As water evaporates from a wet surface it cools the surface; this is why the wind feels cool on sweaty skin. If the bulb of a thermometer is wrapped in a wet wick and air is passed over it, the value it shows falls according to the dryness of the air and stops at a certain point: this is the wet-bulb temperature. If the air is fully saturated (100% humidity) there is no evaporation, and the wet and dry bulbs show the same value. The drier the air, the greater the difference: at 35 °C and 30% humidity the wet bulb is about 21–22 °C, at 35 °C and 60% humidity it is 28 °C.
Its importance in refrigeration is that it sets the limit for all heat rejection equipment that works with water. An air-cooled condenser works with the dry-bulb temperature: in 35 °C air, condensing is at 45–50 °C. An evaporative condenser and a cooling tower, on the other hand, approach the wet-bulb temperature: with a wet bulb of 22 °C, condensing in an evaporative condenser falls to 32–35 °C, and a cooling tower cools the water to 25–27 °C. The 12–15 K difference in condensing between the two means a saving of roughly 25–35% in compressor consumption.
The choice by climate is made accordingly. In regions with hot but dry summers such as Konya and Central Anatolia the design wet-bulb temperature is around 20–22 °C, and evaporative or adiabatic cooling is very efficient. In humid coastal cities the wet bulb rises to 25–27 °C; the gain narrows but it is still superior to air cooling. In a project the design value taken from local meteorological data is used; selecting evaporative equipment by looking at the dry-bulb temperature alone leads to high-pressure trips on humid days.
The classic instrument for measurement is the psychrometer, which has two thermometers, one dry and one with a wet wick; the wick is wetted with pure water and air is passed over it at a speed of at least 2–3 metres per second (the hand-whirled or fan type). From the two readings the relative humidity is found with a table or the psychrometric chart. In a cold room below zero the wick freezes and the reading is misleading; there an electronic humidity sensor is used. Wet-bulb temperature must not be confused with dew point: in unsaturated air the dew point is always lower.