Heat guide
Humidity, Evaporation and Cooling
Evaporation removes heat because liquid water requires substantial latent energy to become vapour. The surrounding air’s humidity affects how readily that vapour can be accepted.
Evaporation requires latent energy
When liquid water evaporates, energy is required to overcome intermolecular attractions. That energy can come from the liquid, a surface or surrounding air, producing cooling.
Humidity reduces drying potential
When air already contains a large fraction of the water vapour it can hold at that temperature, the vapour-pressure difference driving evaporation becomes smaller.
Airflow can increase evaporation
Moving air removes humid boundary-layer air from a wet surface and replaces it with drier bulk air. This can raise evaporation rate when other conditions remain suitable.
Evaporative coolers have a physical limit
Direct evaporative cooling cannot ideally cool air below its entering wet-bulb temperature. Real equipment approaches that limit only according to its effectiveness.
Human cooling is more complex
Sweating cools only when sweat evaporates. Dripping sweat that leaves the skin without evaporating provides much less cooling. Clothing, radiant heat, air speed and metabolic heat also affect human heat balance.
Why airflow changes evaporation
Evaporation adds water vapour to the air immediately above a wet surface. If that moist boundary layer remains in place, the vapour-pressure difference driving further mass transfer falls. Air movement replaces some of that local moist air with surrounding air and can increase evaporation when other conditions allow it.
This is why relative humidity alone cannot predict a drying or cooling rate. Air velocity, exposed area, clothing or surface resistance, liquid availability and radiant and air temperatures can all alter the outcome.
Wet-bulb temperature expresses the evaporative cooling limit of air
If unsaturated air passes over a wetted surface, evaporation can lower the surface and nearby air temperature while raising moisture content. In an idealised adiabatic process, the achievable temperature approaches the wet-bulb temperature rather than falling without limit. Drier air generally offers more evaporative cooling potential than humid air.
This relationship is useful in cooling towers, evaporative coolers and human heat-stress analysis. Actual performance also depends on contact area, airflow, water temperature, equipment effectiveness and heat exchange with the surroundings.
Evaporation depends on a vapour-pressure difference
Liquid water molecules continually leave and return to a surface. Net evaporation becomes stronger when the water-vapour pressure at the surface exceeds the vapour pressure in the surrounding air and when airflow removes humid air from the boundary layer.
Relative humidity helps describe the surrounding air, but surface temperature also matters because saturation vapour pressure changes strongly with temperature.
Evaporation removes latent energy
When liquid becomes vapour, energy is required to separate molecules into the gas phase. If that energy is drawn from the liquid, skin or nearby air, the remaining material cools. This is the physical basis of sweating and direct evaporative cooling.
Cooling rate depends on the evaporation rate as well as latent heat per unit mass. A high latent heat does not guarantee strong cooling if humid air or impermeable clothing prevents much water from evaporating.
Air movement can improve evaporation even without changing humidity
A stagnant layer of humid air can build up next to a wet surface. Air movement thins or replaces that layer, maintaining a larger vapour-pressure difference near the surface and often increasing evaporation.
This is why a fan can make sweating feel more effective in many conditions. When air is already extremely warm and humid, however, the total human heat balance still needs to be considered rather than assuming airflow always makes heat exposure safe.
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Sources and further reading
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