What Is the Heat Index—and How Is That Different from the RealFeel Temperature?

Updated: Jan. 25, 2024

Summer kicked off with record-breaking temps, and more heat is on the way. Here's what you need to know about the heat index and the RealFeel temperature before you head out the door.

If you’re in a dry, arid climate like the Southwest, you might downplay temperatures over 100 degrees by adding, “But it’s a dry heat.” And you wouldn’t be wrong. On the other side of the country, the humidity really does make it feel hotter. Regardless of geography, there’s no question that early July saw record-breaking temperatures across the country and a heat index that left people adjusting their thermostat settings to get some relief.

In fact, it was the earth’s hottest recorded temperature in 125,000 years, according to some scientists and data from the U.S. National Centers for Environmental Prediction. So yeah, the weather is feeling a bit crazy, and we’re just at the beginning of what’s shaping up to be a sweltering summer.

Chances are, you’re wondering how to stay cool without a/c or how to keep your air-conditioned home comfortable without breaking the bank. But there are a few other warm-weather facts to know if you want to stay safe amid high temps this summer. It’s a smart idea to learn about the heat index and the RealFeel temperature—and we have an easy-to-follow explainer for you below.

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What is the heat index?

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The National Weather Service defines the heat index as the apparent temperature, or “what the temperature feels like to the human body when relative humidity is combined with the air temperature.”

While your local weatherman may say the temperature is 90 degrees, it’ll feel hotter or cooler depending on how humid it is. Relative humidity at 80%? It’s going to feel like 113 degrees. With only 10% humidity, though, it’ll feel like 85 degrees.

You might wonder why this matters—and it does matter. The No. 1 reason: The human body can heat up only so much before it reaches a dangerous temperature level. Sometimes staying in the shade isn’t enough, and we need to understand that to avoid heat stroke (or to avoid having our dog experience heat stroke), it’s better to stay in a cool house or skip our afternoon jog. The heat index is a guide that lets us determine how hard we can exert ourselves or what times of day we should stay inside.

Many people don’t realize that heat index calculations are based on readings taken in the shade, and according to the National Weather Service, the heat index value can be up to 15 degrees higher in the sun.

How do you determine the heat index?

To calculate the heat index, you must know the temperature and the relative humidity. You can work it out on your own, and the National Weather Service’s calculator makes that easier.

But there’s an even easier way to find out the heat index. Most weather apps do this for you, including a “feels like” temperature that takes into account the humidity and the wind.

What is the RealFeel temperature?

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The “feels like” temperature you’ll find on a weather app is pretty accurate, but RealFeel goes a few steps further. Created in the 1990s by AccuWeather, RealFeel helps people understand all the components (besides temperature and humidity) that go into what the air feels like by taking into consideration cloud cover, sun intensity, wind speed, precipitation and other factors.

That’s important, especially in dangerous temperatures, because the way the air feels depends on environmental factors. Think about it: The number on your backyard thermometer will read higher if it’s in direct sunlight and lower if it’s in the shade.

You’ll see a similar effect with the thermometer in your car (technically called a thermistor!), which will almost always give a higher-than-accurate reading because of heat radiated off the road. The high-temperature readings from vehicles highlight how the local conditions—the difference between standing on blacktop and grass, for example—impact the way the temperature feels in real time.

How do you determine the RealFeel temperature?

Because RealFeel is exclusive to AccuWeather, which holds the patent, you’ll find RealFeel temperatures on AccuWeather.com as well as the AccuWeather apps—there’s one for iPhones and Android phones.

Which temperature should you pay attention to?

Temperature alone is good for a ballpark, but if you really want to be prepared when you step out the door, focus on the “feels like” temperature on a weather app or the RealFeel temperature on the AccuWeather app in particular.

And keep in mind that while you can’t control the weather, there are ways to make the oppressive heat more bearable and less dangerous. Wearing lightweight, breathable fabrics in light colors will help you keep cool when the sun and heat feel relentless. Packing a reusable water bottle will keep you hydrated. And when all else fails, window air conditioners can keep your home cool when you can’t bear to be outside.

Whether you call it global warming or climate change, it’s arrived, and we need to do what we can to keep ourselves safe, which means paying attention to the heat index and understanding how it impacts our health.