Which term refers to the percentage of water in the air relative to its capacity at the current temperature?

• A. Humidity
• B. Barometric pressure
• C. Relative humidity
• D. Wind

Answer: (C)

The concept being tested is how moisture in the air is measured as a percentage of what the air can hold at the current temperature. This precise measure is relative humidity. It expresses how close the air is to being saturated with water vapor at that temperature: the actual amount of water vapor present compared with the maximum amount the air can hold when it’s at that temperature, shown as a percentage. Temperature matters because warm air can hold more water vapor than cool air, so changing the temperature changes the capacity even if the actual amount of moisture stays the same. If the air reaches its maximum capacity, relative humidity is 100%, and any cooling tends to cause condensation (dew or fog). This is different from absolute humidity, which is about the actual mass of water vapor in a given amount of air, and from humidity in a casual sense, which isn’t tied to capacity. Barometric pressure and wind describe other atmospheric properties and don’t quantify moisture relative to capacity.