Dew-point temperature is defined as?

• A. Temperature of moist air saturated at pressure p with the same humidity ratio W as the sample.
• B. The temperature of dry air in the room.
• C. The temperature at which air becomes fully humid.
• D. The temperature read by a wet-bulb thermometer.

Answer: (A)

Dew-point temperature is the temperature to which moist air must be cooled (at the same pressure) to reach saturation for the existing amount of water vapor in the air, i.e., the humidity ratio remains the same and condensation would begin. In other words, it’s the temperature at which water vapor would start to condense if the air were cooled without changing its moisture content. This is not the room air temperature, nor the temperature at which air becomes “fully humid” in a general sense, and it is not the reading from a wet-bulb thermometer. Knowing the dew point helps assess condensation risk on surfaces and is a key parameter in psychrometric analysis for HVAC design.