Percentage humidity, specifically relative humidity (RH), indicates the amount of moisture present in the air compared to the maximum moisture it can hold at a given temperature. The following formula shows how to calculate it:
Understanding Relative Humidity Calculation
The key formula to calculate relative humidity is:
R.H. = (vapour pressure / saturation vapour pressure) × 100
Let's break down each component:
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Vapour Pressure: This refers to the current amount of water vapor present in the air, measured in units of pressure, like Pascals (Pa) or millibars (mb).
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Saturation Vapour Pressure: This represents the maximum amount of water vapor the air can hold at a specific temperature. It is also measured in pressure units. The saturation vapor pressure increases with the temperature because warmer air can hold more moisture.
Practical Steps to Calculate Relative Humidity
Calculating percentage humidity isn't always straightforward, as you often need specific instruments to measure vapor pressure directly. However, you can understand the process:
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Measure Vapor Pressure: Use a hygrometer or a psychrometer to determine the actual amount of moisture in the air. These devices usually give a reading that can be converted to vapor pressure.
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Determine Saturation Vapor Pressure: This requires knowledge of the air temperature. You can either:
- Use a standard psychrometric chart or table for a given temperature that lists saturation vapor pressure values, or
- Use an empirical formula, such as the Arden Buck equation (which requires the temperature) to estimate the saturation vapor pressure.
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Apply the Formula: Divide the actual vapor pressure by the saturation vapor pressure, and then multiply the result by 100 to express it as a percentage.
Example
Let's say:
- The vapor pressure (measured by a hygrometer) is 1500 Pa.
- The saturation vapor pressure at the current temperature (from a chart) is 3000 Pa.
Then:
R.H. = (1500 Pa / 3000 Pa) × 100 = 50%
This means the relative humidity is 50%, or the air is holding half of the maximum water vapor it can at that specific temperature.
Key Takeaways
- Relative humidity is not a measure of the absolute amount of water in the air but rather how close the air is to being saturated at its current temperature.
- Warm air can hold more moisture than cold air, so the same amount of water vapor will result in different RH values as temperature changes.
