Edit: Thank you all for the detailed answers! You guys are awesome.
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[there’s a pretty good explanation on wikipedia](https://en.wikipedia.org/wiki/Wet-bulb_temperature#Intuition)
If a thermometer is wrapped in a water-moistened cloth, it will behave differently. The drier and less humid the air is, the faster the water will evaporate. The faster water evaporates, the lower the thermometer’s temperature will be relative to air temperature.
Water can evaporate only if the air around it can absorb more water. This is measured by comparing how much water is in the air to the maximum which could be in the air—the relative humidity. 0% means the air is completely dry, and 100% means the air contains all the water it can hold in the present circumstances and it cannot absorb any more water (from any source).
This is part of the cause of apparent temperature in humans. The drier the air, the more moisture it can hold beyond what is already in it, and the easier it is for extra water to evaporate. The result is that sweat evaporates more quickly in drier air, cooling down the skin faster. If the relative humidity is 100%, no water can evaporate, and cooling by sweating or evaporation is not possible.
When relative humidity is 100%, a wet-bulb thermometer can also no longer be cooled by evaporation, so it will read the same as an unwrapped thermometer.
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