AI Assistant
BetaAsk questions about your calculation results
3 free questions per session
AI provides general information, not financial advice. Always consult a qualified professional.
About This Calculator
The heat index combines air temperature with relative humidity to represent the perceived temperature your body experiences. High humidity prevents sweat from evaporating efficiently, making conditions feel significantly hotter. This calculator helps identify dangerous heat levels where heatstroke and heat exhaustion become real threats.
Quick Tips
- 1 Heat index only applies when air temperature exceeds 80°F (27°C).
- 2 At 90°F with 70% humidity, the heat index reaches a dangerous 106°F.
- 3 Direct sunlight can add up to 15°F to the calculated heat index value.
Example Calculation
95F temperature with 65% relative humidity.
Heat index: 117F (47.2C) | Danger level: DANGEROUS | Heat stroke risk with prolonged exposure
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.
Frequently Asked Questions
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.
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.
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.
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.
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.