National Weather Service United States Department of Commerce

Wet Bulb Globe Temperature

(Experimental. Not for operational use)


The Wet Bulb Globe Temperature (WBGT) is an indicator of heat related stress on the human body at work (or play) in direct sunlight. It takes into account multiple atmospheric variables, including:  temperature, humidity, wind speed, sun angle, and cloud cover.  

 

Wet Bulb Globe Temperature

 
 
 
Initial values are estimates
for the selected location
adjust sliders as needed
Month:
 
 
Day:
 
 
Latitude:
 
 
Fcst Max Temp(F):
80
 
DewPoint(F):
70
 
RH(%):
 
 
Wind Speed(mph):
 
 
Cloud Cover(%):
 
 

HeatIndex(F):
88
 
WBGT(F):
88
 

Map, calculator courtesy National Weather Service - Tulsa

MAPS

Wet Bulb Globe Temps

7 am wbgt
7 am
10 am wbgt
10 am
1 pm wbgt
1 pm
4 pm wbgt
4 pm
7 pm wbgt
7 pm
  day 2 max wbgt
 
day 3 max wbgt
 
day 4 max wbgt
 

Heat Index

7 am heat index
7 am
10 am heat index
10 am
1 pm heat index
1 pm
4 pm heat index
4 pm
7 pm heat index
7 pm
  day 2 max heat index
 
day 3 max hi
 
day 4 max hi
 

Wet Bulb Globe Temperature vs Heat Index

While the WBGT and Heat Index both attempt to describe how "hot" it is and the potential for heat related stresses, they go about it in different ways.

‚Ėļ Heat Index is more commonly used and understood by the general public - the higher the values the hotter it's going to feel and the higher the threat for heat related illnesses. It's calculated from the temperature and relative humidity. What's not commonly known is that Heat Index assumes you are in the shade

‚Ėļ WBGT also uses the temperature and humidity in its calculation, but temperatures are measured in direct sunshine. It also factors in wind speed, sun angle, and cloud cover.  

Bottom-line upfront >>>> what value should you use? For day-to-day activities, heat index will serve you well. If you work outside or plan on any sort of vigorous outdoor activity in the full sun, use the WBGT.  

 

Comparing WBGT and Heat Index
 
WBGT
Heat Index
Measured in the sun
Measured in the shade
Uses Temperature
Uses RH
Uses Wind
Uses Cloud Cover
Uses Sun Angle
Examples
Temp F Dew Point F RH % Sky % Wind mph Heat Index F WBGT F
90 65 42 05 03 92 89
90 65 42 05 13 92 83
90 65 42 65 13 92 81
90 70 52 10 06 96 88
90 70 52 60 06 96 86
90 70 52 60 13 96 85
100 70 39 10 13 108 90
100 70 39 10 5 108 94
100 70 39 65 05 108 91

 

WGBT   Heat Index

Origins:

The WBGT date back to the 1950s - specifically the United States Marine Corp Recruit Depot on Parris Island, SC. There, recruits were required to perform high intensity exercise in a high humidity, high temperature environment. Many solders succumbed to heat related illness. In response, a joint effort between the Department of the Navy and Army doctors studied the effects of heat on exercise performance. The result was the WBGT.

WGBT uses several atmospheric variables for its calculations: temperature, humidity, wind speed, sun angle, and cloud cover. Temperatures are measured in the sunlight.

The military uses the WBGT to gauge the potential for heat related stresses to this day. OSHA and many nations also use the WBGT as a guide to managing workload in direct sunlight, as do athletic departments (college and high school) and events. If you work or exercise in direct sunlight, this is a good element to monitor.

Origins:

The Heat Index is based on work carried out by Robert G. Steadman in 1979 ("An Assessment of Sultriness, Parts I and II") where he discussed factors that would impact how hot a person would feel under certain conditions. The National Weather Service developed a "simplified" formula from this work using air temperature and relative humidity as the two inputs. This formula became the "heat index".

It is important to note that the heat index is calculated for shady areas. Direct sunlight can add as much as 15 degrees to the heat index

Inputs:
  • Temperature (in sun)
  • Relative Humidity
  • Wind speed
  • Cloud cover
  • Sun angle
Inputs:
  • Temperature (in shade)
  • Relative Humidity

Equation:

WBGT=0.7Tw+0.2Tg+0.1Td
  • Tw is the wet bulb temperature, which indicates humidity
  • Tg is the globe temperature, which indicates radiant heat
  • Td is the ambient air (dry) temperature

Equation:

Heat Index = -42.379 + 2.04901523T + 10.14333127R – 0.22475541TR – 6.83783(10-3T2) – 5.481717(10-2R2) + 1.22874(10-3T2R) + 8.5282(10-2TR2) – 1.99(10-6T2R2)
  • T = ambient dry temperature (in Fahrenheit)
  • R = relative humidity (percentage)
For more information on the heat index:

PREPARE - SAFETY - RESOURCES

Prepare

Before:

  • Reschedule or postpone outdoor plans during peak heating of the day (usually mid to late afternoon)
  • Check in with family members, friends. Make sure they have a way to keep cool and take necessary precautions from the heat. The elderly and children are especially susceptible to the heat. 
  • Don't forget your pets! Make sure they have adequate shelter (preferably indoors, air conditioned) and ample water. 

During:

  • Take frequent breaks in the shade or in an air conditioned location. Strenuous outdoor activities should be reduced (or eliminated), especially in direct sunlight where there is little ventilation.
  • Drink plenty of water or other non-alcohol fluids. Stay hydrated by drinking plenty of fluids even if you do not feel thirsty. Avoid drinks with caffeine or alcohol.
  • Continue to check on family, friends and your pets. 
  • Don’t get too much sun. Sunburn makes the job of heat dissipation that much more difficult

 

Resources

Guidelines - Charts

While there is not set criterion for WBGT temperatures and related risks/impacts, the American College of Sports Medicine (ACSM) has developed a set of values that have been accepted as a standard to follow. Their original guidelines were developed for running events, but have since been expanded to include intermittent (non-continuous) activities [Medicine & Science in Sports & Exercise: March 2007 - Volume 39 - Issue 3 - p 556-572] . The guidelines are not based on location.

Using ACSM as a starting point, further research by the University of Georgia (UGA) factored in climatology and regional differences in an effort to incorporate acclimatization* into the guidelines. The Korey Stringer Institute and USA Soccer used the regional categories developed by UGA to provide another set of recommend actions.

The military and OSHA also have devised recommendations, as do many university and high school athletic departments.

Below you will find several of these guidelines/recommendations.  

It should be noted that while WBGT will provide solid guidelines, other factors such as an individual’s physical fitness, acclimatization, medical condition, and age also have an impact.

* Acclimatization: the body's natural adaptation to heat/cold. The process usually takes 10 to 14 days.

 

 

NOT based on location - American College of Sports Medicine


Continuous Activities   Intermittent Activities

Activities with little if any break, such as cross country running, 5K runs, and marathons.

Activities that generally have breaks between bursts of high intensity movements/actions, such as football, soccer, lacrosse, etc.

ACSM wbgt guidelines for continuous activities ACSM wbgt guidelines for training or non-continuous activities

Guidelines from the American College of Sports Medicine - not based on location

Guidelines from the American College of Sports Medicine - not based on location

 

Based On Location


The following recommendations use similar criteria as the ACSM, but modified for location and climatology. This adjustment factors in the acclimatization that occurs for those that consistently work and exercise in hot environments. regional cateogories
Regional Categories
     
USA Soccer and KSI guidelines University of Georgia wbgt guidelines for by region

Guidelines from USA Soccer and the Korey Stringer Institute (KSI)

Guidelines from the University of Georgia


 

Other Charts For Reference

wbgt recommendations
A composite of various guidelines and recommendations for those working or exercising in full sun

Work and Rest Water Consumption Table
U.S. Military Recommendations, also used by OSHA

Military Flag Stress Conditions
Military Flag Stress Conditions