‘It’s a dry heat’: How the heat index shows how hot it feels outside

A temperature of over 100 degrees doesn’t paint the whole picture when it comes to determining how hot it feels. The heat index does.

What is the heat index?

“It is essentially what temperature feels like on the human body,” says Nick Novella, a forecaster for NOAA’s Climate Prediction Center.

The air temperature might be 100 degrees, but heat index values might read at 124 degrees, to show how the heat would feel like on the human body.

How is the heat index measured?

“It takes temperature and some measure of moisture content in the atmosphere into account,” says Novella.

Relative humidity is an important measure when considering heat. An air temperature of 105 degrees in a condition with low humidity — such as in Nevada — and 105 degrees with 85 percent humidity will feel different.

“Sweating is the body’s natural response to try to cool itself down,” says Novella. The higher the humidity is, the more difficult it is for the human body’s sweat to evaporate, making it feel hotter.

It can work both ways. On one recent afternoon, the National Weather Service recorded an air temperature of 111 and a relative humidity of 4 percent at its official observation site, but a heat index of only 104 degrees.

There is also a difference in heat index values if people are in the shade or in the sun. Being in direct sunlight can increase the heat index value up to 15 degrees.

Heat index values above 103 degrees can lead to death

People come into the emergency room for milder symptoms including tiredness, but heat can lead to death in extreme cases, says Dr. Ketan Patel, Medical Director of UMC’s Adult Emergency Department.

“Everyone is susceptible to heat. Everyone has a stress point,” says Patel, “Some people, unfortunately, they’ll have a lower stress point than others, because either their underlying medical conditions, their age, or other factors.”

Patel notes that the most vulnerable populations are the elderly, children, those with underlying medical conditions, and homeless individuals who can’t find shelter.

“People should not go outside, if they can help it. If you do go outside, make sure that it’s limited to any lengthy exposure. Also, use sunscreen and sunglasses,” advises Novella.

The most important piece of advice Patel gives is to be aware of any underlying health issues, to avoid alcohol that could put you at more risk of collapsing, to rehydrate, and come into the emergency room if you are feeling any serious effects.

Contact Annie Vong at avong@reviewjournal.com.