Relative humidity (RH) measures the amount of water vapor in the air compared to the maximum amount the air can hold at its current temperature. This percentage indicates how close the air is to the point of saturation, where water condenses into liquid forms like dew or fog. RH is widely used in weather forecasting to predict precipitation. It is also a significant factor in human comfort, as high humidity inhibits the body’s ability to cool itself through sweat evaporation. Maintaining specific RH levels is important in many industries, from agriculture to manufacturing, to prevent damage to products.
Defining the Variables: Actual Vapor Pressure vs. Saturation
Relative humidity calculation requires two distinct measurements of water vapor, both expressed as pressure. The first is the Actual Vapor Pressure (AVP), which represents the pressure exerted by the water vapor molecules currently suspended in the air. This value quantifies the absolute amount of moisture present, regardless of the air temperature.
The second variable is the Saturation Vapor Pressure (SVP), which defines the maximum amount of water vapor the air can hold at a specific temperature before saturation. Warmer air can hold substantially more water vapor than colder air. This means that if the temperature increases, SVP rises rapidly, allowing the air to hold more moisture.
Conversely, when air cools, its SVP drops. This drop can cause the existing water vapor to exceed the maximum capacity, leading to condensation. Therefore, the current air temperature is a critical input for determining the maximum possible water content.
The Calculation: Ratio and Percentage
Relative humidity is calculated by forming a ratio of the two vapor pressure variables. The Actual Vapor Pressure (AVP) is divided by the Saturation Vapor Pressure (SVP) at the current temperature, and the result is multiplied by 100 to express the value as a percentage.
The fundamental formula is: RH = (Actual Vapor Pressure / Saturation Vapor Pressure) x 100%. This ratio indicates the air’s moisture status relative to its potential capacity. For example, 50% relative humidity means the air contains half the water vapor it could possibly hold before saturation.
When the AVP and the SVP are equal, the relative humidity reaches 100%. This indicates the air is completely saturated and at its dew point, meaning the slightest cooling or addition of moisture will cause condensation.
Practical Measurement Tools and Techniques
Determining the vapor pressure inputs for RH calculation is achieved through specialized instruments.
Psychrometer
A common method uses a psychrometer, which consists of a dry-bulb thermometer and a wet-bulb thermometer covered by a water-soaked wick. As water evaporates from the wick, it draws heat away, causing the wet-bulb temperature to drop below the dry-bulb temperature. The difference between the two readings, known as the wet-bulb depression, directly relates to the air’s moisture content. A larger difference indicates lower water vapor, as evaporation is occurring more rapidly. These readings are then used with psychrometric tables or formulas to calculate the actual vapor pressure and relative humidity.
Dew Point Measurement
Another approach uses the Dew Point temperature, which is the temperature air must be cooled to reach 100% relative humidity. Measuring the Dew Point effectively provides the Actual Vapor Pressure (AVP) value. Once the Dew Point and the current air temperature are known, the Saturation Vapor Pressure (SVP) for both temperatures can be calculated using empirical formulas.
Electronic Hygrometer
A third type of instrument, the electronic hygrometer, uses a capacitive or resistive sensor to measure relative humidity directly. A capacitive hygrometer contains a material whose electrical properties change in proportion to the amount of water vapor it absorbs. These changes are then converted into a direct percentage reading of the relative humidity.