|MLA Citation:||Bloomfield, Louis A. "Question 770"|
How Everything Works 21 Jan 2018. 21 Jan 2018 <http://howeverythingworks.org/print1.php?QNum=770>.
A wet-bulb/dry-bulb system measures humidity by looking at the temperature drop that occurs when water evaporates. As water evaporates from the bulb of the wet thermometer and the bulb's temperature drop, the rate at which water molecules leave the bulb's surface decreases. The bulb temperature drops until the rate at which water molecules leave the bulb is equal to the rate at which water molecules return to the bulb from the air. At that point, there is no net evaporation going on. In humid air, water molecules return to the bulb more often so that this balance is reached at a higher temperature than in dry air. The wet bulb temperature is thus warmer on a humid day than it is on a dry day.