Excessive Heat

Wet Bulb Globe Temperature & Heat Index

Map, calculator courtesy National Weather Service - Tulsa

MAPS

Wet Bulb Globe Temps

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10 am

1 pm

4 pm

7 pm

Heat Index

7 am

10 am

1 pm

4 pm

7 pm

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

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

• General WBGT Information (Wiki page)
• American College of Sports Medicine (ACSM), [position stand]
• Korey Stringer Institute
• University of Georgia Research [AMS conference presentation]
• WBGT Research
• OSHA Heat Hazard Assessment and WBGT
• Department of the Army: Prevention of Heat and Cold Casualties
• Department of the Army: WBGT, Guidelines, Prevention

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.
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 Categories

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

Other Charts For Reference

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.

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.

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

• 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.