What Is the Heat Index?

When the summer heat intensifies, all eyes are focused on the temperature. But often, experts say, it’s the heat index that should draw more concern.

While it’s the temperature that ends up on 10-day forecasts and electronic signs outside banks, the heat index is often a better indicator of the intensity of the heat, and the dangers it can present to people.

What is the heat index?

The heat index is a measure of how hot it really feels outside, when humidity and other factors are considered along with the temperature, according to Kimberly McMahon, a meteorologist with the National Weather Service.

“The heat index matters — and this is especially true in the Northeast — because it includes the amount of moisture or humidity that’s in the air, and that moisture or humidity can make the air temperature feel even higher,” Ms. McMahon said.

Why is the heat index used?

Aside from using it to provide a more accurate measure of what it feels like outside, the National Weather Service relies on the heat index to let people know how much heat the human body can handle before it becomes dangerous.

Sometimes, if it’s humid enough, it doesn’t need to be that hot outside for the weather to be harmful.

“With the high humidity, it’s harder to have the human body sweat, and then have that sweat evaporate,” Ms. McMahon said. “With the inability to inefficiently cool off, that increases a person’s heat stress.”

For example, on a day when the temperature outside is 92 degrees and the humidity is 70 percent, the heat index is 112 degrees. When the heat index reaches that level, it’s dangerous, and it can cause sunstroke, muscle cramps and heat exhaustion. For those doing physical activities or who are outside long enough under those conditions, heatstroke is also a risk.

The National Weather Service uses heat index values to know when to issue heat advisories, watches and warnings, which urge residents in an area to avoid being outside during the hottest parts of the day.

While other countries also use the heat index to gauge how hot it feels outside, an additional measure used in some countries, such as Saudi Arabia, is wet-bulb temperature.

The National Weather Service refers to “WetBulb Globe Temperature” as a measure of heat stress under direct sunlight that factors in temperature, humidity, wind speed, cloud coverage and the angle of the sun.

How common are heat-related illnesses?

Weather-related deaths and injuries are often associated with more drastic events like tornadoes and hurricanes, but heat-related illnesses are the leading cause of death related to weather or environmental events, according to the American Public Health Association.

Every year, more than 600 people across the country die because of extreme heat, according to the Centers for Disease Control and Prevention.

Regardless of whether someone lives in an area where they are accustomed to the heat, Ms. McMahon said it’s important to pay attention to heat-related watches and warnings when they are in effect.

“Both heat is dangerous, whether it be heat with high humidities or the dry heat that they get out in the desert Southwest,” she said. “People need to always pay attention, and take it seriously because it is the No. 1 weather-related killer in the United States.”

How is the heat index calculated?

The heat index is measured using a multiple regression analysis that uses the actual temperature and humidity to produce a number that is more representative of how hot it feels outside.

The equation is long (-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), and uses R to represent humidity and T to represent temperature.

Because that’s a long math problem to do on the fly, the National Oceanic and Atmospheric Administration uses a simple chart that anyone can understand.

According to the chart, on a day when temperatures reach 106 degrees, and the humidity is 50 percent, the heat index is 137 degrees.

Heat index values were created assuming shady conditions with a light wind, so full exposure to sunlight can make it feel even hotter by as much as 15 degrees, according to the National Weather Service.