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The heat index—sometimes called the apparent temperature—is what the temperature feels like to your body when you consider relative humidity in relation to the air temperature. In other words, if the temperature outside is 90 degrees Fahrenheit (32 degrees Celsius) (and the humidity is 60%, it’s going to feel like it is 100 degrees Fahrenheit (37 degrees Celsius) (or warmer if you’re in the sun).
The National Weather Service uses the heat index to warn people of extreme conditions so they can take precautions and stay safe. When the humidity is high and the temperature is high, it can be hard for the body to cool itself and the risk of heat-related illnesses increases.
Here’s what you need to know about the heat index, how to determine it, and how to stay safe when it’s high.
If you’re like most people, you probably look at the temperature when checking the weather for the day. However, that only gives you an idea of the air temperature outside. If you factor in the relative humidity as well, you will get the heat index—what the temperature or the air outside actually feels like to your body.
Interestingly, that number represents what it would feel like outside in the shade. If you’re in the direct sun, add about 15 degrees Fahrenheit (-9 degrees Celsius) to that number. In other words, if the heat index is 100, it will feel like 115 degrees Fahrenheit (46 degrees Celsius) in the sun.
Knowing the difference between the air temperature and the heat index—and how it can affect your body—can go a long way in helping keep you safe. For instance, a high heat index is more dangerous for your body because it is harder to cool down.
A high heat index means it feels hotter outside than the thermometer might say. It also means it’s harder for sweat to evaporate and cool your body. This puts you at risk for a number of different heat-related illnesses.
As global warming continues, the high heat index will continue. One team of researchers predicts that people across the globe will experience an increase in heat indexes up to 10 times more than what they are now. By 2100, people living in the tropics will be exposed to extremely high heat indexes most days of the year.
When the heat index is high, it is difficult for your body to function as it should, and it can lose its ability to regulate its internal temperature. This can lead to everything from heat cramps and dehydration to heat exhaustion and heatstroke. A high heat index can also worsen existing medical conditions like heart disease, diabetes, and respiratory diseases.
It can also be particularly risky for children, pregnant people, and older adults, especially because their bodies are less able to regulate their internal body temperatures. People who work outside, those who live in urban areas, and those below the federal poverty line who are less likely to have access to air conditioning are also at greater risk.
Here is an overview of what can happen to your body as the heat index rises, according to the National Weather Service:
| Heat Index | Health Effects |
|---|---|
| 80-90° F | Fatigue possible |
| 90-105° F | Heat cramps, heat exhaustion, or heat stroke possible |
| 105-130° F | Heat cramps or heat exhaustion likely, heat stroke possible |
| 130° F or higher | Heat stroke highly likely |
When calculating the heat index, you must know the air temperature and the relative humidity. You can use the following formula to calculate the heat index. (R represents humidity, and T represents temperature in Fahrenheit.)
Here is the equation:
Heat Index = -42.379 + 2.04901523T + 10.14333127R – 0.22475541TR – 6.83783 x 10-3T2 – 5.481717 x 10-2R2 + 1.22874 x 10-3T2R + 8.5282 x 10-4TR2 – 1.99 x 10-6T2R2
There is also a chart on the National Weather Service website that will show the heat index if you know both the air temperature and the relative humidity. Alternatively, you can plug that data into the calculator on the National Weather Service’s Weather Prediction Center, which will calculate the heat index for you.
According to the American Red Cross, extreme heat causes more deaths than any other weather event—including hurricanes and tornadoes. For this reason, it’s important to prepare for extreme heat and know what to do when it arrives.
Here are some tips on how to stay safe in the heat.
What To Do Before the Heat Increases
Before warmer weather arrives, it’s important to have a plan for staying cool and preventing heat-related illnesses. For instance, if you do not have air conditioning, you need to know where you will go to cool off. You can’t rely on electric fans in extreme heat because they do not cool your body down. Taking a cool shower or bath or moving to an air-conditioned place is a much better way to cool off.
If you do not have air conditioning or if your air conditioner is not working, consider finding places where you can go to cool off, such as a public library, a shopping mall, a grocery store, or a public cooling center. Plan how you will get there and ensure you have plenty of drinking water for everyone in your family.
What To Do Once the Heat Arrives
You need to stay safe on days when the heat index is particularly high or when there is a heat advisory or warning.
Here are some things you can do to prevent heat-related illnesses:
- Drink plenty of water
- Stay in an air-conditioned place as much as possible or take cool showers
- Try not to rely on electric fans to keep you cool
- Go to your designated air-conditioned place (like the library) if you don’t have air conditioning
- Limit the use of your stove or oven
- Wear lightweight, loose clothing
- Limit your time outside, whether for work or for play
- Avoid sugary drinks
Nearly 1,220 people in the United States die because of extreme heat every year, according to the Centers for Disease Control and Prevention (CDC). Knowing how to stay safe in the heat and what to do if you or someone else has a heat-related illness can help prevent severe conditions and complications.
Here are some signs that you, your friend, or your loved one should call 911 right away or go to the nearest emergency room:
- Cool, moist, pale skin
- Rapid pulse
- Elevated or lowered blood pressure
- High body temperature
- Nausea or vomiting
- Fainting or loss of consciousness
- Shallow breathing
If the person is in late-stage heat stroke and has hot, dry skin and a temperature of 104 degrees Fahrenheit (40 degrees Celsius) or more, try to cool their body down with cool compresses or ice packs on the neck, wrists, and ankles, as well as in the groin and armpits. Also, be prepared to perform CPR if they lose consciousness while waiting for emergency medical assistance to arrive.
The National Weather Service uses the heat index to tell you what the temperature outside feels like. This number is calculated using the air temperature and relative humidity. Knowing this number can help you take precautions and stay safe in extreme weather, especially since a high heat index could increase your risk for a heat-related illness.
If you or a loved one does become ill from the heat and shows signs of distress, get immediate medical attention. More people die from extreme heat than any other weather-related event.