What is the percentage of water in the air relative to the maximum amount the air can hold at a given temperature?

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

Answer: (D)

The main idea here is relative humidity—the percentage of water vapor in the air relative to how much water vapor the air could hold at that temperature. The air can hold more water as it warms, so the capacity depends on temperature. Relative humidity is defined as the actual amount of water vapor divided by the maximum amount the air can hold at that temperature, multiplied by 100 to give a percent. In other words, it tells you how close the air is to saturation at that temperature.

That’s why the option describing this concept is correct: it directly expresses the current moisture content as a percent of the air’s moisture-holding capacity at the given temperature. Humidity is a broad term that can refer to different ways of describing moisture (not always this specific percent of capacity), temperature is just the heat level, and barometric pressure is the overall atmospheric pressure, which isn’t a measure of moisture content.