National Weather Service Heat Terms
A period of abnormally and uncomfortably hot and unusually humid weather. Typically a heat wave lasts two or more days.
Excessive Heat Watch
Issued by the National Weather Service when heat indices in excess of 105ºF (41ºC) during the day, combined with nighttime low temperatures of 80ºF (27ºC) or higher, are forecast to occur for two consecutive days.
Issued within 12 hours of the onset of the following conditions: heat index of at least 105°F but less than 115°F for less than three hours per day, or nighttime lows above 80°F for two consecutive days.
Excessive Heat Warning
Issued within 12 hours of the onset of the following criteria: heat index of at least 105°F for more than three hours per day for two consecutive days, or heat index more than 115°F for any period of time.
The heat index is an apparent temperature, or a measure of how hot it feels when temperature and humidity are combined. The heat index is the mostly commonly used method of measuring heat. It is the result of extensive biometeorological studies, taking into account many parameters, including body size and shape, core and body surface temperatures, clothing and the skin's resistance to heat and moisture transfer. The heat index values used in National Weather Service forecasts and warnings and in heat index tables assume an average size adult with clothing, in the shade, with a 5-mph wind. Being in full sun or in an area with no air movement can significantly increase the apparent temperature.