Heat guide
WBGT and Occupational Heat Stress
Wet-Bulb Globe Temperature combines measurements related to humidity, radiant heat and air temperature to support heat-stress assessment in work and activity settings.
WBGT uses different measurements from heat index
Traditional outdoor WBGT uses natural wet-bulb temperature, globe temperature and dry-bulb temperature. Indoor or shaded formulas may omit the dry-bulb term depending on the method.
The globe thermometer captures radiant load
A black globe responds to radiant heating as well as convection. This helps WBGT reflect hot surfaces and solar exposure that a standard air-temperature measurement can miss.
Natural wet-bulb temperature reflects evaporation
The wetted sensor responds to humidity, air movement and evaporation. It is not the same measurement as the psychrometric wet-bulb temperature obtained with forced ventilation.
WBGT is only part of a heat-stress program
Thresholds depend on workload, acclimatisation and clothing. PPE can reduce evaporative cooling and may require adjustments or more conservative controls.
Do not infer an exact WBGT from ordinary weather inputs without a validated method
Approximation formulas exist, but they introduce uncertainty. A site calculator should identify whether a value is measured or estimated and should not present an estimate as equivalent to an instrument reading.
Instrument placement affects WBGT readings
A globe thermometer responds to radiant surroundings as well as air conditions, while a natural wet-bulb sensor depends on evaporation and airflow. Placing the instrument in shade, near a hot wall, over dark pavement or inside a poorly ventilated enclosure can therefore produce readings that represent different microenvironments.
For workplace or sports use, measurements should represent the actual exposure location and follow the relevant procedure. A weather-station estimate can be useful for screening, but it is not identical to an on-site instrument exposed to the same radiant and airflow conditions as the person.
WBGT includes environmental pathways that heat index does not
Wet-bulb globe temperature combines measurements intended to represent evaporative potential, radiant heat and air temperature. Outdoor WBGT commonly uses natural wet-bulb, globe and dry-bulb measurements in a weighted relationship, while indoor or shaded applications can use a different form.
The method is useful because solar and radiant loads can be important even when ordinary air temperature and relative humidity look moderate.
Instrument conditions matter
A valid WBGT measurement depends on appropriate sensors, placement and sufficient time for the instruments to respond. A globe thermometer is designed to capture radiative effects that an ordinary shielded air thermometer does not.
A weather-app estimate based only on standard meteorological data is therefore not identical to a properly measured workplace WBGT, even when it can provide useful screening information.
Workload and clothing are separate from the environmental number
WBGT characterises environmental heat, but acceptable exposure also depends on metabolic workload, acclimatisation, clothing and personal protective equipment. Heavy impermeable garments can greatly reduce evaporative cooling even when the measured environmental WBGT is unchanged.
Occupational guidance therefore combines WBGT with work-rest practices, clothing adjustments and other controls rather than using the environmental reading by itself.
WBGT combines several environmental heat pathways
Wet-bulb globe temperature was developed to combine information related to humidity, air movement, radiant heat and dry-bulb temperature into a practical screening index. Outdoor WBGT with solar load uses natural wet-bulb temperature, globe temperature and dry-bulb temperature with specified weighting factors.
Because the instrument responds to the local environment, two nearby locations can produce different WBGT values if shade, radiant surfaces or airflow differ even when the weather-station air temperature is identical.
Action limits depend on workload, clothing and acclimatization
A WBGT number does not by itself tell every person how long they can work safely. Occupational guidance adjusts interpretation according to metabolic workload, clothing and whether workers are acclimatized to heat. Protective garments can increase heat burden even when the measured environment is unchanged.
This is why a WBGT calculator should be treated as a screening aid rather than a personalised medical or work-rest prescription.
Continue exploring this topic
Sources and further reading
Use the linked primary or authoritative resources for additional detail, standards and source-specific conditions.