According to ASHRAE 55, humidity can be expressed in terms of which of the following?

• A. Vapor pressure, dew point temperature, and humidity ratio
• B. Relative humidity only
• C. Vapor pressure and relative humidity
• D. Humidity ratio only

Answer: (A)

Humidity can be described in several equivalent ways, using different quantities that represent moisture in the air. In ASHRAE 55, you can express moisture content as vapor pressure (the partial pressure of water vapor), dew point temperature (the temperature at which air becomes saturated and condensation begins), or humidity ratio (the mass of water per unit mass of dry air). These forms are all valid, independent representations of humidity that designers and researchers commonly use for comfort calculations and analyses.

The best choice is the one that lists three distinct, widely used expressions—vapor pressure, dew point temperature, and humidity ratio. This reflects the fact that humidity can be quantified in multiple ways beyond relative humidity, giving a more complete set of options for analysis. Relative humidity is indeed a common measure, but it’s not the set described by that particular option.