Difference Between Absolute and Relative Humidity

In the field of physics, humidity usually represents the quantity of water moisture present in the atmosphere. The humidity can be seen in naked eyes but can be estimated by using certain methods. Humidity indicated the weather forecast efficiently. Furthermore, it is classified into two kinds, namely, Absolute and Relative humidity. 

Absolute vs Relative Humidity 

The main difference between Absolute and Relative Humidity is that the absolute humidity of an atmosphere is the increase or decrease in water vapor by proportion to the amount/mass of dry air. However, relative humidity determines the proportion of water vapor in dry air based on how much water vapor is present.

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When manifesting the absolute humidity of the atmosphere, it ignores the temperature factor. In simple words, Absolute humidity is a criterion of how considerably the rate of water vapor is subdued in a given amount of air during that time. The volume of air and water vapor are divided to obtain absolute humidity

On the contrary, Relative humidity, however, is defined as the ratio between the vapor pressure equilibrium for water in a given solvent and the partial fraction of water vapor in the solvent. Here, the temperature condition is taken into account while calculating the relative humidity of the atmosphere. The relative humidity is always in percentage. 

Comparison Table Between Absolute and Relative Humidity 

Parameters of Comparison Absolute Humidity Relative Humidity 
Definition Water vapor content is used as a measurement of absolute humidity in a given volume of air.It is the partial percentage of the stable vapor pressure of water by the level of water vapor in that given mixture.
SI unitThe absolute humidity is expressed in the form of moisture in grams or kilograms per cubic meter of the given air. It results in a percentage value. 
Temperature role The absolute humidity is irrelevant to the temperature available in the atmosphere at that point. Relative humidity relies on the temperature of the atmosphere, such that relative humidity is inversely proportional to the temperature. 
Affected by area The circumstances of the planet affect absolute humidity, such as seasons, land, and water.  In coastal areas, relative humidity is more, while low in desert areas. 
Mathematical expression Absolute humidity can be expressed as the ratio of air volume to water vapor volume. The relative humidity is calculated by dividing the volume of water vapor and the volume of the dry air

What is Absolute?

The absolute humidity of the quantity of air in the atmosphere is defined as the quality of water vapor present in the wet air. As a result of the presence of water vapor in humid air, the volume of water vapor is the same as wet air. Consequently, absolute humidity is often described by its ability to measure the density of water vapor in a moist atmosphere. Meanwhile, the temperature does not affect absolute humidity. 

Furthermore, the symbol of absolute humidity is denoted as ρv. Besides, the value of absolute humidity is not constant, it changes according to the moisture content of the air at various places. As a unit of measurement, absolute humidity is defined as the amount of moisture per cubic meter of air (g/m3). Over and above, due to the precise humidity measurement, absolute humidity is often considered the most accurate scientific method. It can be calculated by dividing the mass of the water vapor capacity in the air by the volume occupied by the air. 

For instance, if 6 grams of water vapor exists in 2 cubic meters of air, then the absolute humidity is equal to 3 grams/m3. Due to its precise calculation of how much water is in the air, absolute humidity helps in defining the humidity in the summer, winter, and various seasons. 

What is Relative Humidity?

When we speak of relative humidity, we are referring to the ratio of water vapor content in the air compared to how much is needed in order to completely saturate the air. Moreover, the role of temperature is very important in relative humidity, since relative humidity is inversely proportional to the temperature, so, when the temperature is increased, the relative humidity of the atmosphere decreases along with it. 

Subsequently, the relative humidity remains constant regardless of the amount of air in a space. And it can be calculated by using the formula; partial percentage ratio between the vapor pressure of pure water over a leveled surface, and the water vapor in the mixture. However, on the other hand, an instrument named a Hygrometer, invented by Swiss physicist and Geologist Horace Bénédict de Saussure in 1783, to calculate relative humidity. The relative humidity is usually expressed in percent and denoted by the symbol φ or RH in physics.

The percentage expressed humidity is used as a significant metric while indicating the weather forecasts; in such a way, that higher percentage means the air is humid. The air conditioning and steam turbines are common examples of relative humidity.

Main Differences Between Absolute and Relative Humidity 

  1. Water vapor content is determined by absolute humidity, whereas relative humidity determines the ratio of water vapor to dry air. 
  2. Absolute doesn’t depend on the temperature, that is, the temperature is neglected while calculating the absolute humidity, whereas, the temperature is taken into consideration. 
  3. It is measured in grams or kg/m3 (grams/kilos per cubic meter). And SI unit of Relative Humidity is expressed in percentage. 
  4. The value of absolute humidity is not constant, as it changes along with changes in the amount of water vapor in the atmosphere, while the value of relative humidity is constant, as it contains absolute moisture of the atmosphere. 
  5. Absolute humidity is used to calculate the ratio of the mass of vapor to mass present in the carrier gas stream, while relative humidity is used in weather forecasts, to indicate the presence of fogs and mists. 

Conclusion

Humidity is an important aspect, as it is what tells the level of moisture in the air, eventually, helping in maintaining the body temperature with respect to the atmosphere. On the whole, humidity is a concept in science, which is classified into two classes, namely, Absolute humidity and Relative humidity. 

Both are used to calculate the amount of water vapor in the air, however, one precisely tells the amount without the interference of temperature, while the other calculates the amount of water vapor relative to temperature in percentage. Consequently, their major differences include temperature factors, formulas, and units to express them. 

References

  1. https://europepmc.org/article/med/1572193
  2. https://www.tandfonline.com/doi/abs/10.1179/sic.1994.39.Supplement-2.32

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