Real Feel:
The Heat Index Explained

We hear the question often from you – what is heat index?  Is it for real?  In recent broadcasts, we’ve been discussing the heat index, also known as the “feels like” temperature, and it seems many of you have questions about what this means and how it’s determined. Some believe it’s malarkey, while others are simply confused. Let’s clear up the confusion!

What Is The Heat Index?

The heat index is a measure that combines air temperature and relative humidity to indicate what the temperature feels like to the human body. It’s designed to provide a better understanding of how hot it truly feels when humidity is factored in, helping people make informed decisions about outdoor activities and health precautions.

How Is The Heat Index Determined?

The heat index is calculated using a formula that incorporates both the actual air temperature and the relative humidity. When humidity levels are high, the body’s ability to cool itself through evaporation of sweat is diminished. This makes it feel much hotter than the actual air temperature.

Matt Noyes’ Explanation: “Our body cools by sweating…but it’s actually the process of that sweat evaporating off our skin that cools us. Evaporation is a cooling process. So, on a very humid day, the sweat doesn’t evaporate nearly as quickly from our skin – it clings to us. As a result, our bodies can’t cool as quickly. So, the heat index essentially says, ‘if the humidity were at a more normal level, it might as well be 105° – because with all the humidity in the air today, our bodies can only cool as well as we could if it was 105° with normal humidity.’”

What Does The Heat Index Mean?

 

The heat index helps us understand the combined effect of heat and humidity on our bodies. Here’s a breakdown of what different heat index values indicate:

  • 80-90°F: Caution – Fatigue possible with prolonged exposure and activity.
  • 90-103°F: Extreme Caution – Heat cramps and heat exhaustion possible.
  • 103-124°F: Danger – Heat cramps and heat exhaustion likely; heatstroke possible with prolonged exposure and activity.
  • 125°F and higher: Extreme Danger – Heatstroke highly likely.

Application In Our Daily Life

Understanding the heat index is crucial for making safe choices during hot and humid weather. Here are some practical applications:

  1. Stay Hydrated: Drink plenty of water to help your body stay cool.
  2. Limit Outdoor Activities: Avoid strenuous activities during peak heat hours, typically from late morning to early evening.
  3. Wear Appropriate Clothing: Light-colored, loose-fitting clothes can help keep you cool.
  4. Take Breaks: If you must be outside, take frequent breaks in the shade or indoors.
  5. Monitor Vulnerable Individuals: Pay extra attention to children, the elderly, and those with preexisting health conditions, as they are more susceptible to heat-related illnesses.

By understanding and respecting the heat index, we can better protect ourselves and our loved ones from the dangers of extreme heat.

Heat Index By The Numbers

For those of you who enjoy the science behind the weather, here’s a look at the equation used to calculate the heat index, though you might enjoy reading the full rundown on the equation by clicking here. The formula involves several variables, including temperature and humidity, and is as follows:

Heat Index (HI) = -42.379 + 2.04901523T + 10.14333127R – 0.22475541TR – 6.83783 × 10^-3T^2 – 5.481717 × 10^-2R^2 + 1.22874 × 10^-3T^2R + 8.5282 × 10^-4TR^2 – 1.99 × 10^-6T^2R^2

where:

  • T = Temperature in degrees Fahrenheit
  • R = Relative Humidity (as a whole number percentage)

This complex formula accounts for the non-linear effects of temperature and humidity on the perceived temperature, giving us the heat index value that better represents what our bodies experience.

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