A humidity unit is a unit used to measure humidity, or the amount of water vapor in the air. Understanding the units of moisture is especially important in industry, because moisture is one of the factors that can damage products and manufacturing processes if a factory cannot keep it under proper control.
In this article, we’ll introduce the different humidity units and how each one works, what units humidity can be expressed in, as well as the calculation formulas and the contexts in which each unit is best applied.
Table of Contents
Humidity Units You Should Know
There are many humidity units to choose from, depending on the application and the results you need to analyze. Here are the key ones to know:
Relative Humidity
Relative humidity is the most basic and widely used unit for measuring humidity in the air. It expresses the amount of water vapor in the air as a percentage of the maximum amount of water vapor the air can hold before it becomes saturated at that temperature.
RH, or %RH, is the unit of relative humidity and is calculated with the following formula:
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RH (%) = (Vapor pressure of water in the air / Saturation vapor pressure) x 100% |
Or, using symbols:
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RH (%) = (e / es) x 100% |
In other words, a relative humidity of 50% means that, at the current temperature, the air contains half of the total water vapor it can hold without becoming saturated.
Specific Humidity
Specific humidity is a humidity unit that measures the ratio of the mass of water vapor to the mass of moist air (dry air mass + water vapor mass). It is usually expressed as grams of water vapor per kilogram of air (g/kg). Unlike relative humidity, this unit does not take temperature into account.
The formula for specific humidity is:
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Specific Humidity (SH) = Mass of water vapor / Mass of moist air |
Specific humidity is commonly used in meteorology, as it makes it easy to analyze and compare moisture levels at different temperatures.
Absolute Humidity
Absolute humidity is a humidity unit that measures the total mass of water vapor contained in a given volume of air. It is usually expressed in grams per cubic meter (g/m³) and is used to analyze the density of water vapor per unit volume of air at a given temperature and air pressure.
The formula for absolute humidity is:
|
Absolute Humidity (AH) = Mass of water vapor / Volume of air |
However, absolute humidity is not well suited to meteorology. Air masses constantly contract and expand depending on external factors while the mass of water vapor stays the same, which makes it difficult to analyze atmospheric moisture from absolute humidity alone.
Vapor Pressure
Vapor pressure is the portion of air pressure exerted by water vapor. It is measured in units of pressure, namely pascals (Pa).
Dew Point
The dew point is a type of humidity measure that indicates the temperature at which water vapor becomes saturated. If the temperature drops below the dew point, water vapor condenses into dew (condensation is the change of a substance from a gas to a liquid). In other words, the dew point is the temperature at which relative humidity reaches 100%.
Moisture Content
Moisture content is a fairly broad term for a humidity measure that can be applied in many contexts depending on the industry. It generally refers to the percentage by weight of water or other liquid contained in an item or material.
The formula for moisture content is:
| Moisture Content (%) = (Mass of water / Total mass of material) x 100 |
Comparison of Humidity Units
Comparison Table of Humidity Units
|
Humidity Measure |
Unit |
| Relative Humidity | % |
| Specific Humidity | g/kg |
| Absolute Humidity | g/m³ |
| Vapor Pressure | Pa |
| Dew Point | °C or °F |
| Moisture Content | % |
Summary: Why Do Humidity Units Matter?
Moisture is a condition that can cause damage across many industries. Excessive moisture can degrade the quality of materials, equipment, and many types of products. A solid understanding of the different humidity units is therefore essential for properly studying, analyzing, and controlling moisture levels.
If you’re looking for a high-quality, safe moisture meter that measures humidity accurately, contact PICO for more information and to explore our products for industrial plants:
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