Heat Index Calculator
Calculate the heat index based on actual air temperature and relative humidity. The heat index represents how hot it actually feels, which is often higher than the actual air temperature due to humidity.
Enter Weather Conditions
Range: -50°C to 60°C (-58°F to 140°F)
Range: 0% to 100%
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Enter weather conditions and click calculate to see the heat index.
About Heat Index Calculator
What is Heat Index?
The heat index is a measure of how hot it actually feels when combined with humidity. When humidity is high, the body cannot cool itself effectively through perspiration, making it feel hotter than the actual air temperature. Understanding heat index is crucial for outdoor activities and public health.
Key Features of Heat Index
- Combines air temperature and relative humidity to calculate perceived temperature
- Most accurate for temperatures above 26.7°C (80°F) with humidity above 40%
- Helps determine heat-related health risks and safety guidelines
- Used by weather services and health organizations worldwide
Benefits of Understanding Heat Index
- Better preparation for outdoor activities during hot and humid weather
- Understanding health risks associated with heat stress and heat illness
- Making informed decisions about physical activity, clothing, and hydration
How to Use the Heat Index Calculator
Using this calculator is simple and straightforward. Follow these steps to get accurate heat index measurements.
Step-by-Step Instructions
- Enter the current air temperature in your preferred unit (Celsius or Fahrenheit)
- Select your temperature unit from the dropdown menu
- Enter the relative humidity percentage (0-100%)
- Click 'Calculate Heat Index' to get your results
Helpful Tips
- Heat index is most relevant for temperatures above 26.7°C (80°F)
- Accurate humidity measurements are important for reliable heat index calculations
- Use humidity readings from weather stations rather than guessing for better accuracy
Heat Index Formula
The heat index formula calculates how hot it feels by combining air temperature and relative humidity. The National Weather Service formula is the most widely used and accepted internationally.
National Weather Service Heat Index Formula
HI = -42.379 + 2.04901523T + 10.14333127RH - 0.22475541T·RH - 0.00683783T² - 0.05481717RH² + 0.00122874T²·RH + 0.00085282T·RH² - 0.00000199T²·RH²
This formula calculates heat index in degrees Fahrenheit when air temperature is above 26.7°C (80°F) and relative humidity is above 40%.
Variables in the Formula
HI: Heat Index in degrees Fahrenheit
T: Air temperature in degrees Fahrenheit
RH: Relative humidity as a percentage (0-100%)
Important Notes About the Formula
- The formula is most accurate for temperatures above 26.7°C (80°F)
- Relative humidity should be between 40% and 100% for accurate results
- For temperatures below 80°F or humidity below 40%, the heat index equals the actual air temperature
Heat Index Examples
Here are some real-world examples showing how heat index is calculated and how it affects perceived temperature.
Example 1: Moderate Heat with Moderate Humidity
Given:
Air temperature: 32°C (90°F)
Relative humidity: 60%
Result:
Heat index: 41°C (106°F) - Caution: Heavy exertion should be limited
Example 2: High Heat with High Humidity
Given:
Air temperature: 38°C (100°F)
Relative humidity: 75%
Result:
Heat index: 51°C (124°F) - Danger: Heavy exertion should be discontinued
Example 3: Extreme Heat with Very High Humidity
Given:
Air temperature: 40°C (104°F)
Relative humidity: 85%
Result:
Heat index: 57°C (135°F) - Extreme Danger: Heat stroke risk is very high
Frequently Asked Questions About Heat Index
Get answers to common questions about heat index, how it is calculated, and its effects on your health and safety.
What is the difference between heat index and actual temperature?
The actual temperature is the true air temperature, while heat index represents how hot it feels. High humidity reduces the body's ability to cool through perspiration, making it feel hotter than the actual temperature. Heat index is always equal to or higher than the actual temperature.
At what temperature does heat index become important?
Heat index becomes relevant for temperatures above 26.7°C (80°F) with relative humidity above 40%. Below these thresholds, the heat index is essentially the same as the actual air temperature.
How quickly can heat-related illness develop?
Heat illness can develop rapidly, especially during high heat index conditions. Heat exhaustion symptoms can appear within 30 minutes to 2 hours of exposure. Heat stroke is a medical emergency requiring immediate attention.
Is heat index the same everywhere?
No, heat index varies based on local air temperature and humidity conditions. Two locations with the same temperature can have different heat indices due to differences in humidity levels. Wind and sunlight exposure also affect perceived temperature.
Why does humidity make heat feel worse?
The human body cools itself primarily through perspiration and evaporation. High humidity reduces evaporation efficiency, preventing the body from cooling effectively. This causes the body to retain more heat, making it feel much hotter than the actual temperature.