What Is the Heat Index?
The heat index is the temperature it “feels like” outside when both air temperature and relative humidity are taken into account. On hot, humid days, your body struggles to cool itself through sweat evaporation, because the air is already saturated with moisture, perspiration can’t evaporate efficiently. The result is that your body retains more heat than it would in dry conditions, making the air feel significantly hotter than the thermometer reads.
The heat index was developed by meteorologist Robert Steadman in 1979 and later refined by the National Weather Service into the formula we use today. It is the summer equivalent of wind chill: both are “feels like” indexes designed to communicate the real-world impact of weather conditions on the human body.
How Is the Heat Index Calculated?
The NWS uses the Rothfusz regression equation to calculate heat index, which takes air temperature and relative humidity as inputs. The formula is valid when temperatures are at or above 80°F (27°C) and relative humidity is above 40%. Below those thresholds, humidity has a negligible effect on how hot you feel.
At lower humidity levels (below 13%) a correction factor is applied because very dry air actually makes the heat index slightly lower than the air temperature. At very high humidity levels above 85%, an upward correction is applied.
One important clarification: the heat index assumes shaded conditions. Direct sunlight can make you feel an additional 10–15°F hotter on top of the calculated heat index value.
Heat Index Danger Levels
The NWS uses four heat index danger categories to communicate risk:
Caution (80–90°F / 27–32°C): Fatigue is possible with prolonged exposure or physical activity. Limit strenuous activity during peak afternoon hours.
Extreme Caution (90–103°F / 32–39°C): Heat cramps and heat exhaustion are possible. Drink water regularly, take shade breaks, and avoid prolonged outdoor exertion.
Danger (103–124°F / 39–51°C): Heat cramps and heat exhaustion are likely. Heat stroke is possible with prolonged exposure. Limit outdoor activity and monitor for symptoms.
Extreme Danger (above 125°F / 51°C): Heat stroke is highly likely. Avoid all outdoor activity. This is a life-threatening condition requiring immediate action.
Signs of Heat Exhaustion and Heat Stroke
Knowing the heat index danger level is only useful if you know what warning signs to watch for.
Heat exhaustion symptoms include heavy sweating, cold and clammy skin, a fast and weak pulse, nausea, dizziness, muscle cramps, and fatigue. Move to a cool location, drink water, and apply cool wet cloths to the skin. If symptoms worsen or persist beyond one hour, seek medical attention.
Heat stroke is a medical emergency. Symptoms include a body temperature above 103°F, hot and red skin, a rapid and strong pulse, and confusion or loss of consciousness. Call 911 immediately. Do not give fluids to someone who is unconscious.
Heat Index vs. Wind Chill
Heat index and wind chill are two sides of the same coin — both are “feels like” temperature indexes used by the National Weather Service to communicate the real-world impact of weather on human health.
Wind chill applies in cold weather, measuring how quickly wind strips heat away from exposed skin. Heat index applies in hot weather, measuring how humidity prevents your body from cooling itself. Together they give a complete picture of thermal comfort across all seasons.
Use our Wind Chill Calculator for cold weather conditions.
Tips for Staying Safe in High Heat Index Conditions
Hydrate before you’re thirsty. By the time you feel thirsty, you’re already mildly dehydrated. Drink water consistently throughout the day, not just when you feel the need.
Schedule outdoor activity wisely. The heat index peaks between noon and 4pm. Exercise and outdoor work are significantly safer in the early morning or after sunset.
Wear loose, light-colored clothing. Dark colors absorb heat. Loose, breathable fabrics allow air circulation and improve sweat evaporation.
Never leave children or pets in parked cars. Interior car temperatures can reach lethal levels within minutes on a hot day, even with windows cracked.
Check on vulnerable people. The elderly, young children, and those with chronic illness are most at risk from extreme heat. Check in on neighbors and family members during heat waves.
Use your weather station. A home weather station gives you real-time temperature and humidity readings at your exact location – the two inputs you need to calculate heat index accurately. See our picks for the best home weather stations of 2026.
Reviewed by Ed Oswald
Lead Reviewer, Weather Station Advisor
Ed has covered consumer technology and weather instruments for Digital Trends, PC World, and the New York Times for over 20 years. He has personally tested every station recommended on this page.