Difference Between Dewpoint and Wet Bulb Temperature

The key difference between dewpoint and wet bulb temperature is that dewpoint temperature is the temperature to which we should cool the air to saturate the air with water vapor whereas wet bulb temperature is the temperature that we get from a moistened thermometer bulb that is exposed to air flow.

Dew point and wet bulb temperatures are very important in indicating the state of the humid air. Another important method is the measurement of dry bulb temperature which is the most common method of determining the temperature of the surrounding. When we give the temperature at the moment, it refers to the dry bulb temperature. However, dew point and wet bulb temperature are useful to determine the temperature of the moist air. There are several differences between these two temperatures, including their value; the value of wet bulb temperature is always between dry bulb temperature and dewpoint temperature.

CONTENTS

1. Overview and Key Difference
2. What is Dewpoint Temperature
3. What is Wet Bulb Temperature
4. Side by Side Comparison – Dewpoint vs Wet Bulb Temperature in Tabular Form
5. Summary

What is Dewpoint Temperature?

Dewpoint temperature is the temperature at which the air becomes saturated with water vapor. In other words, it is the temperature to which we should cool the air to saturate the air with water vapor. Therefore, when further cooled, water vapor starts to condense and form dew drops. But when the temperature is below the freezing point of water, then we call the dewpoint, “frost point” because then frost forms rather than dew.

When the dewpoint temperature equals the air temperature, it is the state of saturation of the air with water vapor. But this temperature never exceeds the air temperature. Therefore, if the air tends to further cool down, moisture removes from the air via condensation.

When considering the relationship between relative humidity and dewpoint;

• If the dewpoint is close to the dry air temperature, the relative humidity is high.
• If the dewpoint is well below the dry air temperature, the relative humidity is low.

What is Wet Bulb Temperature?

Wet bulb temperature is the temperature that we get from a moistened thermometer bulb that is exposed to air flow. We use a thermometer covered with a water-soaked cloth to measure this temperature. The evaporation of water from this cloth indicates the temperature. Therefore, if the water vapor content in the air is high, the evaporation is low. It indicates a low temperature.

When considering the relationship between relative humidity and wet bulb temperature;

• At 100% of relative humidity, the wet bulb temperature equals the dry air temperature.
• At a lower relative humidity, the wet bulb temperature is low.

We can determine this temperature without using a thermometer, using slide rule method: to measure the temperature with this method, we must know the dry bulb temperature and relative humidity (measured using a hygrometer). There are three steps to indicate the temperature;

1. Using a slide rule, first, indicate the relative humidity (using the upper scale).
2. Then we can obtain the dry bulb temperature using the lower scale of the slide rule.
3. This gives the wet bulb temperature using the interior scale of the slide rule.

What is the Difference Between Dewpoint and Wet Bulb Temperature?

The dewpoint temperature and wet bulb temperature are different from each other in several ways as indicated below. However, the key difference between dewpoint and wet bulb temperature is that the dewpoint temperature is the temperature at which the air becomes saturated from water vapor, but on the contrary, wet bulb temperature is the temperature which we measure from a moistened bulb. However, both these terms relate to the relative humidity in the air.

Summary – Dewpoint vs Wet Bulb Temperature

Dewpoint and wet bulb temperature are two forms of temperature that gives an idea about the humidity of the air. The difference between dewpoint and wet bulb temperature is that dewpoint temperature is the temperature to which we should cool the air to saturate the air with water vapor whereas wet bulb temperature is the temperature that we get from a moistened thermometer bulb that is exposed to air flow.

Reference:

1. Lallanilla, Marc. “What Is Dew Point?” LiveScience, Purch, 11 Feb. 2014. Available here  
2. “Dry Bulb, Wet Bulb and Dew Point Temperatures.” Densities of Solids. Available here  
3. “Wet-Bulb Temperature.” Wikipedia, Wikimedia Foundation, 28 May 2018. Available here  

Image Courtesy:

1.’Dewpoint-RH’By Easchiff – Own work, (CC BY-SA 4.0) via Commons Wikimedia 
2.’Slide rule scales back’ (Public Domain) via Commons Wikimedia