Excessive heat warning

An Excessive Heat Warning is issued by the National Weather Service of the United States within 12 hours of the heat index reaching one of two criteria levels. In most areas, a warning will be issued if there is a heat index of at least 105 °F for more than 3 hours per day for 2 consecutive days, or if the heat index is greater than 115 °F for any period of time. Note that local offices, particularly those where excessive heat is less frequent or in areas with deserts or mountainous terrain, often have their own criteria. High values of the heat index are caused by temperatures being significantly above normal and high humidities, and such high levels can pose a threat to human life through conditions such as heat stroke, Heat exhaustion, and other heat-related illnesses.

Due to the extreme illnesses that can occur, athletes should limit the amount of physical activity that they participate in when an excessive heat warning is issued. According to the National High School Sports-Related Injury Surveillance System, High School RIO, an estimated number of 51,943 excessive heat illnesses occurred in nine of the high school sports observed between 2005 and 2011.

The following is an example of an Excessive Heat Warning issued by the National Weather Service office in Wilmington, Ohio on July 20, 2011 during a severe heat wave.

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