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What Is the Heat Index? What Does the "Feel Like" Temperature Mean?

Updated on April 4, 2013
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Why Does the Heat Index Matter?

Until July, 2012, I dismissed heat index values as made up numbers used by local weathermen to sensationalize their programs. Nothing could be further from the truth! A brush with heat exhaustion that left me ready to vomit and incapable of function for the rest of the day convinced me it's really important to pay attention to those heat index values!

Particularly if you live in a humid area, the heat index can be even more important the thermometer's temperature reading. The heat index temperature doesn't simply "feel like" a number - it is a measure of how effectively your body can exchange heat. Because of this, the heat index is frequently also called the apparent temperature.

If you ignore the heat index value when planning your day's outdoor activities, you could place yourself in danger of heat exhaustion, or even heat stroke. Heat index calculators use a combination of factors, including the temperature, relative humidity, and the dew point, to accurately determine how the weather affects the body's ability to regulate its temperature.

Did you Know?

Each summer, the United States experiences over 1,300 heat-related deaths?

What is Relative Humidity?

People say "It's not the heat, it's the humidity" for a reason. Humidity is a measurement of how much water vapor is present in the air. Warm air is capable of holding more moisture than cool air, so the summer tends to have higher humidity levels than the winter.

The relative humidity indicates the rate of water evaporation at the specific air temperature. When you sweat, your body is trying to use evaporative cooling to reduce your body temperature, but high humidity slows the rate of evaporation and leaves you feeling uncomfortable. Evaporative cooling is also why a quick swim feels fantastic on a hot summer day. Also, think about stepping out of a shower. Even if your room's temperature was comfortable before the shower, you will feel chilly before you towel dry.

Unfortunately, air can only hold so much water. The relative humidity tells you how much of the air's holding capacity is already 'used.' If the temperature were 75 degrees with a relative humidity of 100%, a pool of water that was also 75 degrees would not evaporate. If the relative humidity is 90%, the water evaporates very, very slowly because it, essentially, has nowhere to go. When sweat cannot evaporate off your skin, you are left feeling hot and sticky, even if the thermometer temperature is otherwise pleasant. 75 degrees can feel pretty gross with 90% relative humidity!

Dew Point Table

Temperature, Degrees Fahrenheit
Dew Point, Degrees Fahrenheit
Relative Humidity
85
80
80%
90
80
67%
100
80
47%
110
80
33%

What is the Dew Point

The dew point is related to the relative humidity. The dew point is the temperature at which the air would have 100% relative humidity and atmospheric water vapor would condense into actual water droplets. Calculating it takes some complicated math, but online dew point calculators exist. Because warm air holds more water than cool air, a very hot day with a high dew point can have a lower relative humidity than a cooler, more comfortable day. Look at the table to the right to see how increasing temperatures with the same dew point lead to lower relative humidity.

By the time the dew point reaches 70 degrees, most people are very uncomfortable, but humidity is perceptible even when the dew point is in the upper 60s. "Oppressive" is frequently used to describe days with a dew point in the 70s!

Holmes HPF1010A-NM Outdoor Misting Fan
Holmes HPF1010A-NM Outdoor Misting Fan

Use your patio or porch more often with a large misting fan to keep you cool.

 

The Heat Index

A scientist named George Winterling developed the heat index in 1978 and, just one year later, the National Weather Service adopted it for official use. The actual calculation sounds like a bunch of gibberish unless you are an atmospheric scientist. Suffice it to say the heat index takes the temperature, humidity, and dew point into consideration to figure how effectivly the human body can cool down outside on a given day. To calculate the heat index easily, visit the NOAA website.

The heat index scale was devised to describe the temperature in the shade with a light breeze. Placing yourself in direct sunlight or a lack of breeze can cause the heat index value to rise by up to 15 degrees Fahrenheit.

Temperatures vs Heat Index Values

Click thumbnail to view full-size
Humidity of 49% with a 69 degree dew point means a heat index of 96 for Atlanta - 5 degrees higher than the actual temperature.50% humidity with a 63 degree dew point make Boston feel only 1 degree warmer than its actual temperature.
Humidity of 49% with a 69 degree dew point means a heat index of 96 for Atlanta - 5 degrees higher than the actual temperature.
Humidity of 49% with a 69 degree dew point means a heat index of 96 for Atlanta - 5 degrees higher than the actual temperature. | Source
50% humidity with a 63 degree dew point make Boston feel only 1 degree warmer than its actual temperature.
50% humidity with a 63 degree dew point make Boston feel only 1 degree warmer than its actual temperature. | Source

Heat exhaustion can 'build' in your body. If you are exposed to heat for several days in a row, you can slowly become increasingly dehydrated from day to day. Heat exhaustion can catch up to you days later, even if the temperature has dropped slightly. Protect yourself by staying hydrated!

How to Use the Heat Index to Stay Safe

According to NOAA, a heat index of 105 or more means you need to limit outdoor exposure and activity, as much as possible. High heat index values, regardless of the actual temperature, can lead to heat exhaustion, and even heat stroke. Sunburn reduces your body's ability to lose heat, so wearing sunscreen can actually help you stay cool by preventing burns. Because your body's reaction to extreme heat changes as you age, a teenager could feel fine but someone over 40 could suffer heat stroke under the same conditions.

If you go outside in the heat, make sure to monitor yourself and others in your group for signs of heat exhaustion. Symptoms of heat exhaustion include: heavy sweating, nausea, cramping, headache, fatigue, and more. To see a comprehensive list of symptoms, visit the Mayo Clinic website. In a first aid class, I learned you should move a person displaying these symptoms to the shade or an air conditioned building, loosen their clothing, fan them, and give them cool (but not cold) water to drink. Do not give them anything alcoholic or with caffeine - both alcohol and caffeine are diuretics and stimulate urine production, which causes further dehydration. If the person does not visibly improve, or his/her condition deteriorates, call 911 before heat exhaustion becomes potentially deadly heat stroke. Heat stroke can lead to seizures and even death.

Heat injury and illness can have lifelong consequences. Even after cooling down and rehydrating, many people report sensitivity to lights, particularly sunlight, and smells for weeks after suffering heat exhaustion. More severe heat stroke can leave you sensitive to heat and humidity for years to come. Due to a heat stroke seizure many years ago, one individual I know cannot do many of her favorite outdoor activities for any length of time because direct sun exposure for more than 10 minutes leaves her with crippling headaches for days to come.

Don't fall into the trap of only looking at the big temperature on the weather page with the temperature reading. Take a moment to look at the dew point and the heat index to plan your day - it could literally save your life.

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    • profile image

      joanwz 4 years ago

      Really great information and laid out in an easy to read format. Great job.

    • twinstimes2 profile image

      Karen Lackey 4 years ago from Ohio

      Very informative and easy to read. I learned quite a bit. Our heat index yesterday was 105. Maybe that is why it felt and was so hot!

    • jellygator profile image

      jellygator 4 years ago from USA

      Great information. Even after suffering a significant heat injury, I've only recently been learning more about the index and how it works.

    • Natashalh profile image
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      Natasha 4 years ago from Hawaii

      I really did think it was fake for a long time. "Feels like" sounds so subjective! I really prefer calling it the apparent temperature - it sounds more real that way. Thanks, everyone, for stopping by! I hope you enjoyed it and can use this information to stay safe.

    • teaches12345 profile image

      Dianna Mendez 4 years ago

      Great information to have and one that will keep people safe when venturing outdoors this summer. In Florida, humidity is always a factor, but the number of people who get skin cancer here is higher than most US states. I like your safety advice and hope people take note! Well designed hub.

    • Natashalh profile image
      Author

      Natasha 4 years ago from Hawaii

      I'm not surprised there are higher skin cancer rates there. As you near the equator, the UB factor increases. That's why you can burn so horribly in the USVI even though the weather feels fantastic.

      Thanks for stopping by and commenting, Teaches, and I appreciate the complement.

    • CyclingFitness profile image

      Liam Hallam 4 years ago from Nottingham UK

      Its not a scale we hear a lot of in the uk as salt our weather doesn't tend to go to extremes except on rare occasions but this a great well plotted info. Thanks for share cf

    • Natashalh profile image
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      Natasha 4 years ago from Hawaii

      Strangely, Canada and the US actually use slightly different versions of the formula I didn't realize it isn't really used in the UK, but that kind of makes sense. Thanks for stopping by!

    • wilderness profile image

      Dan Harmon 4 years ago from Boise, Idaho

      I never knew that sunburn decrease the cooling ability of the body, but it makes total sense. Even after cooling down, the sunburned area is hot and won't cool off.

    • Natashalh profile image
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      Natasha 4 years ago from Hawaii

      I had no idea, either, until my boss mentioned it the other day. I looked it up and, sure enough! Your burned skin just can't shed heat the way it should.

    • Vinaya Ghimire profile image

      Vinaya Ghimire 4 years ago from Nepal

      In the town I'm staying at the moment, there is awful heat wave. Thanks for this comprehensive article.

    • Natashalh profile image
      Author

      Natasha 4 years ago from Hawaii

      Stay safe! I'm lucky, it's been cooler for me than elsewhere in the country. It's still been really hot, though. I actually got some heat exhaustion last week, which is what inspired me to research the heat index a little more.

    • Winsome profile image

      Winsome 4 years ago from Southern California by way of Texas

      Hi Natasha, thanks for explaining that the way I feel outside--hotter or cooler than my thermometer reads--is an old fashioned way of measuring and I can save myself the trouble of walking outside by using the heat index. I saved a lot of time with rain too, I just follow the prediction rather than looking out the window or noticing I am wetter than usual when outside. The other day I was sopping wet but there was no prediction of rain so I left my umbrella in the car. It's amazing what these scientists can come up with to make our lives safe and predictable.

      Of course you know I'm just messin' with you, but seriously, the other day I was going to write a song for the open mic I go to and I thought of the phrase "its not the heat, its the humidity" and decided to challenge myself to write a song based on it. I did and it was not bad. The chorus goes like this:

      " I said it didn’t matter

      I said I didn’t mind

      Call me any name you want

      The worst that you can find

      Your tears were the reason

      That I had to leave

      I said It’s not the heat Babe…..

      It’s the humidity"

      Copyright Winsome Publishing 2012

      =:)

    • Natashalh profile image
      Author

      Natasha 4 years ago from Hawaii

      Yep, I pretty much ignore the weather forcast when it comes to rain. If it looks and feels like rain, I plan for it. If the weather forcast calls for rain but it looks clear to me, I don't stress.

      A whole song based on a simple phrase - wow! Thanks for sharing this part of it.

    • billybuc profile image

      Bill Holland 4 years ago from Olympia, WA

      That was interesting. Since I taught science I'm aware of the different things like humidity, but I didn't know how they determined the heat index. Thanks for educating this old teacher. You live in a brutal part of the country during the summer. The humidity is off the charts. My one visit to D.C. and Virginia, I thought I would feint several times.

      Great hub Natasha!

    • Natashalh profile image
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      Natasha 4 years ago from Hawaii

      It certainly is brutal in the southeast during the summer! At least I'm in a costal location. The inland states are even worse. Once summer I went to Huntsville, Alabama and even I thought it was too hot!

    • BlissfulWriter profile image

      BlissfulWriter 4 years ago

      Typically, climate is more comfortable when living near water. That is why population center congregate in area near large body of water.

    • Natashalh profile image
      Author

      Natasha 4 years ago from Hawaii

      Absolutely. Being near the coast makes all the difference! And water temperature is slower to change than air temperature, so bodies of water have a moderating effect on the climate.

    • phdast7 profile image

      Theresa Ast 4 years ago from Atlanta, Georgia

      Natahsa - What a great and information packed hub. Very interesting and very well done. Sharing,

    • Natashalh profile image
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      Natasha 4 years ago from Hawaii

      Thank you, phdast.

    • CriticalMessage profile image

      Murphy 4 years ago from Chicagoland, Illinois

      Greatly appreciating this... *smiles*

    • Natashalh profile image
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      Natasha 4 years ago from Hawaii

      Thank you, CriticalMessage.

    • penlady profile image

      penlady 4 years ago from Sacramento, CA

      Information definitely worth knowing for these hot summer months. Voted up and useful.

    • Natashalh profile image
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      Natasha 4 years ago from Hawaii

      Thank you, penlady.

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