Heat index is a measurement of how hot it really feels outside. The heat index formula relies on environmental data including the ambient air temperature and relative humidity, and is only calculated when temperatures exceed 80 degrees Fahrenheit (27 degrees Celsius).
The heat alerts initiated by the National Weather Service are mainly based on heat index values. Excessive heat can be a safety and health hazard. For an example of heat index, let's say your weather station records an outdoor temperature of 90 degrees Fahrenheit (32.2 degrees Celsius) and a humidity reading of 60%. The temperature at which these two values intersect is the heat index. In this example, it will feel like it is 100 degrees Fahrenheit (38 degrees Celsius) outside.