In some cases, the combination of heat and humidity (what creates the 'heat index' ) is driving the index to over 105-degrees. At that point, the National Weather Service issues a heat advisory.
A heat advisory is issued within 12 hours of the onset of the following conditions: heat index ('feels like ' temperature) of at least 105°F but less than 115°F for less than 3 hours per day, or nighttime lows above 80°F for 2 consecutive days.
In this weekend's case for the Nation's heartland and for some areas in North Carolina, we're dealing with about a 2-day threat of taxing temperatures.
According to the National Weather Service, heat kills by taxing the human body beyond its abilities. In a normal year, about 175 Americans succumb to the demands of summer heat. Human bodies dissipate heat by varying the rate and depth of blood circulation, by losing water through the skin and sweat glands, and as a last resort, by panting, when blood is heated above 98.6°F. Sweating, by itself, does nothing to cool the body, unless the water is removed by evaporation. High relative humidity retards evaporation.
Heat disorders generally have to do with a reduction or collapse of the body's ability to shed heat by circulatory changes and sweating. When heat gain exceeds the level the body can remove, the temperature of the body's inner core begins to rise and heat related illnesses may develop.
As a result, the "Heat Index" (HI) has been developed. The HI is the temperature the body feels when heat and humidity are combined. The table below correlates the actual temperature and relative humidity, producing a HI. (This chart is based upon shady, light wind conditions. Exposure to full sunshine can increase the HI by up to 15°F.)
Here's the basics of what you need to know:
(courtesy: NWS Louisville)
Any value less than 80 is considered comfortable.
Any value greater than 90 is considered extreme.
Any value greater than 100 is considered hazardous.
Any value greater than 110 is considered dangerous.
When the heat index is forecast to be:
80°F to 90°F : Fatigue is possible with prolonged exposure and physical activity.
90°F to 105°F : Sunstroke, heat cramps and heat exhaustion is possible.
105°F to 130°F : Sunstroke, heat cramps, and heat exhaustion are likely, and
heatstroke possible.
and while we're not expecting heat indices of this magnitude, I still included the top of the scale:
130°F or greater : Heat stroke highly likely with continued exposure.
You already know about keeping cool. The basics for dealing with the heat include drinking plenty of fluids…stay in an air-conditioned room…stay out of the sun…and check up on relatives and neighbors. Bring pets indoors and provide plenty of water for them as well.
Here's also a couple of links with more information:
http://www.bt.cdc.gov/disasters/extremeheat/faq.asp
http://www.nia.nih.gov/HealthInformation/Publications/hyperthermia.htm
Your Weather on the Ones forecast center will keep you up to date if any advisories are issued or extended for your area. Your latest 7-day forecast for North Carolina is available at news14.com
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