Edit: Thank you all for the detailed answers! You guys are awesome.
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Wet-bulb temperature is the temperature that an old-fashioned liquid bulb thermometer will read if you wrap it in cotton gauze, wet it, and swing it around in the air. (The “bulb” is the little reservoir at the end of the thermometer that holds the mercury or alcohol.)
Wet-bulb temperature by itself doesn’t tell you much. Wet-bulb temperature compared to dry-bulb temperature is useful. It’s a measure of how much moisture is already in the air, and how easy it is for moisture to evaporate into the air.
So if it’s 95 degrees F out and the wet-bulb thermometer reads 94 degrees F, that means very little evaporation is occurring on the little bulb of the thermometer. So it’s very humid (actually, almost raining), and evaporation will not occur easily.
If it’s 95 degrees F out and the wet-bulb thermometer reads 70 degrees F, that means a lot of evaporation is occurring on the little bulb of the thermometer…..because it’s the water evaporating on the literal wet-bulb that is lowering the temperature of the fluid in the thermometer below the actual air temperature. It’s very dry, and evaporation will occur easily.
When evaporation doesn’t occur easily, all sorts of processes are affected: sweating is less effective – so your body overheats more easily, cooling towers don’t work as well – so you spend more energy to cool your building or power plant, manufacturing processes that require things to evaporate (films, adhesives, etc.) may not work well.
When it evaporation does occur easily, processes that rely on evaporation occur easily. Your sweat evaporates – so you feel cool even in 105 F heat….until you realize that you’re sunburned like a lobster.
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