How is relative humidity defined?

• A. The absolute mass of water in the air.
• B. The dew point temperature.
• C. The amount of moisture the air can hold at a given temperature.
• D. The percentage of moisture air contains relative to maximum at the same temperature.

Answer: (D)

Relative humidity is a measure of how close the air is to being saturated with water vapor. It compares the current amount of moisture in the air to the maximum amount the air can hold at the current temperature, expressed as a percentage. Temperature matters because warmer air can hold more moisture, so the same actual amount of water vapor can produce different relative humidity readings at different temperatures.

For example, if the air contains half of what it could hold at a given temperature, the relative humidity is about 50%. If you heat the air with the same moisture, the relative humidity drops, since the air’s capacity increases. If you add moisture until the air holds as much as possible at that temperature, the relative humidity reaches 100%.

This differs from absolute humidity, which is simply the actual mass of water vapor in the air, and from the dew point, which is the temperature at which air must be cooled to become saturated. Relative humidity is the percentage of the air’s moisture capacity currently used at the present temperature.