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  • Writer's pictureMitchell Fotheringham

Absolute vs. Relative Humidity – What’s the Difference?



Humidity is a measure of the amount of water vapour present in the air. It affects our daily lives in many ways, from how we feel indoors to how plants grow outdoors. When discussing humidity, two common terms that often come up are absolute humidity and relative humidity. While they may sound similar, they have distinct differences that are important to understand. In this blog post, we'll explore the differences between absolute humidity and relative humidity to help you better understand these concepts and how they impact our daily lives.


Absolute Humidity: Absolute humidity is a measure of the actual amount of water vapour present in the air, expressed in terms of the mass of water vapour per unit volume of air. It is usually measured in grams of water vapour per cubic meter of air (g/m³) or grains of water vapour per cubic foot of air (gr/ft³). Absolute humidity is a direct measurement of how much moisture the air contains, regardless of the air's temperature or pressure. It is an important parameter in various industrial and scientific applications, such as in meteorology, air conditioning, and manufacturing processes.


Relative Humidity: Relative humidity, on the other hand, is a measure of the amount of water vapour in the air compared to the maximum amount of water vapour the air could hold at a given temperature and pressure, expressed as a percentage. It is the ratio of the actual water vapour pressure in the air to the saturation water vapour pressure at the same temperature and pressure. In simpler terms, relative humidity tells us how much moisture the air is holding relative to its capacity to hold moisture at a particular temperature and pressure. For example, if the air is holding half of the moisture, it could hold at a particular temperature and pressure; the relative humidity would be 50%.


Differences: The key difference between absolute humidity and relative humidity is that absolute humidity is an actual measurement of the amount of water vapour in the air, while relative humidity is a comparison of the amount of water vapour in the air to its capacity to hold moisture at a particular temperature and pressure. Absolute humidity is expressed in units of mass per unit volume, whereas relative humidity is expressed as a percentage.


Another important difference is that absolute humidity remains constant as long as the amount of water vapour in the air remains unchanged, regardless of changes in temperature and pressure. On the other hand, relative humidity changes with temperature and pressure variations. When the temperature of air decreases, its capacity to hold moisture decreases, resulting in an increase in relative humidity even if the actual amount of water vapour in the air remains the same. Conversely, when the temperature of air increases, its capacity to hold moisture increases, resulting in a decrease in relative humidity even if the actual amount of water vapour in the air remains constant.


Importance: Both absolute humidity and relative humidity are important in various applications. Absolute humidity is often used in industrial processes to control the moisture content in materials, such as in food production, pharmaceutical manufacturing, and paper manufacturing. It is also used in meteorology to study weather patterns and predict precipitation. On the other hand, relative humidity is commonly used in indoor environments to assess comfort levels and to control moisture-related issues, such as mould growth, condensation, and damage to buildings and furniture. Relative humidity is also a critical factor in determining weather conditions, such as the formation of fog, dew, and clouds.


Conclusion: In conclusion, while absolute humidity and relative humidity may sound similar, they have distinct differences. Absolute humidity is a direct measurement of the amount of water vapour in the air, expressed in units of mass per unit volume, and remains constant regardless of changes in temperature and pressure. On the other hand, relative humidity is a comparison of the amount of water vapour in the air to its capacity to hold moisture at a particular temperature and pressure, expressed as a percentage. It changes with temperature and pressure variations. Both absolute humidity and relative humidity are important in various applications, from industrial processes to weather prediction and indoor comfort control. Understanding the differences between absolute humidity and relative humidity can help us better comprehend how moisture behaves in the air and its impact on our environment.


In conclusion, while absolute humidity and relative humidity are related concepts, they have distinct differences in how they are measured and how they change with varying conditions. Absolute humidity is a direct measure of the actual amount of water vapour in the air, while relative humidity is a comparison of the amount of water vapour in the air to its capacity to hold moisture at a given temperature and pressure. Both absolute humidity and relative humidity are crucial in various fields, from meteorology to indoor comfort control, and understanding their differences can enhance our understanding of humidity and its effects on our daily lives.

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