Adiabatic humidification of unsaturated air at constant pressure: which property remains essentially constant while water evaporates into the air?

Introduction / Context:
Adiabatic humidification and cooling are central to air–water psychrometrics, evaporative coolers, and packed-tower humidifiers. When unsaturated air contacts liquid water in an adiabatic device at essentially constant pressure, the air warms or cools and its humidity changes in a coupled way along lines closely tied to the wet-bulb temperature.

Given Data / Assumptions:

• Unsaturated air enters; water evaporates into the air.
• Process is adiabatic (no heat exchange with surroundings) and at nearly constant pressure.
• Negligible heat capacity of liquid compared with gas stream, or well-mixed water at constant temperature representative of the wet-bulb state.

Concept / Approach:
The wet-bulb temperature (T_wb) is the temperature a water-wetted thermometer reaches due to evaporative cooling under adiabatic conditions. In adiabatic saturation of air with water at constant pressure, the air state moves along a path of nearly constant T_wb: the reduction in sensible heat balances the gain in latent heat. The dry-bulb temperature (T_db) decreases as evaporation proceeds, while absolute humidity increases until an approach to the adiabatic saturation state is reached.

Step-by-Step Solution:

Verification / Alternative check:
On a standard psychrometric chart, adiabatic humidification trajectories are nearly lines of constant wet-bulb temperature, confirming the invariance of T_wb, while T_db shifts downward and humidity ratio increases toward saturation at the adiabatic saturation temperature.

Why Other Options Are Wrong:

• Dry-bulb temperature: changes markedly (cools) during evaporation.
• Humidity and relative saturation: both increase, not constant.
• Dew-point temperature: increases as moisture is added.

Common Pitfalls:
Confusing isothermal humidification (contact with a very large water reservoir at fixed temperature) with adiabatic humidification; in the latter, the key invariant is the wet-bulb temperature, not dry-bulb or dew point.

Final Answer:
wet-bulb temperature