Atmospheric thermodynamics: A fixed volume of air at constant barometric pressure can be brought to its dew point by which action?

Introduction / Context:
Dew point is the temperature at which air becomes saturated with water vapor for a given pressure and moisture content. In hydrology and meteorology, understanding dew point helps in estimating condensation, fog formation, and precipitation potential.

Given Data / Assumptions:

• Barometric (ambient) pressure is constant.
• The air parcel volume is considered, with fixed water vapor mixing ratio before cooling begins.
• No external moisture is added or removed during the temperature change.

Concept / Approach:
For a given pressure and actual water vapor content, saturation vapor pressure depends on temperature. Dew point is reached when the actual vapor pressure equals the saturation vapor pressure at the cooled temperature. Therefore, at constant pressure and humidity ratio, the only way to reach saturation is to lower temperature until saturation occurs.

Step-by-Step Solution:
At constant pressure, saturation vapor pressure e_s(T) decreases as temperature decreases.The actual vapor pressure e_a is fixed by the moisture present.Cooling the air until e_s(T_dew) = e_a yields the dew point temperature T_dew.Therefore, the correct action to achieve dew point is decreasing temperature.

Verification / Alternative check:
Psychrometric charts show that, for a fixed humidity ratio and pressure, horizontal movement to the left (cooling) reaches the saturation curve; this intersection temperature is the dew point.

Why Other Options Are Wrong:
Increasing temperature increases e_s(T), moving the state away from saturation.Options “neither” or “both” contradict the thermodynamic definition at constant pressure and constant moisture content.

Common Pitfalls:
Confusing relative humidity changes with absolute humidity; mixing constant-volume vs. constant-pressure processes; assuming addition of moisture—which is not permitted in this question.

Final Answer:
Decreasing the temperature.