When areas of the U.S. experience extreme heat, the National Oceanic and Atmospheric Administration, along with the National Weather Service and meteorologists produce heat watches, advisories and warnings so populations living in areas affected by the heat will not be harmed.
The NOAA and the NWS both use a heat index, which measures how hot it feels to someone--in degrees Fahrenheit--along with the relative humidity forecast. For instance, if the air temperature is 100 degrees Fahrenheit and the relative humidity is 50 percent, the heat index is 118 degrees Fahrenheit, which is dangerous. In contrast, if the air temperature is 80 degrees Fahrenheit and the relative humidity is 100 percent, the heat index is 87 degrees Fahrenheit.
When you add exposure to full sunshine and windy conditions, heat index values increase to 154 degrees Fahrenheit--this is classified as “extreme danger.”
Image: Man and Fan FDP Credit graur razvan ionut: http://www.freedigitalphotos.net/images/view_photog.php?photogid=987
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