Find out how hot it really feels.
Enter the air temperature and the relative humidity and see the true apparent temperature. High humidity blocks sweat from evaporating, so the same reading can feel many degrees hotter and cross into dangerous territory.
The conditions
The heat index applies to shade with a light wind. Direct sun can add up to 15°F to what your body feels.
What this means for you
How humidity drives the heat index
Heat index as humidity climbs, at 90°F
Reading the result
Heat index reference table
Apparent temperature in °F for a range of air temperatures and humidity levels. Your closest match is highlighted.
Heat and your body, explained
How the Heat Index Is Calculated
The heat index uses the Rothfusz regression equation, which combines air temperature and relative humidity to determine how hot it actually feels. High humidity reduces the body ability to cool through sweat evaporation, making the apparent temperature higher.
Heat Index Danger Levels
Caution (80-90°F): fatigue possible. Extreme Caution (90-103°F): heat stroke, cramps, and exhaustion possible. Danger (103-124°F): heat cramps and exhaustion likely, heat stroke possible. Extreme Danger (125°F+): heat stroke imminent.
Heat-Related Illness Prevention
Stay hydrated, wear light-colored and loose-fitting clothing, limit outdoor activity during peak heat (10 AM to 4 PM), take frequent breaks in shade or air conditioning, and never leave children or pets in parked vehicles.
Humidity and Its Effects
Humidity above 60% significantly impacts heat index. At 95°F with 50% humidity, the heat index is about 107°F. At 80% humidity, it jumps to about 133°F. Desert heat with low humidity is more tolerable than tropical heat with high humidity.
Common questions
What is the heat index?
The heat index, also called the apparent temperature or feels-like temperature, combines air temperature and humidity to indicate how hot it actually feels to the human body. It is the warm-weather counterpart to wind chill.
When should I worry about heat index?
Take precautions when the heat index exceeds 90°F. At 103°F or above, heat-related illnesses are likely with extended exposure. At 125°F or above, heat stroke is imminent. Vulnerable populations should be extra cautious at lower thresholds.
Does the heat index apply in the shade?
The heat index is calculated for shady conditions with light wind. Direct sun exposure can increase the heat index by up to 15°F. Always add 10-15°F to the calculated value if you will be in direct sunlight.
Why is humid heat more dangerous than dry heat?
Your body cools itself by evaporating sweat. High humidity slows evaporation, reducing your body cooling ability. In dry heat, sweat evaporates quickly, providing effective cooling. This is why 95°F in Arizona feels more tolerable than 95°F in Florida.
What temperature and humidity trigger the heat index?
The heat index calculation begins when the air temperature reaches 80°F and relative humidity is at least 40%. Below these thresholds, the actual temperature adequately represents how hot it feels.
Estimates for general awareness only. The heat index assumes shade and a light wind, and it is not a substitute for medical advice or official National Weather Service alerts. People who are older, very young, pregnant, or managing chronic illness feel heat sooner. When in doubt, get out of the heat.