Wet bulb temperature is the lowest air temperature due to evaporative cooling, measured with a psychrometer or glass bulb thermometer. It is estimated using dew point and dry bulb temperature and is used to determine the efficiency of evaporative coolers in dry regions.
Wet bulb temperature is the lowest measure of air temperature that results from evaporative cooling and can be thought of as the temperature that wet skin feels when exposed to moving air. It is typically measured with a psychrometer or glass bulb thermometer wrapped in a damp 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 whole equation used to calculate it using other measurements is complicated, but it can be estimated using dew point and dry bulb temperature. One of the common uses of this measurement is in determining the efficiency of evaporative coolers in dry regions.
In general, a psychrometer or mercury glass bulb thermometer is used to measure the wet bulb temperature of a place. A psychrometer uses a thin layer of water applied to a thermometer bulb which is swirled in air and, for a mercury glass bulb thermometer, is usually wrapped in a damp cloth or muslin. When moist parts are exposed to air, the water evaporates and extracts heat from the thermometer, which lowers the recorded temperature. The lowest measure of evaporative cooling is wet bulb temperature.
Since 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 evaporation rate has an inverse relationship with the air saturation level, also known as the relative humidity level. When humidity is high, less water can evaporate and cool the wet bulb, so the temperature won’t be much different than dry bulb. On the other hand, lower humidity means more evaporation and cooling, making the wet temperature much lower than the dry one. The two temperatures will be equal when the air is 100% saturated because no evaporation can occur.
One can calculate wet bulb temperature without having a wet bulb thermometer by using an equation that combines other measurements such as dry bulb temperature, latent heat, atmospheric pressure, and relative humidity. An easier way to estimate this is using just the dry bulb temperature and the dew point. First, you subtract the dew point from the dry temperature and divide that number by three. The number from that calculation is then 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 which is moistened with water and air is then blown through the material. The process adds moisture to the air, making it cooler. Evaporative coolers can generally achieve 70-95% of the wet bulb temperature of the outside air. Chillers don’t produce the same results in humid regions because not much water can evaporate, so they don’t reduce core temperatures by a significant amount.
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