There’s a lot of data that we use in order to provide you with a good idea of what the weather outside feels like. In colder weather, we incorporate wind chill for this purpose, and in the summer, we use heat index. But what is the heat index?

The heat index was developed in 1979 in order to create a simple chart that people can use to determine when heat gain will reach a point that exceeds our body’s ability to regulate it. This is important because above that level, body temperature begins rising, and heat illnesses can develop.

The heat index is determined by a formula that uses the air temperature combined with relative humidity to tell us what it really feels like outside to the human body. The combination of these two factors makes the heat index an extremely helpful measurement because our bodies have a hard time cooling themselves when the relative humidity is 60% or higher. Why is that? Well, it has to do with sweating.

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The Importance Of Sweat

Sweat is made of water mixed with a little salt. Our bodies sweat in order to regulate our temperature. The way it works is actually pretty cool (no pun intended): as the water in sweat evaporates, it has a cooling effect on the skin beneath it. High humidity impacts our ability to keep our temperature regulated because it keeps our sweat from evaporating. When sweat can’t evaporate, it no longer serves its actual purpose.

What 'Feels Like' Means

The “feels like” temperature that the heat index gives us allows us to take precautions before going into excessive heat. When our bodies can’t regulate their temperatures, those temperatures can rise extremely rapidly. For example, when heat stroke occurs, body temperature can rise to 106 degrees or higher in just 10 to 15 minutes. When the heat index reaches 100 degrees or higher, being outdoors for any significant amount of time can be dangerous.

The Leading Cause Of Weather-Related Death

Heat exhaustion is an illness stemming from water or salt depletion. It can occur after exposure to dangerous levels of heat and can progress to heat stroke, organ injury and even death if not properly treated.

According to NOAA, excessive heat was the leading cause of weather-related death in the United States from 1992 to 2021. In 2024, excessive heat took 199 lives in the U.S., more than floods (89) and hurricanes (78) combined. In order to warn of dangerous heat situations, the NWS issues excessive heat warnings. These warnings can vary based on location and time of year, but generally occur when the heat index will be 103 degrees or above for at least two consecutive hours.

An Important Note About Direct Sunlight And Heat

It’s important to note that in urban areas, population density and building construction can exacerbate the impacts of excessive heat, meaning that you should take extra precautions in such areas when the feels-like temperature is high. You should also know that the heat index was designed for use in the shade, with light wind conditions. This means that in direct sunlight, the heat index (and the feels-like temperature) can increase by up to 15 degrees.

Key Takeaways:

• The heat index is used to calculate how hot it actually feels.
• When the heat index reaches 100 degrees or higher, being outdoors for any significant amount of time can be dangerous.
• The heat index gives an idea of what it will feel like in the shade, meaning that if you are in direct sunlight, the heat index can increase by up to 15 degrees.

Senior writer Chris DeWeese edits Morning Brief, The Weather Channel’s newsletter.