Military heat flag conditions explained

Heat flag conditions in the military describe the risk of suffering heat illness (heat cramps, heat exhaustion, heat injury, and heat stroke) during work or exercise outside in the heat. They’re currently based on “wet-bulb globe temperature” (WBGT), which is a calculation of ambient temperature, humidity, sunlight exposure, and wind speed. WBGT is different from heat index, which only takes into account temperature and humidity.

The U.S. Marine Corps first used flag conditions in the late 1940s through early 1950s to address the high rates of heat illness at their training sites. However, the temperature calculations that flag conditions were based on were relatively inaccurate because they didn’t consider sunlight and wind. WBGT was then developed as a more accurate assessment of temperature, taking those factors into account. It’s important to note that WBGT doesn’t factor in substantial clothing—it was validated for a standard, khaki-colored, military uniform from the 1950s, and not the current working uniforms, special uniforms (such as EOD bomb suits), or body armor. When using WBGT to inform flag conditions, decisions about modifying training exercises should include guidance—from your branch and local installation—about uniform modifications or training load reductions if the uniforms can’t be reduced.

Currently, there are 5 flag conditions in use by the U.S. military: no flag, green, yellow, red, and black. After no flag, each color corresponds with increasing restrictions to work or training based on personnel’s heat acclimatization status and training intensity. Though the specifics of each condition vary by branch, the basics all remain the same. The graphic below highlights the key points for each flag condition, and shouldn’t be considered policy for any one Service.

The WBGT index determines heat flag conditions. The color of the flag corresponds to specific precautions you must take to avoid heat illness.  A white flag (or no flag) indicates a WBGT index of less than 82 degrees. No additional precautions. A green flag indicates a WBGT index of 82–84.9 degrees. Recommended precautions include limiting physical training (PT) or strenuous work outdoors to 30–50 minutes at a time, with 10–30 minutes of rest. And use caution when training individuals newly arrived to hot environments (less than 2 weeks). A yellow flag indicates a WBGT index of 85–87.9 degrees. Recommended precautions include limiting PT or strenuous work outdoors to 30–40 minutes at a time, with 20–30 minutes of rest. Limit outdoor activity for those who aren’t fully heat acclimatized. A red flag indicates a WBGT index of 88–89.9 degrees. Recommended precautions include limiting PT or strenuous work outdoors to 20–30 minutes at a time, with 30–40 minutes of rest. Or move physical training indoors. A black flag indicates a WBGT index of 90 degrees or hotter. Recommended precautions include suspending all PT and training exercises (usually excludes non-training missions).

Learn more about the different branch policies:

While USCG doesn’t describe flag conditions in the manual, individual USCG installations might use them as part of local policy. For example, TRACEN Cape May uses flag conditions to inform outdoor recruit training evolutions.

For more information, read HPRC’s article on unpacking “heat illness,” our blog post on heat acclimatization for military training, and our list of resources for operating in hot environments.


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References

Budd, G. M. (2008). Wet-bulb globe temperature (WBGT)—its history and its limitations. Journal of Science and Medicine in Sport, 11(1), 20–32. doi:10.1016/j.jsams.2007.07.003

Parsons, K. (2006). Heat stress standard ISO 7243 and its global application. Industrial Health, 44(3), 368–379. doi:10.2486/indhealth.44.368

Ready Marine Corps. Emergency preparedness program: Flag conditions. Retrieved April 9, 2020 from https://www.ready.marines.mil/Stay-Informed/Natural-Hazards/Extreme-Heat/Flag-Conditions/