What is Heat Index?

Summer sun with lens flare
Posted in: Weather 101

What is Heat Index?

Weather conditions are more than just hot or cold, dry or wet, windy or still.

With all of the different conditions interacting with each other, there are many factors that affect how you actually “feel” when you’re outside. Perhaps more importantly, it’s crucial to know what conditions, when combined, can create a situation where your health and safety are at risk.

A great example is the old adage “it’s not the heat, it’s the humidity.” A person who is suffering on a dry, but 110-degree day in Arizona might scoff at this saying, but there is some truth to it. Specifically, moisture in the air reduces your body’s ability to cool itself through sweating. So, the higher the humidity, the hotter it feels. And in extreme cases of high heat and humidity, the safety of you and your family can become a consideration.

It’s because of this phenomenon that the “heat index” calculation was invented.

Heat index is based on any temperature of 80 degrees Fahrenheit (27 degrees Celsius) or above, combined with the relative humidity (%RH). Simply put, an 80-degree day will feel like 80 degrees when there is 40% humidity. But as the moisture level increases, the temperature you feel will also rise.

For example, on an 80-degree day, a 60% relative humidity will create a heat index of 82 degrees Fahrenheit (28 degrees Celsius). Increase that to 80% relative humidity, and the perceived temperature rises to 84 (29 degrees Celsius).

As you can see, at cooler temperatures, humidity isn’t that large of a factor. But when it’s very hot outside, humidity can really make a big difference in how you feel. For example, on a 100-degree Fahrenheit day (38 degrees Celsius), a relatively low 40% humidity already raises the heat index to 109 (42 degrees Celsius). At 55% humidity, you’re feeling a scorching (and dangerous) heat index of 124 degrees Fahrenheit (51 degrees Celsius).

To get a better idea of how heat index works, the National Weather Service published this very helpful Heat Index Calculator.

To help you stay safe and comfortable, many weather products track humidity and provide heat index calculations among their host of helpful insights.

What is Heat Index Used For?

Calculations of heat index have two primary applications:

  • Weather Observation - One of the joys of owning a weather station is keeping track of the weather, and trying to predict what the weather will do next. During the summer months, it's essential to monitor and track heat index.
  • Better Understanding Outdoor Conditions - Heat index offers insight as to how it feels outside, which helps you get a sense of how heat and humidity interact.

Why Is It Beneficial to Know Heat Index?

Heat index calculations help you better prepare yourself before going outside:

  • Comfort - If you’re just going on temperature alone, you don’t have an accurate idea of how you will likely feel when you’re outside. The heat index calculation can help you better decide clothing and activities that will keep you feeling cool and comfortable.
  • Safety - When dangerous temperatures with extreme humidity are present, the more you’re at risk of heatstroke, heat exhaustion, and other serious problems. Knowing the heat index level will help you make more informed decisions better for properly gearing up, or making the decision to reschedule an outdoor activity.

AcuRite offers a wide variety of weather stations that calculate heat index.

  • Personal Weather Stations - AcuRite weather stations measure temperature and humidity in your exact location, with select models providing reliable heat index calculations and alerts, based on temperature monitoring right in your back yard.
  • My AcuRite Platform - Any AcuRite weather sensor can also stream temperature and humidity readings through the AcuRite Access™ to the My AcuRite platform. When temperatures rise above 80 degrees, My AcuRite gives you convenient access to heat index calculations on mobile devices like your smartphone, tablet, and preferred web browser.

References and Resources

What is the Heat Index and Why is it Used? – Weather.com

What is the heat index? – National Weather Service

Effects of Humidity on Your Body – UPMC HealthBeat

Heat Index Calculator – National Oceanic and Atmospheric Administration

April 5, 2017
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