The temperature to which humid air must be cooled at constant pressure to become saturated is called

• A. Relative humidity
• B. Saturation temperature
• C. Dew point
• D. Dew point temperature

Answer: (C)

The dew point is the temperature to which humid air must be cooled at constant pressure to become saturated. As the air cools, its capacity to hold water vapor decreases, while the actual moisture content stays the same. When the temperature reaches the dew point, the actual vapor pressure equals the saturation vapor pressure, and condensation begins, indicating saturation. Relative humidity is a ratio, not a temperature, so it isn’t the correct term. The common term is dew point (also called dew point temperature in some contexts).