What is Wet Bulb Temperature?

A wet bulb temperature is the lowest measurement of air temperature that results from evaporative cooling, and it can be thought of as the temperature that wet skin feels when exposed to moving air. It is typically measured with either a psychrometer or a glass bulb thermometer wrapped in a wet cloth. As the water evaporates from the cloth, it lowers the temperature measured on the thermometer, although the rate of evaporation depends on the relative humidity level. The full equation used to calculate it using other measurements is complicated, but it can be estimated using the dew point and dry bulb temperature. One of the common uses for this measurement is determining the efficiency of evaporative coolers in dry regions.

In general, a psychrometer or mercury glass bulb thermometer is used for measuring the wet bulb temperature of a location. A psychrometer uses a thin layer of water applied to a thermometer bulb that is spun around in the air, and for a mercury glass bulb thermometer, it is usually wrapped in moist cloth or muslin. When the wet parts are exposed to air, water evaporates and draws heat out from the thermometer, which lowers the temperature recorded. The lowest measurement from the evaporative cooling is the wet bulb temperature.

As evaporation has a cooling effect, a wet bulb temperature will always be less than or equal to the dry bulb temperature, which is the heat content in the air. The rate of evaporation has an inverse relationship with the saturation level of the air, also known as the relative humidity level. When the humidity is high, less water can evaporate and cool the wet bulb, so the temperature will not be much different than the dry bulb one. On the other hand, lower humidity means more evaporation and cooling, making the wet temperature much lower than the dry. The two temperatures will be equal when the air is 100% saturated because no evaporation can occur.

One can calculate the wet bulb temperature without having a wet bulb thermometer by using an equation that combines other measurements such as dry bulb temperatures, latent heat, barometric pressure, and relative humidity. A simpler way to estimate it uses just the dry bulb temperature and dew point. First, one must subtract the dew point from the dry temperature, and divide that number by three. The number from that calculation then gets subtracted from the dry temperature.

An example of how a wet bulb temperature is used in daily life can be seen with evaporative coolers, sometimes called swamp coolers, commonly used in dry locations to cool the inside of buildings and homes. The cooler contains absorbent material that is moistened with water, and air is then blown through the material. The process adds moisture to the air, which makes it feel cooler. Evaporative coolers can generally get to within 70-95% of the outside air’s wet bulb temperature. The coolers do not produce the same results in humid regions because not as much water can evaporate, so they do not reduce the indoor temperatures by a significant amount.