Measurements · Meteorology
Wet Bulb Temperature
Also known as wet bulb
Wet-bulb temperature: an input for evaporative cooling and heat-stress calculations.
What it is
What is wet bulb temperature?
Wet bulb temperature is the lowest temperature that can be reached by evaporating water into the air: the temperature read by a thermometer whose bulb is wrapped in a wet wick and ventilated. As water evaporates from the wick, it draws heat from the bulb and cools it below the ordinary (dry-bulb) air temperature. The drier the air, the more evaporation and the greater the difference. In fully saturated air no evaporation is possible, and the wet-bulb and dry-bulb temperatures coincide. It therefore encodes both temperature and humidity in a single value and represents the physical limit of evaporative cooling.
Health and operational effects
Wet bulb temperature is fundamental to heat physiology because the human body cools itself chiefly by sweating, that is, by evaporating water. When the wet-bulb temperature approaches skin temperature (around 35 °C, a threshold much discussed in climate research), sweat can no longer evaporate and the body cannot shed metabolic heat. Sustained exposure is considered unsurvivable regardless of shade, water, or rest. Well below that absolute limit, high wet-bulb temperatures already mark severe heat-stress conditions for anyone working or exercising.
Operationally, wet bulb is the input to the wet-bulb globe temperature (WBGT) heat-stress index and to psychrometrics generally, and it sets the performance ceiling of evaporative (swamp) coolers and cooling towers, which cannot cool below the ambient wet-bulb temperature.
How it is measured
How Wet Bulb Temperature Is Measured
Classically it is measured with a psychrometer: two matched thermometers, one dry and one with a water-moistened wick, ventilated either by whirling (a sling psychrometer) or by a fan, as in an aspirated or Assmann psychrometer. The ventilated wet-bulb reading is the measurement, and the pair together give humidity. In modern monitoring, wet-bulb temperature is more often derived from measured dry-bulb temperature, relative humidity, and barometric pressure using standard psychrometric equations, avoiding the upkeep of a wetted wick. Dedicated WBGT instruments use a natural, unaspirated wet-bulb sensor as one of their three channels.
An instrument on Thiamis2
A third-party instrument connected to a Thiamis gateway, reporting into the same Environet account.
Placing wet bulb monitoring
Where a physical wet-bulb sensor is used, it must be kept wetted with clean (ideally distilled) water, adequately ventilated, and shielded appropriately for the index being computed: the natural wet-bulb used in outdoor WBGT is deliberately unaspirated and sun-exposed, whereas a psychrometric wet-bulb is aspirated and shaded. For a derived value, site the temperature and humidity sensors to represent the conditions people experience, in the same sun and wind as the work location, since a shaded reading understates outdoor heat exposure.
Where it is monitored
Solutions That Monitor Wet Bulb Temperature
Each one links to the solution page showing what that industry measures it for.
Heat Stress & Outdoor Work
Often addedWBGT for worksites, athletics, and events
Weather Stations & Meteorology
Often addedProfessional multi-parameter weather monitoring
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Questions
Wet Bulb Temperature FAQ
What is the difference between wet bulb and dry bulb temperature?
Dry bulb is the ordinary air temperature from a standard shielded thermometer; wet bulb is the lower temperature reached when evaporation cools a wetted, ventilated thermometer. The difference between them, the wet-bulb depression, reflects the humidity: large in dry air, zero when the air is saturated.
Why is a wet bulb temperature of 35 °C significant?
It is close to human skin temperature, the point at which sweat can no longer evaporate and the body loses its main cooling mechanism. A sustained wet-bulb temperature near 35 °C is considered a survivability limit even for a healthy, resting person in shade, which is why climate studies track it closely.
How is wet bulb temperature related to WBGT?
Wet bulb temperature is one of the three inputs to the wet-bulb globe temperature index, and in outdoor WBGT the natural wet-bulb term carries the largest weight. WBGT then adds a black-globe radiant term and the dry-bulb temperature to give the full occupational heat-stress index.
How do I monitor wet bulb temperature?
Wet bulb is measured or derived through an external instrument rather than the built-in Aethair sensors, so pair a Thiamis gateway with a WBGT heat-stress monitor or a weather station that reports wet bulb. Use the configurator to combine it with WBGT, air temperature, and humidity for a complete heat-stress panel.
Monitor Wet Bulb Temperature on the Record
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