | Symbol | Meaning |
|---|---|---|
| Actual vapor pressure | e | Water vapor partial pressure in the air |
| Saturation vapor pressure | es(T) | Maximum vapor pressure possible at temperature T |
e = RH × es(Tdb) (with RH as a fraction, not percent).
2) Iterative solution for Twb
Wet-bulb temperature is found by solving a psychrometric balance that links:
- Latent heat of vaporization (energy used to evaporate water)
- Humidity ratio (how much water vapor is in the air)
- Thermal energy difference between Tdb and Twb
The calculator implements this numerically, because Twb appears inside saturation vapor pressure terms. The result converges quickly for typical weather and HVAC ranges.
How to Use the Wet Bulb Calculator
To get a reliable estimate:
- Enter your dry-bulb temperature (°C or °F).
- Enter your relative humidity (0–100%).
- Keep pressure at the default unless you know you are far from sea level or using a different reference.
- Press Calculate to compute wet-bulb temperature.
If inputs are outside realistic ranges, the calculator flags them so you can correct values before using the result.
Practical Examples
Example 1: Heat stress planning for outdoor work
Suppose the air is 32°C with 60% RH. Evaporative cooling potential is moderate, so wet-bulb temperature will be meaningfully below dry-bulb temperature. A wet-bulb estimate helps decide whether to increase rest breaks and hydration.
Use the calculator to translate conditions into Twb, then compare with your organization’s heat guidance thresholds.
Example 2: HVAC and evaporative cooling performance
For an evaporative cooler, the achievable cooling is constrained by the wet-bulb temperature. If you know outdoor Tdb and RH, you can estimate Twb and predict how low supply air temperature can realistically go.
Enter outdoor conditions into the Wet Bulb Calculator to estimate Twb and size your system expectations.
Limitations You Should Know
Wet-bulb temperature depends on measurement method and assumptions. Real instruments may include radiation effects, airflow differences, and wick saturation behavior.
- This calculator estimates Twb from Tdb and RH using standard psychrometric assumptions.
- For very cold conditions, or for unusual aerosols and salinity (not pure air-water), errors can increase.
- Pressure changes matter at high altitude; use the pressure input if you need better accuracy.
Frequently Asked Questions
What does wet-bulb temperature tell you that relative humidity doesn’t?
Wet-bulb temperature combines temperature and moisture into a single evaporation-cooling metric. Relative humidity shows how close air is to saturation, but it doesn’t directly describe how much cooling you can get from evaporation. Twb is tied to the energy balance of evaporating water.
Is wet-bulb temperature always lower than dry-bulb temperature?
In typical outdoor conditions with evaporative cooling, wet-bulb temperature is less than or equal to dry-bulb temperature. When air is fully saturated, evaporation stops and Twb equals Tdb. If measurements are affected by radiation or instrument setup, readings can differ.
How accurate is a Wet Bulb Calculator for everyday use?
A calculator using standard psychrometric relations is accurate enough for HVAC estimates, heat risk screening, and general engineering planning. Accuracy depends on correct inputs, especially relative humidity and pressure. For critical safety decisions, use certified instruments and validated methods.
Can I use wet-bulb temperature to estimate how cold an evaporative cooler can get?
Yes, wet-bulb temperature is the practical limit for evaporative cooling. In ideal conditions with perfect contact and no losses, outlet air approaches Twb. Real systems have inefficiencies, so the actual temperature is usually higher than the calculated wet-bulb temperature.
What units should I use in the calculator?
You can enter temperature in either °C or °F, and the calculator converts internally. Relative humidity should be entered as a percent from 0 to 100. If you change pressure, use the unit dropdown so the calculator interprets the value correctly.
Conclusion: Use Wet Bulb Temperature for Better Decisions
Wet-bulb temperature is a practical way to quantify evaporation cooling and moisture stress in the air. With a Wet Bulb Calculator, you can estimate Twb from temperature and relative humidity in seconds.
Use the result to improve planning for comfort, equipment performance, and safety. When precision matters, verify your inputs and consider local calibration and measurement conditions.