what are black flag conditions

Black flag conditions are a type of heat warning system used by the U.S. military and other organizations to indicate extreme heat conditions. When the outdoor temperature exceeds 90℉, a black flag is in effect. Black flag conditions are characterized by a Wet Bulb Globe Temperature Index (WBGTI) of 90 degrees Fahrenheit or above. During black flag conditions, all nonessential physical activity is halted. The U.S. military uses five flag conditions: no flag, green, yellow, red, and black. The black flag is the most severe warning and indicates that the heat is at a dangerous level and poses a significant risk to health and safety.