What is defined as the amount of moisture in the air compared to how much it can hold?

The term that accurately describes the amount of moisture in the air compared to how much moisture that air can actually hold is relative humidity. This measurement is expressed as a percentage and indicates how close the air is to being fully saturated with moisture. For example, if the relative humidity is 50%, it means the air is holding half of the moisture it is capable of holding at a certain temperature.

Relative humidity is a critical concept in HVAC because it affects both thermal comfort and the efficiency of air conditioning systems. When relative humidity is too high or too low, it can lead to discomfort, problems with condensation, and potential damage to building materials.

This term is distinct from absolute humidity, which measures the total amount of water vapor in the air regardless of temperature, specific humidity, which is the mass of water vapor per unit mass of air (including the water vapor), and vapor pressure, which refers to the pressure exerted by water vapor in the air. Each of these measures describes different aspects of moisture in the air but does not capture the concept of moisture relative to the maximum amount the air can hold like relative humidity does.