Here's some pics of an antique wet bulb thermometer and mercury thermometers that I use. Exteremely accurate!Relative humidity or wet bulb temperature. Just depends on what the scale is calibrated for, no? I believe we have conversion charts.
The water is in equilibrium between liquid and the air. Takes heat to evaporate so the thermometer is being cooled by the evaporation. Wick maximizes surface area for cooling the thermometer bulb. Hope that helps.
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