Relative humidity is defined as the ratio of the amount of water vapor in the air to the maximum amount that could exist at the current temperature. What is this maximum value most commonly expressed as?

• A. The maximum amount that could exist at that temperature
• B. The current wind direction
• C. The air's temperature only
• D. The pressure at sea level

Answer: (A)

Relative humidity compares how much water vapor is actually in the air to how much could be there if the air were saturated at that temperature. The maximum amount air can hold at that temperature is the saturation value, often referred to in practice as the saturation vapor pressure. Relative humidity is this actual vapor content divided by that saturation amount, times 100 percent. The choice describing the maximum amount that could exist at that temperature best matches this concept of saturation capacity. The other options—wind direction, the air’s temperature itself, and sea-level pressure—do not describe the limit of water vapor the air can hold at the current temperature.