Concept of Wet Bulb Temperature

The wet-bulb temperature is the lowest temperature that air may be lowered to by the evaporation of water into the air at a constant pressure. This is the temperature at which the temperature of a wet-bulb can be measured. As a result, it is measured by winding a damp wick around the bulb of a thermometer, and the temperature that is obtained this way corresponds to the temperature of the thermometer’s wet bulb. The temperature of the dry bulb corresponds to the temperature of the surrounding air. The relative humidity of the air can be calculated using the difference in temperature between these two readings. The difference between these temperatures has a direct correlation to the level of humidity in the air.

Wet Bulb Temperature

The wet-bulb temperature (WBT) is the temperature that is read by a thermometer that is covered in a fabric that has been soaked in water that is at the same temperature as the ambient air. This type of thermometer is called a wet-bulb thermometer. At a relative humidity of one hundred percent, the wet-bulb temperature is the same as the air temperature (the dry-bulb temperature). However, when the humidity is lower, evaporative cooling causes the wet-bulb temperature to be lower than the dry-bulb temperature.

The temperature of a parcel of air that has been cooled to saturation (100 percent relative humidity) by the evaporation of water into it is referred to as the wet-bulb temperature. This temperature is calculated using the latent heat that is supplied by the parcel. The temperature that is displayed on a wet-bulb thermometer is very near to the actual (thermodynamic) temperature of a wet bulb. The wet-bulb temperature is the lowest temperature that can be attained under the existing conditions of the environment only by the evaporation of water. This is the temperature at which the wet-bulb temperature scale begins.

Even persons who are used to the heat won’t be able to participate in routine outdoor activities once the wet-bulb temperature reaches 32 degrees Celsius (90 degrees Fahrenheit), which is comparable to a heat index of 55 degrees Celsius (130 degrees Fahrenheit). The wet-bulb temperature of 35 degrees Celsius (95 degrees Fahrenheit) is the theoretical limit for human survival for more than a few hours in the shade, even with an unlimited supply of water. This is theoretically equivalent to a heat index of 70 degrees Celsius (160 degrees Fahrenheit), though the heat index does not go that high.

Wet-bulb temperature and Health

Only within a narrow temperature window are living things able to maintain their existence. The production of sweat in humans and horses, saliva and water in dogs and other mammals, and other forms of evaporative cooling allow many species of animals to reduce their body temperature below that of the surrounding environment when the ambient temperature is excessive. This helps to prevent potentially fatal hyperthermia caused by heat stress. Wet-bulb temperature, or more complex calculated quantities such as wet-bulb globe temperature (WBGT), which also takes into account solar radiation, gives a useful indication of the degree of heat stress and is used by several agencies as the basis for heat stress prevention guidelines. Evaporative cooling is only effective when the relative humidity is high.

At a wet-bulb temperature that is maintained at or above 35 degrees Celsius (95 degrees Fahrenheit), it is highly likely that even people who are physically fit and healthy will perish, even if they are unclothed, in the shade, next to a fan. This is because at this temperature, human bodies switch from losing heat to the environment to gaining heat from it.

In reality, such ideal conditions for humans to cool themselves will not always exist; this is the primary reason for the high number of fatalities that occurred during the heat waves that hit Europe in 2003 and Russia in 2010, both of which had wet-bulb temperatures that did not exceed 28 degrees Celsius.

Temperature Measured Using the Thermodynamic Wet-Bulb Method

The thermodynamic wet-bulb temperature, also known as the adiabatic saturation temperature, is the temperature that a volume of air would have if it were cooled adiabatically to saturation by the evaporation of water into it, with the volume of air supplying all of the latent heat.

The temperature of an air sample that has passed over a large surface of the liquid water in an insulated channel is referred to as the thermodynamic wet-bulb temperature. This temperature is determined after the air sample has become saturated by travelling through an ideal, constant-pressure, adiabatic saturation chamber.

It is possible for meteorologists and other individuals to refer to the “thermodynamic wet-bulb temperature” by using the term “isobaric wet-bulb temperature.” It is also referred to as the “adiabatic saturation temperature,” although meteorologists also use the term “adiabatic saturation temperature” to mean “temperature at the saturation level,” which refers to the temperature that the parcel would reach if it expanded adiabatically until it was saturated.

Conclusion

The temperature at which adiabatic saturation occurs is referred to as the Wet Bulb temperature. This is the temperature that is displayed on a thermometer that has a wet bulb that is exposed to the air passage. The temperature of a wet bulb can be determined by wrapping the bulb in damp muslin and placing it inside a thermometer. A wet-bulb thermometer is used to determine the degree to which a surface has cooled as a result of the moisture evaporating from it (evaporative cooling). The temperature measured using the wet bulb is almost always going to be lower than the temperature measured using the dry bulb, with the exception of situations in which the relative humidity is 100 percent.