Calculate Heat Index
The heat index measures how hot it feels when humidity is combined with actual air temperature. High humidity reduces evaporative cooling from sweat, making heat more dangerous.
Heat Index Reference Chart
This chart shows approximate heat index values for various temperature and humidity combinations (Fahrenheit):
| Temp (F) \ RH | 40% | 50% | 60% | 70% | 80% | 90% |
|---|
Values assume shade and light wind. Direct sunshine can increase heat index by up to 15F.
About the Heat Index
What is the Heat Index?
The heat index, also known as the "apparent temperature" or "feels-like temperature," combines air temperature and relative humidity to determine how hot conditions feel to the human body. It was developed by the National Weather Service (NWS) to help assess heat-related health risks.
The NWS Formula
The heat index uses the Rothfusz regression equation, which is accurate for temperatures at or above 80F with moderate to high humidity:
HI = -42.379 + 2.04901523T + 10.14333127RH - 0.22475541TRH - 0.00683783T² - 0.05481717RH² + 0.00122874T²RH + 0.00085282TRH² - 0.00000199T²RH²
Where T is temperature in Fahrenheit and RH is relative humidity in percent. Adjustments are applied for low humidity at high temperatures and high humidity at moderate temperatures.
Why Humidity Matters
The human body cools itself primarily through evaporation of sweat. When relative humidity is high, the air is already saturated with moisture, which slows evaporation. This forces the body to work harder to maintain a safe core temperature, increasing the risk of heat-related illness.
For example, a temperature of 95F with 50% humidity feels like 107F, but the same temperature with 80% humidity feels like 129F — a dangerous difference.
Limitations and Assumptions
The heat index assumes:
- Shaded conditions (direct sunlight can add up to 15F)
- Light wind or air movement
- Average adult body mass and hydration
- Light-colored, lightweight clothing
Individual factors such as age, fitness level, medical conditions, medications, and activity intensity can significantly affect heat tolerance. Children, elderly individuals, and those with chronic conditions are at higher risk.
Danger Levels and Health Risks
Safe (Below 80F)
No significant heat stress for most people during normal activities.
Caution (80-90F)
Fatigue is possible with prolonged exposure or physical activity. Stay hydrated and take breaks.
Extreme Caution (90-103F)
Heat cramps and heat exhaustion are possible with prolonged exposure or physical activity. Avoid strenuous outdoor work. Drink plenty of water, take frequent breaks, and watch for signs of heat illness.
Danger (103-125F)
Heat cramps and heat exhaustion are likely. Heat stroke is possible with continued exposure. Minimize outdoor activity, especially for vulnerable populations. Seek air conditioning when possible.
Extreme Danger (125F and above)
Heat stroke is highly likely with continued exposure. This is a life-threatening emergency. Avoid all outdoor activity. Stay in air-conditioned spaces. Check on vulnerable individuals.
Frequently Asked Questions
What is the heat index?
The heat index is a measure of how hot it feels when relative humidity is factored into the actual air temperature. It represents the apparent temperature experienced by the human body due to the combined effects of heat and humidity. The heat index was developed by the National Weather Service to communicate heat-related health risks.
How is the heat index calculated?
The heat index uses the NWS Rothfusz regression equation, which includes multiple terms accounting for temperature, humidity, and their interactions. The formula is most accurate for temperatures at or above 80F with moderate to high humidity. Additional adjustments are applied for very low humidity at high temperatures and very high humidity at moderate temperatures.
What are the heat index danger levels?
The NWS defines four danger levels based on heat index values: Caution (80-90F) with possible fatigue during prolonged exposure; Extreme Caution (90-103F) with heat cramps and exhaustion possible; Danger (103-125F) with heat cramps and exhaustion likely and heat stroke possible; and Extreme Danger (125F and above) with heat stroke highly likely.
Why does humidity make heat feel worse?
Humidity reduces the body's ability to cool itself through evaporation of sweat. When the air is already saturated with moisture, sweat does not evaporate as readily from the skin. Since evaporative cooling is the primary mechanism for heat dissipation during hot conditions, high humidity forces the body to work much harder to regulate core temperature, making heat feel more oppressive and increasing health risks.
At what temperature does the heat index apply?
The heat index formula is most accurate for temperatures at or above 80F (27C) with relative humidity of 40% or higher. Below these thresholds, the heat index is typically equal to or very close to the actual air temperature, as evaporative cooling is still effective.
How does direct sunlight affect the heat index?
The standard heat index assumes shaded conditions with light wind. Direct exposure to sunlight can increase the heat index by up to 15F (8C). This means the actual feels-like temperature in full sun may be significantly higher than the reported heat index, especially during midday hours.
How accurate is the heat index?
The heat index is a reliable estimate for typical conditions and average adults, but individual experience varies. Factors like age, fitness level, hydration status, clothing, activity level, wind speed, and sun exposure all affect how heat is perceived and tolerated. The heat index is a general guideline, not a precise measure of individual risk.
What should I do when the heat index is high?
When the heat index reaches Extreme Caution or higher, limit outdoor activities, especially during peak heat hours (10 AM to 4 PM). Stay in air-conditioned spaces when possible, drink plenty of water even if not thirsty, wear light-colored and loose-fitting clothing, take frequent breaks in shade, and never leave people or pets in parked vehicles. Watch for signs of heat illness: confusion, nausea, dizziness, rapid pulse, or cessation of sweating.
Does this calculator store my data?
No. All calculations run entirely in your browser. No temperature, humidity, or heat index data is sent to any server, and nothing is stored.
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Privacy & Accuracy
Privacy: This calculator runs entirely in your browser. No temperature, humidity, or heat index data is transmitted or stored anywhere.
Accuracy: This calculator uses the official NWS Rothfusz regression equation with standard adjustments. Results assume shaded conditions with light wind. Individual heat tolerance varies based on age, health, fitness, hydration, and other factors. This tool is for informational purposes and should not replace professional weather forecasts or medical advice.
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Heat Index Calculator FAQ
What is the heat index?
The heat index is a measure of how hot it feels when relative humidity is factored into the actual air temperature. It represents the apparent temperature experienced by the human body due to the combined effects of heat and humidity.
How is heat index calculated?
The heat index uses the NWS Rothfusz regression equation: HI = -42.379 + 2.04901523T + 10.14333127RH - 0.22475541TRH - 6.83783e-3T^2 - 5.481717e-2RH^2 + 1.22874e-3T^2RH + 8.5282e-4TRH^2 - 1.99e-6T^2RH^2, with adjustments for specific humidity and temperature ranges.
What are the heat index danger levels?
The NWS defines four danger levels: Caution (80-90F) with possible fatigue, Extreme Caution (90-103F) with heat cramps and exhaustion possible, Danger (103-125F) with heat cramps and exhaustion likely, and Extreme Danger (125F+) with heat stroke highly likely.
Why does humidity make heat feel worse?
Humidity reduces the body's ability to cool itself through evaporation of sweat. When the air is already saturated with moisture, sweat does not evaporate as readily, making the body work harder to regulate temperature and causing heat to feel more oppressive.
At what temperature does the heat index apply?
The heat index formula is most accurate for temperatures at or above 80F (27C) with relative humidity of 40% or higher. Below these thresholds, the heat index is typically equal to the actual air temperature.
How accurate is the heat index?
The heat index assumes shaded conditions with light wind. Direct sunshine can increase the heat index by up to 15F. Individual factors like age, hydration, clothing, and activity level also affect how heat is experienced.
Does this calculator store my data?
No. All calculations run entirely in your browser. No data is sent to any server, and nothing is stored.