Heat Index Chart and Tips on Heat Safety
When summer heats up, most folks look at the temperature. However, a better indicator is the “heat index,” 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.
–Weather proverb
What is the Heat Index?
Ever wondered why it feels so much hotter than the thermometer reads? That’s called humidity.
Read Next
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 “felt air temperature.”
- For example, the heat you feel when the actual temperature is 90ºF with a relative humidity of 70% is 105ºF.
Sometimes, when it’s extremely humid, it doesn’t 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’s 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’s fairly easy to follow. For instance, if the air temperature is 96°F and the relative humidity is 65%, the heat index—how hot it feels—is 121°F. The Heat Index assumes shady conditions with a light wind since it’s 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.
NOAA also has a Heat Index calculator.
The National Weather Service will issue alerts when the Heat Index is expected to exceed 105° to 110°F (depending on local climate) for at least 2 consecutive days.
Extreme heat strikes for a number of reasons. El Niño, 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’t wait until you’re thirsty to drink. Bring a reusable water bottle with you to maintain hydration. If you’re at work, you could bring a drinking glass and pitcher and fill it every morning.
- If you’re 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’t 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’t 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 “broad spectrum” 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’s 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 NOAA Website.
Catherine Boeckmann
ADVERTISEMENT
07/15/2024, in our area of Indiana we had 88 degrees with 76% humidity, along with 78 dewpoint & a "Feels like" temp of 100. How does the Dewpoint figure into this equation or was just another stat that the Weather Channel threw into the mix?
If you have the temperature and relative humidity, then you don’t need the dew point to find the heat index. However, you can find calculators for the heat index that just use the temperature and dew point. That’s because the three factors are interrelated.
The dew point is the temperature at which the air would need to be cooled, at constant pressure, so that the relative humidity would be 100%; the dew point can not be higher than the actual temperature. If the temperature were to cool below the dew point, then the water in the air in gas form (vapor) would condense into liquid, such as via rain or fog.
Warmer air can hold more moisture before it becomes saturated. A higher dew point means that there is more moisture in the air than there would be at a lower dew point. A dew point above 75F can feel oppressively muggy.
Relative humidity is the ratio of the amount of moisture present in the air relative to the amount that would be present if the air were completely saturated. It is dependent on dew point and temperature.
Relative humidity at 100% will feel less humid if the dew point temperature is 30F, for example, compared to, say, 60F.
So, it sounds like the weather report gave an extra statistic to help viewers to know the entire picture of how hot and muggy it was outside that day, although all of those stats are related.
In scrutinizing my email, it seemed to be discombobulated. Seems I mentioned in the TX winter "Regs wouldn't allow us to remove our fatigue tops. We often connected an AC unit hose into the B-52 cockpit and get relief."
Hope you figured out it was in the summer heat, not crazy to do so in winter. Sorry and glad I caught it.
Heat and cold extremes are interesting to study and be aware of. I've been a info sucker-upper many years and this is interesting. I've been at both extremes.
For the heat) Joined the Air Force 9/9/1958 and Basic Training at Lackland AFB in San Antonio. It was a hot summer in that region. They had us take salt tablets very often. At 5am in full dress at 5amon the street it was still in the 90's. But that's what was the "protection" at that time. At Shepard AFB in Wichita Falls TX it would "cool off" into the 90's at night. Opened the screenless windows mosquitoes the size of hornets would attack you. There was no AC in either places. Assigned to SAC and in that Texas summer it's been 123 deg in the shade. The sun reflected off the concrete flight line, a double whammy direct and reflected. Sheppard also in winter often was in the 10's and windy, nothing to stop the wind across the plains. Regs wouldn't allow us to remove our fatigue tops. We often connected an AC unit hose into the B-52 cockpit and get relief.
Believe the tabulation on the charts. I have sought and calculated the data and made my own charts. Discovered the data points when plotted form a tangential curve and do for each increment of degrees. If a temperature is in between interpolate. Hard to know any wind speed unless you have that instrument, which I had at one time. The curves vary at various points of temp & either humidity (heat index) or wind speeds (wind chill).
Parking airplanes on Alton Bay NH in February during our fly-ins on the ice, (FAA sanctioned runway) Google it for videos) I've been in 40 below temps all day with at least 40 mph winds, t'was chilly. Got use to it and didn't seek warmth. The flying club (Alton Bay Flying Club) has the fly-ins each February, conditions being right. Pilots come from MI, VA and in between to experience landing on ice, I've done it. Mentioned to inform, the cold isn't so bad. This informs about the cold side of temperatures.
I am looking for how many 100 or above days in a row we have had in the area this summer also how many we have had this year. We are located in the DFW area TX
Cowell, You can check the daily Weather History tool for temps last year; it's free. There's also a Customized Weather History tool for a small fee which makes it easier if you wish to pull multiple days and look at trends. http://www.almanac.com/weather/history
