The wet bulb temperature (WBT) is the lowest temperature air can reach through the evaporation of water. This metric combines the effects of air temperature and atmospheric moisture, measuring how much water vapor the air can still absorb. Because WBT is directly linked to evaporative cooling, it is a significant indicator for understanding atmospheric moisture content and calculating heat stress on living organisms. It is a more accurate gauge of thermal comfort and safety than the standard air temperature alone.
Measuring Wet Bulb Temperature Using a Psychrometer
The most direct and accurate way to determine the wet bulb temperature involves using a psychrometer, which is an instrument containing two thermometers. One is the standard dry bulb thermometer, which measures the ambient air temperature. The second is the wet bulb thermometer, which has its bulb covered by a cotton wick saturated with distilled water.
The principle relies on evaporative cooling, where water evaporating from the wick draws latent heat from the thermometer bulb, causing its temperature to drop. The rate of this evaporation, and thus the degree of cooling, is directly related to the air’s humidity. Drier air allows more rapid evaporation and a greater temperature drop, while highly humid air slows evaporation, resulting in a smaller drop.
To get a precise reading, the psychrometer must be properly ventilated, often by manually “slinging” the instrument or using an aspirating fan. Once the wet bulb temperature stabilizes, both the dry bulb and wet bulb temperatures are recorded. The difference between the two readings, known as the wet-bulb depression, is then referenced against a psychrometric chart or table to find the corresponding relative humidity.
Estimating Wet Bulb Temperature from Existing Data
In many meteorological and engineering applications, the wet bulb temperature is calculated from readily available data. This method requires two inputs: the standard air temperature (dry bulb temperature) and the relative humidity (RH). This estimation is necessary because many automated weather stations only record these two variables.
The estimation involves using complex mathematical formulas or specialized psychrometric charts, which graphically represent the relationship between temperature, humidity, and WBT. These equations often rely on principles of thermodynamics and vapor pressure to convert the two known values into an estimated wet bulb temperature. The Stull formula, for example, is one such complex empirical equation that provides a highly accurate estimate over a wide range of conditions.
Digital sensors and online calculators frequently use these embedded formulas to instantly provide a wet bulb reading. While physical measurement with a psychrometer is considered the gold standard for calibration, the calculated WBT is a practical and reliable substitute for real-time reporting and digital applications. The accuracy of the estimation is high, with some empirical equations reporting a mean absolute error of less than 0.3°C across their valid range.
The Importance of Wet Bulb Temperature Thresholds
The significance of the wet bulb temperature lies in its direct correlation with the human body’s ability to regulate its internal temperature. Humans cool down primarily by sweating, a process which is itself a form of evaporative cooling. When the surrounding air’s WBT is high, it means the air is already saturated with moisture, significantly reducing the rate at which sweat can evaporate from the skin.
If the WBT rises too high, the body can no longer effectively dissipate internal heat, leading to a rise in core body temperature, which can result in heatstroke and organ failure. The theoretical limit for human survival has traditionally been cited at a WBT of 35°C (95°F). However, recent studies suggest the true physiological threshold may be significantly lower, closer to 30.6°C to 31.5°C (87°F to 88.7°F). This threshold represents the maximum combination of heat and humidity the body can endure before the environment prohibits necessary cooling. Monitoring WBT is an increasingly used metric for public health warnings, especially as climate change increases the frequency of extreme heat and humidity events globally.