Dew-point temperature is defined as the temperature at which first liquid droplets form when a vapor is cooled at which condition?

• A. The temperature at which the first liquid droplets form when vapor is cooled at constant pressure
• B. The temperature at which humidity is 100%
• C. The boiling point of water
• D. The freezing point of water

Answer: (A)

Dew point is the temperature at which air becomes saturated with water vapor and condensation begins, for a given pressure. As you cool a parcel of vapor while keeping the pressure fixed (usually at atmospheric pressure), the vapor can no longer hold as much moisture, so the vapor pressure equals the saturation vapor pressure at that temperature and liquid droplets start to form. That temperature is the dew point.

The key idea is that condensation happens when the actual vapor pressure reaches the saturation vapor pressure at that temperature, which occurs during cooling at constant pressure. Humidity reaching 100% is a sign of saturation, but the dew point specifies the temperature at which that saturation occurs under a fixed pressure. The boiling point or freezing point are not involved in defining dew point; they are distinct phase-change temperatures.