National Weather Service United States Department of Commerce

Excessive Heat in Southern California; Tropical Storm Ian Expected to Rapidly Intensify

Excessive heat is possible through Wednesday over southern California. Strong thunderstorms may pose some risk for severe weather across parts of the upper Ohio Valley into the northern Mid Atlantic coast. Tropical Storm Ian is forecast to approach the Florida peninsula at major hurricane strength by midweek, and poses a risk of dangerous storm surge, hurricane-force winds, and heavy rainfall. Read More >


Wet Bulb Globe Temperature

(Prototype -- not official -- availability not guaranteed) 


Note: If the below images are not up to date, you may be able to access the latest Wet Bulb Globe Temperature Forecast using this:

Experimental Viewer



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.

► 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

► 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.  

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
Heat Index
Measured in the sun
Measured in the shade
Uses Temperature
Uses RH
Uses Wind
Uses Cloud Cover
Uses Sun Angle
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


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.


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

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


  • Tw = Natural wet-bulb temperature (combined with dry-bulb temperature indicates humidity)
  • Tg = Globe thermometer temperature (measured with a globe thermometer, also known as a black globe thermometer)
  • Td = Dry-bulb temperature (actual air temperature)


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:

NSAA Wet Bulb Globe Temperature (WBGT) Guidelines


The NSAA Board of Directors approved the Wet Bulb Globe Temperature (WBGT) as the recommended measurement practice and device for measuring acceptable heat/ humidity levels for practices and contests. The use of WBGT is recommended throughout the calendar year when the ambient temperature is above 80°.

Region 1 Region 2 Activity Guidelines
<76.1 <79.7

Normal activities

3 separate - 3 to 5 minute rest / water breaks per hour

76.2-81 79.8-84.6

Use discretion for intense / prolonged practice

3 separate - 4 to 6 minute rest / water breaks per hour

Monitor at risk athletes closely

Cold water immersion available

81.1-84 84.7-87.6

Maximum practice time is 2 hours

Helmets and shoulder pads only

Remove helmets and shoulder pads if conditioning

4 separate - 4 to 6 minute rest /water breaks per hour

Monitor at risk athletes closely

Cold water immersion available

Contests: Per NFHS rules, implement additional/extended timeouts for rest / water breaks

84.1-86.1 87.7-89.7

Maximum practice time is 1 hour

Helmets and shoulder pads prohibited

No conditioning

4 separate - 5 to 7 minute rest / water breaks per hour

Monitor at risk athletes closely

Cold water immersion available

Contests: Per NFHS rules, implement additional/extended timeouts for rest / water breaks

Contests: Consider delaying / postponing start times

>86.1 >89.7

No outdoor activities

Cancel or delay outdoor practices / contests until lower WBGT is observed.

Local WBGT Forecast Maps

The Wet Bulb Globe Temperature (WBGT) is a measure of heat stress in direct sunlight, which is based on temperature, humidity, wind speed, sun angle, and cloud cover (solar radiation). This differs from the heat index, which is based only on temperature and humidity and is calculated for shady areas. Scroll down for latest observations and forecasts





Nebraska WBGT Forecast Maps


Custom Wet Bulb Globe Temperature

Use the map below and sliders to calculate the wet bulb globe temperature for a location and parameter values of your choosing

Initial values are estimates
for the selected location
adjust sliders as needed
Fcst Max Temp(F):
Wind Speed(mph):
Cloud Cover(%):