Difference Between Humidity and Relative Humidity

The key difference between humidity and relative humidity is that humidity is the amount of water vapor present in the air at a given moment whereas relative humidity is the ratio between the amount of water vapor present in the air and the amount of water vapor required to saturate the air.

Humidity represents the amount of water vapor in the unit gram per cubic meter (g/m³). However, we express the relative humidity as a percentage value. It is a subcategory of humidity which includes absolute humidity, relative humidity, and specific humidity.

1. Overview and Key Difference
2. What is Humidity
3. What is Relative Humidity
4. Side by Side Comparison – Humidity vs Relative Humidity in Tabular Form
5. Summary

What is Humidity?

Humidity is the amount of water vapor present in the air at a given moment. Water vapor is the vapor phase of water, which is invisible to us because it is totally transparent. The humidity is responsible for situations such as precipitation, dew, and fog. In addition, when the humidity is high, it reduces the sweating from our skin. That is because a high humidity means the warmth in the surrounding is low. However, at some point, the atmosphere can get saturated from water vapor. This means a saturated atmosphere has the maximum amount of water vapor that is can hold. Moreover, the amount of water vapor required for the saturation increases with the increase of temperature.

Difference Between Humidity and Relative Humidity

Figure 01: Fog occurs due to High Humidity

There are three types of humidity;

Relative humidity
Absolute humidity
Specific humidity

Relative humidity is a comparable measurement. It is a ratio which we usually express as a percentage value. More details are below under the subtopic “relative humidity”.

Absolute humidity is the total water vapor content in the atmosphere. It gives the total mass of water vapor in a given volume of air (sometimes we consider the mass of air in place of volume). The unit for the measurement is gram per cubic meter (g/m3). The equation for this relationship is as follows.

AH = m_water / V_net

Specific humidity, on the other hand, is the ratio between the mass of water vapor present in a given mass of air. it is approximately equal to the “mixing ratio” (the ratio between the mass of water vapor in a given volume of air and the mass of water vapor in the same volume of air when it has dry air).

What is Relative Humidity?

Relative humidity is the percentage ratio between the amount of water vapor present in the air and the amount of water vapor required to saturate the air. Moreover, it represents the ratio between the partial pressure of water vapor and the equilibrium vapor pressure of water vapor (at a specific temperature). It is denoted by RH. The relationship between water vapor content and temperature is as follows.

At low temperature, air requires a low water vapor content to obtain a high RH
At high temperature, air requires a high water content to obtain a high RH

The mathematical expression for RH is as follows;

RH or φ = P_H2O/ P*_H2O

What is the Difference Between Humidity and Relative Humidity?

Humidity is the amount of water vapor present in the air at a given moment. Relative humidity is the percentage ratio between the amount of water vapor present in the air and the amount of water vapor required to saturate the air. Humidity is measured by gram per cubic meter (g/m³) while relative humidity is measured as a ratio and it is presented as a percentage.

The mathematical expression to obtain the value of humidity is by dividing the mass of water vapor in air by the volume of air considered at a given temperature. Similarly, to obtain the value of relative humidity we have to divide the partial pressure of water vapor and the equilibrium vapor pressure of water vapor at a given temperature.

Summary – Humidity vs Relative Humidity

Humidity is the total amount of water vapor in air at a given temperature. It represents the likelihood of precipitation, dew, and fog. Relative humidity is one of three forms of humidity. The difference between humidity and relative humidity is that humidity is the amount of water vapor present in the air at a given moment whereas relative humidity is the ratio between the amount of water vapor present in the air and the amount of water vapor required to saturate the air.

Reference:

1. “Humidity.” Wikipedia, Wikimedia Foundation, 12 May 2018. Available here
2. “Relative Humidity.” Wikipedia, Wikimedia Foundation, 16 May 2018. Available here
3. Contributors, HowStuffWorks.com. “What Is Relative Humidity and How Does It Affect How I Feel Outside?” HowStuffWorks Science, HowStuffWorks, 8 Mar. 2018. Available here