National Weather Service United States Department of Commerce

Extreme heat is a major weather-related hazard. Even a short period of exposure can be so taxing on the body that it results in a heat-related illness. Although outdoor physical activity may be unavoidable, heat-related illnesses and fatalities are preventable by using NWS forecast tools like Heat Index (which tells you how temperature feels to the human body in a shady area) or alternatively, Wet Bulb Globe Temperature (WBGT), before heading outside.

What is WBGT?

WBGT is an experimental forecast tool indicating expected heat stress on the human body when in direct sunlight. It estimates the effect of temperature, relative humidity, wind speed, and solar radiation on humans using a combination of temperatures from three thermometers:

  • A Wet bulb measures the temperature read by a thermometer covered in a wet cloth. As water evaporates from the cloth, evaporation cools the thermometer. This mirrors how the human body cools itself with sweat.
  • A black globe is used to measure solar radiation. Solar radiation heats the globe and wind blowing across it cools the globe.
  • A Dry bulb calculates the air temperature measured in the shade. It is the temperature you would see on your thermometer outside.

With WBGT forecasts available by region up to 7 days in advance, it is a useful tool for planning ahead.

Who should use WBGT?

This experimental tool is most useful for active, acclimatized people such as outdoor workers, athletes, and anyone else performing strenuous outdoor activities — and has been used for decades by military agencies, OSHA, and marathon organizers.

What should you do if WBGT deems outdoor activity dangerous?

Always follow the advice of your local officials before engaging in outdoor activities, as WBGT guidelines vary geographically since temperatures fluctuate by region and so does the body’s response to exercising or working in heat. OSHA recommends the following protective measures for outdoor workers:

  • Acclimatize workers starting the first day working in the heat and after any extended absences
  • Provide shade for outdoor work sites
  • Schedule work earlier or later in the day
  • Use work/rest schedules
  • Limit strenuous work (e.g., carrying heavy loads)

For more heat safety tips and resources, visit the NWS safety page.