How to Calculate for Wet Bulb

Wet-bulb temperature measures the temperature where water evaporates and the air reaches saturation---the point at which air can't hold any more water vapor. You can measure this temperature directly using a thermometer with a wet cloth wrapped around the bulb of the thermometer. Alternatively, you can use a psychrometric chart, which contains information about several physical and thermal properties of air as long as you know two other properties.

Things You'll Need

• Psychrometric chart
• Local weather report

Instructions

1. Find the air temperature and the relative humidity percentage. A local weather report should provide this information.

2. Locate the air temperature along the bottom axis of the psychrometric chart.

3. Follow the temperature line vertically upward until it intersects the curved line for the appropriate relative humidity reading.

4. Find the diagonal line nearest this intersection and follow it upward to the left until you reach the curved wet-bulb or saturation temperature axis. The number you read at this point is the wet-bulb temperature.

Tips & Warnings

• You can determine wet-bulb temperature from any other two properties on the psychrometric chart. Find the intersection of the lines for the other two properties, then follow the diagonal line to the curved left axis to determine wet-bulb temperature.

References

• University of Connecticut: Psychrometric Chart Use
• Ohio State University: Using a Psychrometric Chart to Describe Air Properties
• University of Arizona: Psychrometric Chart