When the difference between wet and dry bulb temperature is zero, regardless of the unit of measurement, the humidity is 100% meaning the air is fully saturated with all the water molecules it can contain as gas.
When the humidity is less than 100%, meaning the air can accept more gaseous water, the wet bulb temperature is lower than the dry bulb temperature because water releasing H2O gas molecules also releases heat during the process, thus cooling the water, which in a perfect measurement device such as a psychrometer, is called the wet bulb temperature.
This principle accounts for the fact that bodies of water are normally cooler than the ambient temperature (dry bulb) since they're releasing heat to the air through evaporation.
Devices such as cooling towers and evaporative coolers use this principle to cool water for air conditioning and other purposes such as condensing steam produced by nuclear reactors. The big towers depicted in pictures of nuclear power plants are just large water evaporators that cool plain water to near wet bulb temperature for this purpose. They contain no radioactive substances.
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