Final answer:
Increasing relative humidity in the air leads to a larger wet-bulb depression as it causes reduced evaporation rates; this is due to a lower water vapor pressure gradient between surfaces and the atmosphere.
Step-by-step explanation:
As the amount of moisture suspended in the air increases, the wet-bulb depression increases. This happens because increasing relative humidity leads to reduced evaporation rates due to a decreased difference in water vapor pressure between the leaf and the atmosphere. When relative humidity is high, the air is close to saturation and has a diminished capacity to accept more water vapor, thereby slowing down the evaporation process. This is reflected in a greater wet-bulb depression since the wet-bulb temperature, which takes into account evaporative cooling, will be closer to the dry-bulb temperature, indicative of high humidity levels. Thus, the wet-bulb depression, which is the difference between the dry-bulb and wet-bulb temperatures, becomes more pronounced.