If you’ve ever stepped outside on a hot Colorado summer day and thought, “It feels hotter than that,” you’re not imagining things — you’re feeling the heat index at work. And while Coloradans often enjoy low humidity, heat index becomes a life-threatening factor when high temperatures and moisture team up.
On July 31, 2025, the highest heat index ever recorded was a staggering 182°F at Qeshm International Airport in Iran — a combination of blistering heat and extreme humidity. It’s a sobering reminder that our bodies respond to more than just temperature when it comes to heat stress.
What Is the Heat Index?
The heat index is a measure of how hot it actually feels to your body when you combine air temperature with relative humidity. The National Weather Service developed the scale based on research by R.G. Steadman in 1979, to help communicate the risk of heat-related illness during hot, humid weather.
Why Heat + Humidity Matters
There’s a reason people say, “It’s not the heat, it’s the humidity.” In truth, it’s both.
Our bodies cool down by radiating heat and sweating. But once air temperatures reach around 95°F, the body’s ability to cool by radiation and convection shuts down. At that point, sweating becomes the main way to release heat. However, sweat only works if it evaporates.
When humidity is high, the air is already holding a lot of moisture, so sweat doesn’t evaporate as efficiently — leaving your body hotter and more vulnerable to heat stress.
How the Heat Index Works
The heat index uses a chart that takes the temperature and relative humidity to calculate what the temperature actually feels like. For example:
- At 100°F with 40% humidity, the heat index is 109°F
- At 100°F with 70% humidity, it feels more like 136°F
And this is in the shade. In direct sun, you can add up to 15°F to these values.
Why You Should Care in Colorado
While we don’t usually see the intense humidity levels of the Midwest or Gulf Coast, heat waves are still a serious danger here — especially during stagnant high-pressure systems that trap heat for days. Under these domes, little air movement and cloud cover allow temperatures to soar and stay elevated.
And despite our dry air, heat is the deadliest weather-related hazard in the U.S. — even more than tornadoes, floods, or hurricanes. Between 2013 and 2022, an average of 153 people died each year from heat-related causes.
Tips to Stay Safe During a Heat Wave
- Drink water regularly — don’t wait until you’re thirsty.
- Limit outdoor activities during peak heat, especially between 11 AM and 4 PM.
- Wear light, loose-fitting clothing and a hat.
- Take breaks in the shade or A/C, especially if you’re working or exercising outdoors.
- Never leave children or pets in parked vehicles, even for a few minutes.
Final Thought
Whether you’re hiking the Flatirons, walking the dog in Denver, or camping in the foothills, keep the heat index in mind this summer. It’s not just a number — it’s a life-saving guide to how your body experiences heat. Stay cool, stay hydrated, and stay safe, Colorado.