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.
Learn more about the different branch policies:
- Army TB MED 507/AFPAM 48-152 (I): Heat Stress Control and Heat Casualty Management
- Navy NAVMED-P-5010-3: Prevention of Heat and Cold Stress Injuries (Ashore, Afloat, and Ground Forces)
- Air Force Instruction 48-151: Aerospace Medicine Thermal Injury Prevention Program
- Marine Corps NAVMED P-5010-9: Preventive Medicine for Ground Forces and NAVMED P-5010-3 as described in MARADMIN 111/15: Marine Corps Heat and Cold Stress Injury Prevention Program
- Coast Guard COMDTINST M5100.47C: Safety and Environmental Health Manual
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.