Wet Bulb Globe Temperature: What It Is and How To Use It

Wet bulb globe temperature! Say that tongue-twisting name three times and you’ll likely see anyone close by inch away, then cut and run. But knowing what wet bulb globe temperature is and how to use it is important. It can help you and your family avoid the dangers of heat stress and dehydration.

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Photo: U.S. OSHA

The Oklahoma Mesonet added Wet Bulb Globe Temperature as another map product in the air temperature group earlier this year, through the efforts of Brad Illston. This index has been used by the US military, business, and sports teams to avoid human heat illness. It provides a guide to outdoor activities, liquid intake, and the work/rest time balance.

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The wet bulb globe temperature was created by the US Marines in 1956 at their Parris Island, South Carolina training facility. It provided a weather-based tool to let drill sergeants and officers know how far they could push recruits during outdoor activities and still avoid heat illness. The categories were set to be a guide to activity level and the balance between activity and rest times. Bright colors were assigned to each category, so that solid color flags could be flown over training areas to alert all to the wet bulb globe temperature status.

The wet bulb globe temperature index takes into account air temperature, relative humidity, wind speed, cloud cover (solar radiation), and sun angle to calculate the heat stress risk from working outdoors.

Traditionally, we have used air temperature adjusted only by relative humidity as a measure of human heat stress. That doesn’t take into account the added heat load from work or exercise in sunlight or the reduction in heat load from wind.

How we respond to heat exposure is highly dependent on our level of heat acclimation and hydration. The table below provides a guide to work and rest times in minutes based on wet bulb globe temperature category and whether you are acclimated or unacclimated to the heat level. It also provides a guide to the amount of water intake per hour by heat category.

WBGT Work Rest chart

Recommendations above are for healthy, hydrated humans fully clothed with lightweight summer working clothes. Increase Wet Bulb Globe Temperature (WBGT) by 2 units, when wearing cotton coveralls. Increase WBGT by 4 units, when wearing heavy winter-type clothing. Increase WBGT by 6 units, when wearing permeable, water barrier clothing. Increase WBGT by 10 units, when wearing full-body, impermeable, protective clothing (e.g. Tyvek coveralls and hood) while conducting “Light Work” and increase WBGT by 20 units for “Moderate to Hard Work” tasks. Heat tolerance can be impacted by hydration, overall health, medications, and level of acclimation.

Getting acclimated to high heat conditions typically takes 5 days of heat exposure. It is recommended that you start at the unacclimated level on day one and then increase work and decrease rest times by 20 percent each day.

Rest periods in the table above assume that a person is in the same outdoor conditions. Persons should shade themselves during rest breaks, whenever possible.

Water intake can vary for individuals (+/– 1?4 quart/hr) and exposure to full sun or full shade (+/– 1?4 quart/hr). Sports drinks can be used for a portion of your water intake. The balance between the two depends on the outdoor activity and length of activity.  Avoid high sugar soda pop and alcoholic beverages.

A couple  of important take away lessons from wet bulb globe temperature and outdoor activities:

- Wet bulb globe temperature is a new heat stress index for most of us, so you’ll need to monitor it under moderate and high heat conditions to learn how to use it.

- Most of us spend so much time in air conditioned buildings or vehicles, few of us are acclimated to high heat conditions.

- We need to let the wet bulb globe temperature and our physical condition dictate how long we work, not the task.

- We need to stay ahead of dehydration by hydrating before outdoor activities and drinking enough water during outdoor activities.

***Printable Mesonet Wet Bulb Globe Temperature info sheet: WBGT Mesonet Work Rest Aug 2015


WebBulb Globe Temperature information, National Weather Service Tulsa, Oklahoma Forecast Office. Website: www.srh.noaa.gov/twa/?n=wbgt

Prevention of Heat and Cold Stress Injuries. Manual of Naval Preventive Medicine, Chapter 3, pp. 3-28. NAVMED P-5010-3 (Rev. 2-2009). Website: http://www.med.navy.mil/directives/Pub/5010-3.pdf

Heat Illness Prevention, U.S. Army Public Health Command. Website: phc.amedd.army.mil/topics/discond/hipss/Pages/HeatinjuryPrevention.aspx

Heat Stress. U.S. Department of Labor OSHA Technical Manual Section III: Chapter 4. Website: www.osha.gov/dts/osta/otm/otm_iii/otm_iii_4.html

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