Relative humidity limit on evaporation

Definition

The relative humidity of the surrounding air caps how much water can evaporate, and therefore how much heat an evaporative system can remove.

When air is already near saturation, the driving force for evaporation weakens, so evaporative coolers and cooling towers reject less heat in humid climates. This is why swamp coolers work well in the arid Southwest and poorly in humid regions, and why a tower cannot cool water below the air’s wet-bulb temperature.

Related concepts

Approach (cooling tower) · Evaporative Cooling · Wet-bulb temperature

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Part of The Blueprint of Water · 🔥 thermodynamics