Difference Between Absolute and Relative Humidity

Before we jump into the difference between absolute and relative humidity, first we must understand the concept behind humidity and how it can be measured. We know that humidity greatly affects living organisms, including us humans as well as materials. But what exactly is humidity? Water exists on earth in three forms – solid (ice), liquid (water), and gas (water vapor). On average, water vapor makes about 4% of the molecules in the air. The amount of water vapor in the atmosphere or in a gas is commonly referred to as the humidity of air. The determination of humidity is of great importance. Thus, humidity control becomes imperative in all fields. But how exactly humidity is measured?

That being said, humidity can be measured in two main ways: Absolute and Relative. The absolute humidity is the actual amount of water vapor per unit volume of gas at given temperature and pressure, and is expressed in g/m³. The relative humidity, on the other hand, is the ratio of the amount of water vapor present in the air to the amount present at a given temperature, and is expressed as a percentage. Well, there are various ways the humidity can be expressed depending on the particular use. We explain the two ways of measuring humidity in detail by providing a detailed comparison between the two.

What is Absolute Humidity?

Absolute humidity is the actual amount of water vapor present in the air, regardless of the temperature. More precisely, the absolute humidity is the ratio of mass of water vapor to the mass of dry air. The absolute humidity is also referred to as the humidity ratio, and is expressed in grams of moisture per cubic meter of air (g/m³). It is derived from the specific humidity and the density of the air. The absolute humidity is independent of the temperature. It is largely determined by the nature of the surface under the air mass. For example, if the water vapor in 1 cubic meter of air weighs 35 grams, the absolute humidity of the air is 35 g/m³.

Absolute humidity = Mass of water vapor/Volume of air

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What is Relative Humidity?

Relative humidity is the ratio, expressed as a percentage, of the amount of water vapor present in the air to the amount present at a given temperature. It is the ratio of vapor pressure to saturated vapor pressure and is expressed in percentage. In simple terms, relative humidity is the amount of water vapor that is present in the air, relative to the amount it could hold at that given temperature. It is a percentage of the amount of moisture the air could possibly hold. The value of relative humidity does not specify the moisture content of the air unless accompanied by the temperature of the air. If the moisture content in the air remains constant, the relative humidity decreases as the temperature increases and vice-versa.

Difference between Absolute and Relative Humidity

Definition

– The actual amount of water vapor (moisture) present in the air is commonly referred to as absolute humidity. In simple terms, the absolute humidity is the ratio of mass of water vapor to the mass of dry air. This is also referred to as the humidity ratio. The relative humidity, on the contrary, is the amount of water vapor that is present in the air, relative to the amount it could hold at that given temperature. Simply put, the relative humidity is a percentage of the amount of moisture the air could possibly hold.

Measurement

– The absolute humidity is the actual amount of water vapor per unit volume of gas at given temperature and pressure, and is expressed in grams of moisture per cubic meter of air (g/m³). It is derived from the specific humidity and the density of the air. The relative humidity, on the other hand, is expressed in percentage, as the amount of water vapor relative to the temperature of the air. Unlike absolute humidity which is independent of the temperature, relative humidity depends on the current temperature of the air.

Temperature Impact

– Absolute humidity is totally independent of the temperature. It is purely about how much water is present in the air, regardless of the temperature. Relative humidity is totally dependent on the temperature meaning the value of relative humidity only specifies the moisture content of the air when accompanied by the temperature of the air. So, if the moisture content in the air remains constant, the relative humidity decreases as the temperature increases or relative humidity increases as temperature decreases. In any given day, if the absolute humidity remains unchanged but the temperature goes up, the relative humidity goes down eventually.

Absolute vs. Relative Humidity: Comparison Chart

Summary of Absolute vs. Relative Humidity

In a nutshell, relative humidity is the ratio of moisture content in the air relative to the highest amount of moisture at a given temperature, and is expressed in percentage. This measurement is required in a wide range of applications including air-conditioning (HVAC) applications and drying processes. Absolute humidity, on the contrary, is the amount of moisture present in the air, and is expressed in g/m³. It is derived from the specific humidity and the density of the air. Unlike relative humidity which is dependent on the temperature of the air, absolute humidity is totally independent of the temperature. So, if the temperature warms, relative humidity goes down and vice-versa.

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