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What Is the Heat Index, and Why Does It Matter?

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Kylee Monroe

Heat Index Chart and Tips on Heat Safety

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When summer heats up, most folks look at the temperature. However, a better indicator is the 鈥渉eat index,鈥 which also considers humidity. Heat is the number one weather-related killer in the United States. Check out the heat index chart to keep safe this summer.

As the days begin to shorten,
the heat begins to scorch them.
鈥撁厶伊等 proverb

What is the Heat Index?

Ever wondered why it feels so much hotter than the thermometer reads? That鈥檚 called humidity.

The Heat Index is closer to how it really feels outside, since it measures not only temperature, but also how hot it feels when humidity is factored in with the actual air temperature. The Heat Index also called the 鈥渇elt air temperature.鈥 

  • For example, the heat you feel when the actual temperature is 90潞F with a relative humidity of 70% is 105潞F. 

Sometimes, when it鈥檚 extremely humid, it doesn鈥檛 need to be that hot outside for the weather to be harmful. Dry heat can also be dangerous because the body can only handle so much heat. The combination of high temperature and high humidity reduces your body鈥檚 ability to cool itself.

Heat is the top weather-related killer, causing more fatalities than lightning, tornadoes, and hurricanes combined. 

Heat Index Chart

NOAA (National Oceanic and Atmospheric Administration) will issue heat alerts based on Heat Index Values. Pay attention to heat-related watches and warnings when they are in effect.

The math equation is very long, so the NOAA created a simple chart (below). It鈥檚 fairly easy to follow. For instance, if the air temperature is 96掳F and the relative humidity is 65%, the heat index鈥攈ow hot it feels鈥攊s 121掳F.  The Heat Index assumes shady conditions with a light wind since it鈥檚 important to be in the shade during high heat.

When the index measures anywhere from 103 to 125 degrees Fahrenheit, experts label it as dangerous heat. The darkest grey area below indicates extreme danger. Such temperatures carry a higher risk for heat stroke, exhaustion, and heat disorders.

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The National 蜜桃恋人 Service will issue alerts when the Heat Index is expected to exceed 105掳 to 110掳F (depending on local climate) for at least 2 consecutive days.

 

heat index chart

Extreme heat strikes for a number of reasons. El Ni帽o, a Pacific weather pattern, increases global temperatures. A warming climate is another reason for the rising heat. Over the past 30 years, summer temperatures have increased.

To practice heat safety, review this checklist:

What To Do When Heat Index is High

Drink More Fluids

  • Drink more fluids, regardless of your activity level. Don鈥檛 wait until you鈥檙e thirsty to drink. Bring a reusable water bottle with you to maintain hydration. If you鈥檙e at work, you could bring a drinking glass and pitcher and fill it every morning.
  • If you鈥檙e older, be especially careful to drink fluids because the amount of water retained by the body decreases with age.
  • Avoid salt tablets, if possible. If your doctor has you on water pills, ask him or her how much you should drink.
  • Don鈥檛 drink liquids with dehydrating caffeine, alcohol, or large amounts of sugar. Avoid very cold drinks.
  • At family events and reunions, bring a cooler of cool drinks.
  • Cut back on exercising on hot days; if you must exercise, drink two to four glasses of cool, nonalcoholic fluids each hour.

Keep Your Body Cool

  • Dress in lightweight, light-colored clothing and sun-reflective shirts. Wear loose clothing.
  • Wear wide-brimmed hats and sunglasses. Keep a stash of hats on hand for friends and family!
  • According to the Centers for Disease Control and Prevention (CDC), fans may provide comfort, but they do not prevent heat-related illnesses when the temperature is in the high 90s. Take a cool shower or bath, or move to an air-conditioned place to cool off.
  • Stay inside, or stay in the shade, especially between 11 A.M. and 3 P.M.
  • Avoid outdoor exercise during the heat of the day.
  • Never leave anyone in a parked vehicle with the windows closed or open.
  • To feel cooler, eat cooler. Reduce your protein intake.
  • Open your house to the breeze after sunset and leave it open until dawn.
  • In the early morning, draw the shades before the temperature starts to rise.

Protect Yourself From the Sun

  • Avoid direct sunlight when the heat index is high.
  • Don鈥檛 get sunburnt, which makes it difficult for your body to dissipate heat.
  • In addition to a hat and sunglasses, wear sunscreen SPF 15 or higher. The most effective products say 鈥渂road spectrum鈥 or UVA/UVB on the label.

Stay Alert

  • Be aware of heavy sweating, hot and dry skin, rapid pulse, pale or clammy skin, and/or cramps in your leg or abdomen muscles. All could be signs of heat disorder.
  • If you are older, it鈥檚 wise to keep medical information readily available, including phone numbers of health care providers and copies of your prescription and health insurance cards.

To see guidelines on how to respond to heat disorder symptoms, go to the .

About The Author

Catherine Boeckmann

Catherine Boeckmann loves nature, stargazing, and gardening so it鈥檚 not surprising that she and The Old Farmer鈥檚 蜜桃恋人 found each other. She leads digital content for the 蜜桃恋人 website, and is also a certified master gardener in the state of Indiana. Read More from Catherine Boeckmann