National Weather Service United States Department of Commerce

An Active Weather Wednesday

Strong to severe thunderstorms are possible on Wednesday in parts of the Ohio Valley, Upper Mississippi Valley and south-central Plains. These storms may produce heavy rain and flooding in the Ohio Valley. Meanwhile, a strong cold front will sweep through the Northwest U.S. with rain, mountain snow and high winds. Finally, heat and record high temperatures are likely across the Southern U.S. Read More >

EXCESSIVE HEAT: Wet Bulb Globe Temperature & Heat Index


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.

The Heat Index is more commonly known and used as an indicator of extreme heat and potential health related impacts but only takes into account temperature (measured in the shade) and relative humidity. 

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.

Wet Bulb Globe Temperature & Heat Index

 
 
 
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(%):
 
 

Heat Index (F):
88
 
WBGT (F):
88
 

Map, calculator courtesy National Weather Service - Tulsa

MAPS

Need HOURLY forecast maps for WBGT, Heat Index and other weather elements? Check this out.

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.

► 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.  

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

 

WBGT   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

WBGT 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

 

Heat Index Guidelines - Charts

How to read the chart: find the temperature on the left hand side, then move to the right until you find the column for the approximate relative humidity. That number will be the temperature that it will "feel" like. Example: A temperature of 95 and relative humidity of 50% will "feel" like 107 degrees.

 

  0% 5% 10% 15% 20% 25% 30% 35% 40% 45% 50% 55% 60% 65% 70% 75% 80%
115 103 107 111 115 120 127 135 143 151                
110 99 102 105 108 112 117 123 130 137 143 151            
105 95 97 100 102 105 109 113 118 123 129 135 142 149        
100 91 93 95 97 99 101 104 107 110 115 120 126 132 136 144    
95 87 88 90 91 93 94 96 98 101 104 107 110 114 119 124 130 136
90 83 84 85 86 87 88 90 91 93 95 96 98 100 102 106 109 113
85 78 79 80 81 82 83 84 85 86 87 88 89 90 91 93 95 97
80 73 74 75 76 77 77 78 79 79 80 81 81 82 83 85 86 86
75 69 69 70 71 72 72 73 73 74 74 75 75 76 76 77 77 78
70 64 64 65 65 66 66 67 67 68 68 69 69 70 70 70 70 71

 

 

IMPORTANT: Heat index values were devised for shady, light wind conditions. Exposure to full sunshine can increase values by up to 15 degrees! Also, strong winds, particularly with very hot, dry air, can be extremely hazardous.

 

Or, if you know the temperature and the dewpoint OR relative humidity, figure out the heat index using the calculators below.

 

HEAT INDEX using Temp and Dew Point
Air Temperature
F C
Dew Point Temperature
F C
Heat Index:
Relative Humidity:
HEAT INDEX using Temp and RH
Air Temperature
F C
Relative Humidity
%  
Heat Index:
 

Dew Point vs. Humidity

The dew point is the temperature the air needs to be cooled to (at constant pressure) in order to achieve a relative humidity (RH) of 100%. At this point the air cannot hold anymore water in the gas form. If the air were to be cooled even more, water vapor would have to come out of the atmosphere in the liquid form, usually as fog or precipitation.

The higher the dew point rises, the greater the amount of moisture in the air. This directly effects how "comfortable" it will feel outside. Many times, relative humidity can be misleading. For example, a temperature of 30 and a dew point of 30 will give you a relative humidity of 100%, but a temperature of 80 and a dew point of 60 produces a relative humidity of 50%. It would feel much more "humid" on the 80 degree day with 50% relative humidity than on the 30 degree day with a 100% relative humidity. This is because of the higher dew point.

So if you want a real judge of just how "dry" or "humid" it will feel outside, look at the dew point instead of the RH. The higher the dew point, the muggier it will feel.

General comfort levels that can be expected during the summer months:

  • less than or equal to 55: dry and comfortable
  • between 55 and 65: becoming "sticky" with muggy evenings
  • greater than or equal to 65: lots of moisture in the air, becoming oppressive

 


 

Heat Index/Heat Disorders

Heat Index Possible heat disorders for people in higher risk groups
80-90 Fatigue possible with prolonged exposure and/or physical activity.
90-105 Sunstroke, heat cramps and heat exhaustion possible with prolonged exposure and/or physical activity.
105-130 Sunstroke, heat cramps or heat exhaustion likely, and heat stroke possible with prolonged exposure and/or physical activity.
130 or higher Heatstroke/sunstroke highly likely with continued exposure.

 

 

Heat Disorder Symptoms First Aid
Sunburn Redness and pain. In severe cases swelling of skin, blisters, fever, and headaches. Ointments for mild cases if blisters appear and do not break. If breaking occurs, apply dry sterile dressings. Serious, extensive cases should be seen by a physician.
Heat Cramps Painful spasms usually in muscles of the legs and abdomen possible. Heavy sweating. Firm pressure on the cramping muscles, or gentle massaging to relieve the spasm. Give sips of water. If nausea occurs, discontinue use.
Heat Exhaustion Heavy sweating, weakness, skin cold, pale and clammy. Pulse thready. Normal temperature possible. Fainting and vomiting. Get victim out of sun. Lay down and loosen clothing. Apply cool, wet cloths. Fan or move victim to air conditioned room. Sips of water. If nausea occurs, discontinue use. If vomiting continues, seek immediate medical attention.
Heat Stroke (sunstroke) High body temperature (106 F or higher). Hot dry skin. Rapid and strong pulse. Possible unconsciousness. Heat stroke is a severe medical emergency. Summon emergency medical assistance or get the victim to a hosiptal immediately. Delay can be fatal.

Move the victim to a cooler environment. Reduce body temperature with cold bath or sponging. Use extreme caution. Remove clothing, use fans and air conditioners. If temperature rise again, repeat process. Do not give fluids.

 

 

Safety Tips

  • Slow down. Strenuous activities should be reduced, eliminated, or rescheduled to the coolest time of the day. Individuals at risk should stay in the coolest available place, not necessarily indoors.
  • Dress for summer. Lightweight, light-colored clothing reflects heat and sunlight, and helps your body maintain normal temperatures.
  • Foods (like proteins) that increase metabolic heat production also increase water loss.
  • Drink plenty of water or non-alcoholic fluids. Your body needs water to keep cool. Drink plenty of fluids even if you don't feel thirsty.
  • Do not drink alcoholic beverages.
  • Spend more time in air-conditioned places. Air conditioning in homes and other buildings markedly reduces danger from the heat. If you cannot afford an air conditioner, spending some time each day (during hot weather) in an air conditioned environment affords some protection.
  • Don't get too much sun. Sunburn makes the job of heat dissipation that much more difficult.