Wet-Bulb & Wet-Bulb Globe Temperature

The Wet-bulb temperature is the temperature read by a thermometer that has its bulb wrapped in a distilled water-soaked fabric and ventilated. When air is at 100% relative humidity, the wet-bulb temperature is equal to the normal air temperature (dry-bulb temperature). As the relative humidity falls, the wet-bulb temperature becomes lower than the normal air temperature.

The Wet-Bulb Globe Temperature (WBGT) is a measure of heat stress in direct sunlight, which takes into account temperature, humidity, wind speed, sun angle, and cloud cover (solar radiation). Uniquely, the WBGT includes sunlight as part of the calculation. It is an important heat stress metric for human health safety and guide to managing workload while working or exercising in direct sunlight. The WBGT index was developed in 1957 as a basis for environmental heat stress monitoring to control heat casualties at military training camps.

The WBGT differs from the heat index, which ONLY takes into consideration temperature and humidity and is calculated for shady or interior areas.

This CDC report has a lot of detail that may be of interest especially if you have staff working outside during the summer.

The First Dog on the Moon provides a delightful overview of wet-bulb temperature.