The dewpoint is the temperature at which air reaches a state where it can hold no more water.

• A. Relative humidity
• B. Saturation pressure
• C. Temperature of vaporization
• D. The temperature at which air reaches a state where it can hold no more water

Answer: (D)

Dewpoint is the temperature to which air must be cooled at a given pressure for water vapor to become saturated and begin condensing. When air is cooled to its dewpoint, the relative humidity reaches 100 percent, meaning the air can hold no more water vapor at that pressure. This description matches the idea that condensation starts once saturation is reached. Relative humidity is a percentage of how full the air is with vapor, not a temperature. Saturation pressure is the vapor pressure at which the air would be saturated for the current temperature, not the temperature itself. Temperature of vaporization relates to the energy needed to turn liquid water into vapor, which doesn’t define dewpoint.