# Does Atmospheric Pressure Affect Humidity?

## Relationship Between Pressure and Relative Humidity

Does atmospheric pressure affect relative humidity? The question is important to archivists who preserve paintings and books, as water vapor can damage priceless works. Many scientists say there is a relationship between atmospheric pressure and humidity, but describing the nature of the effect is not so simple. Other experts believe pressure and humidity are unrelated.

In a nutshell, pressure likely does affect relative humidity. However, the difference between atmospheric pressure at different locales likely doesn't impact humidity to a significant degree. Temperature is the primary factor affecting humidity.

## The Case for Pressure Affecting Humidity

1. Relative humidity (RH) is defined as a ratio of mole fraction of actual water vapor, to a mole fraction of water vapor that can be saturated in dry air, where the two values are obtained at the same temperature and pressure.
2. Mole fraction values are obtained from water density values.
3. Water density values vary with atmospheric pressure.
4. Atmospheric pressure varies with altitude.
5. The temperature boiling point of water varies with atmospheric pressure (or altitude).
6. Saturated Water Vapor pressure value is dependent on the boiling point of water (such that the values of the boiling point of water is lower at higher altitudes).
7. Humidity in any form is the relationship between the saturated water vapor pressure, and the sample-air's partial water vapor pressure. Partial water vapor pressure values are dependent on pressure and temperature.
8. Since both saturated water vapor property values and partial water pressure values are observed to non-linearly change with atmospheric pressure and temperature, then the absolute value of atmospheric pressure is required to accurately calculate the water vapor relationship as it applies to the perfect ideal gas law (PV = nRT).
9. To accurately measure humidity and use the principles of the perfect gas law, one must obtain the absolute atmospheric pressure value as a fundamental requirement for calculating relative humidity values at higher altitudes.
10. Since the majority of the RH sensors do not have built-in pressure sensor, they are inaccurate above sea level, unless a conversion equation is used with a local atmospheric pressure instrument.

## The Argument Against a Relationship Between Pressure and Humidity

1. Nearly all humidity related processes are independent of total air pressure, because water vapor in air does not interact with oxygen and nitrogen in any way, as first demonstrated by John Dalton early in the nineteenth century.
2. The only RH sensor type that is sensitive to air pressure is the psychrometer, because air is the carrier of heat to the wet sensor and the remover of evaporated water vapor from it. The psychrometric constant is quoted in tables of physical constants as a function of total air pressure. All other RH sensors should not need adjustment for altitude. However, the psychrometer is often used as a convenient calibration device for HVAC installations, so if it is used with the constant for the wrong pressure to check a sensor that is in fact correct, it will indicate a sensor error.
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