What Does
Wet Bulb Temperature Mean?
Wet bulb temperature is a measure of atmospheric conditions that combines air temperature, air velocity, and humidity. It is meant to represent the cooling effect of sweating and identify the temperature at which sweat can no longer evaporate and regulate the body’s temperature.
While dry bulb temperature is measured by exposing a thermometer to the air, measuring wet bulb temperature also involves wrapping the thermometer bulb in a wet cotton or muslin cloth.
Safeopedia Explains Wet Bulb Temperature
Hot environments can put workers at risk of heat-related illnesses, such as heat exhaustion and heat stroke. While heat risks are commonly associated with air temperature, other factors like humidity also play a significant role.
The Importance of Wet Bulb Temperature
Wet bulb temperature is an important indicator of when the human body is vulnerable to overheating.
The human body’s natural temperature range is 36.5°C to 37.5°C (97.7°F to 99.5°F). When the air temperature is too high, the body regulates its internal temperature by evaporative cooling. Simply put, it produces sweat which then cools down the body as it evaporates against the skin. If the wet bulb temperature is higher than the body’s natural temperature, however, sweat cannot evaporate. This means that the body is no longer able to cool itself and will undergo a state of hyperthermia (absorbing more heat than it is able to release).
A wet bulb temperature of over 35°C (95°F) is considered deadly for humans. This is equivalent to 35°C with 100% humidity, 39°C (102°F) with 75% humidity, or 45°C (113°F) with 50% humidity.
Above this wet bulb threshold, the body’s temperature will continue to rise. Initial symptoms might include heavy sweating, fast pulse, and rapid breathing. If not addressed immediately, this can progress to nausea, dizziness, headache, vomiting, and low blood pressure. That low blood pressure can, in turn, cause loss of consciousness, seizures, organ failure, and death.
Calculating Wet Bulb Temperature
Wet bulb temperature is calculated using a thermometer with its bulb covered in a cotton or muslin cloth. The cloth is kept wet by connecting it to a water tank with a cotton thread. The thermometer is then placed in an air stream, allowing the water to evaporate. The evaporating water extracts heat from the environment and the thermometer bulb, decreasing the measured temperature.
The rate of evaporation will depend on the humidity in the surrounding environment. The lower the humidity, the faster the water in the cloth will evaporate, resulting in a greater cooling effect.
Evaporation ceases (along with its cooling effect) when the relative humidity is at 100%. At that point, the wet bulb temperature will be equal to the dry bulb temperature.
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