Heat guide
Evaporative Cooling and Wet-Bulb Temperature
Direct evaporative cooling lowers air dry-bulb temperature by using sensible heat from the air to evaporate water, increasing humidity in the process.
The wet-bulb limit
For an ideal direct evaporative process, the outlet dry-bulb temperature approaches the entering-air wet-bulb temperature. Real equipment stops above that limit.
Effectiveness describes approach to the limit
A common simplified definition is ε = (Tdb,in - Tdb,out)/(Tdb,in - Twb,in). Higher effectiveness means the outlet comes closer to the entering wet-bulb temperature.
Dry air offers more cooling potential
Low relative humidity generally produces a larger difference between dry-bulb and wet-bulb temperatures, which creates more room for evaporative cooling.
Humidity rises
Direct evaporation adds water vapour to the airstream. This can be desirable in dry climates but problematic in already humid spaces.
Water and hygiene matter in real systems
Practical evaporative coolers require water management, maintenance and hygiene controls. A psychrometric estimate does not address those operational requirements.
Direct evaporative cooling raises moisture content
When water evaporates directly into an air stream, the air gains water vapour while its dry-bulb temperature can fall. This tradeoff can be beneficial in dry climates but may be undesirable where indoor humidity is already high.
Indirect evaporative systems separate the product air from the wetted stream so they can reduce sensible temperature without adding the same amount of moisture directly to the supplied air.
Water quality and maintenance affect real systems
Evaporating water leaves dissolved minerals behind, so scale can accumulate on wetted media and equipment. Biological growth and drift control also require appropriate maintenance in many installations.
A thermodynamic estimate of cooling potential therefore describes only part of system performance. Water use, hygiene, maintenance and fan energy are practical design considerations.
Wet-bulb temperature sets an important practical limit
For direct evaporative cooling, the entering-air wet-bulb temperature provides a useful lower bound that an ideal adiabatic process approaches but does not cross. Real equipment normally leaves the air warmer than that limit because of finite contact, bypass, fan heat and other inefficiencies. The smaller the dry-bulb to wet-bulb difference, the less sensible cooling potential is available.
This is why the same evaporative cooler can perform strongly on a hot dry afternoon and poorly in warm humid weather even when the dry-bulb temperature looks similar.
Evaporation rate must be supported by heat and mass transfer
Water does not evaporate at a fixed rate simply because liquid is present. The rate depends on surface area, air movement, vapour-pressure difference and the energy available to supply latent heat. As the surface cools, that energy balance can reduce the evaporation rate unless heat arrives from the air or another source.
Practical systems therefore combine airflow, wetted-media design and water distribution to sustain useful transfer instead of relying on an exposed reservoir alone.
Where the cooling energy comes from
Evaporation requires latent energy. In direct evaporative cooling, that energy comes largely from sensible energy in the air-water system, lowering the air dry-bulb temperature while increasing its humidity ratio.
The process does not create cold energy. It redistributes energy while converting liquid water to vapour. The entering wet-bulb temperature provides a useful lower-bound reference for an ideal direct process.
Why humid weather limits the effect
When air already contains substantial water vapour, its capacity to accept additional vapour is reduced. The dry-bulb and wet-bulb temperatures move closer together, leaving less potential for direct evaporative cooling.
This is why an evaporative cooler can be effective in a hot dry climate but provide little temperature reduction in a hot humid climate, even if the dry-bulb temperatures are similar.
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Sources and further reading
Use the linked primary or authoritative resources for additional detail, standards and source-specific conditions.