# Heat Index Calculator

> Calculate the heat index (feels-like temperature) from air temperature and relative humidity using the NWS formula

**Category:** Utility
**Keywords:** heat index, feels like, temperature, humidity, weather, apparent temperature, NWS
**URL:** https://complete.tools/heat-index-calculator

## How it calculates

The heat index (HI) is calculated using the following formula: 
HI = c1 + c2T + c3R + c4TR + c5T² + c6R² + c7TR² + c8T²R + c9TR² 
Where: 
- HI = Heat Index 
- T = Air temperature in degrees Fahrenheit 
- R = Relative Humidity (in percentage, expressed as a whole number) 
- c1 = -42.379 
- c2 = 2.04901523 
- c3 = 10.14333127 
- c4 = -0.22475541 
- c5 = -6.83783 × 10^-3 
- c6 = -5.481717 × 10^-2 
- c7 = 1.22874 × 10^-3 
- c8 = 8.5282 × 10^-4 
- c9 = -1.99 × 10^-6 
This formula incorporates both temperature and humidity to provide a more accurate representation of perceived heat, highlighting the non-linear relationship between these variables and the resulting heat index.

## Who should use this

Examples of specific use cases for the Heat Index Calculator include: 
1. Meteorologists assessing weather conditions for public safety announcements. 
2. Event planners determining the best time for outdoor activities to ensure participant comfort. 
3. Occupational health professionals evaluating heat stress risks for workers in construction or agriculture. 
4. Athletes and coaches planning training sessions to prevent heat-related illnesses during high-temperature days.

## Worked examples

Example 1: A meteorologist records an air temperature of 90°F and relative humidity of 70%. Using the formula: 
HI = -42.379 + (2.04901523 × 90) + (10.14333127 × 70) + (-0.22475541 × 90 × 70) + (-6.83783 × 10^-3 × 90²) + (-5.481717 × 10^-2 × 70²) + (1.22874 × 10^-3 × 90 × 70²) + (8.5282 × 10^-4 × 90² × 70) + (-1.99 × 10^-6 × 90 × 70²) results in a heat index of approximately 105°F. 

Example 2: An outdoor event planner is monitoring conditions with a temperature of 85°F and 60% humidity. The calculation follows: 
HI = -42.379 + (2.04901523 × 85) + (10.14333127 × 60) + (-0.22475541 × 85 × 60) + (-6.83783 × 10^-3 × 85²) + (-5.481717 × 10^-2 × 60²) + (1.22874 × 10^-3 × 85 × 60²) + (8.5282 × 10^-4 × 85² × 60) + (-1.99 × 10^-6 × 85 × 60²) yields a heat index of around 95°F, indicating potential discomfort for attendees.

## Limitations

The Heat Index Calculator has several limitations. First, the formula is most accurate for temperatures between 80°F and 110°F and relative humidity levels between 40% and 100%. Outside of these ranges, the calculated heat index may not accurately reflect human perception of heat. Second, the formula does not account for factors such as wind speed and direct sunlight, which can influence heat perception. Third, it assumes a steady state of temperature and humidity, which may not be the case in rapidly changing weather conditions. Lastly, the tool does not provide adjustments for individual physiological responses, which can vary based on age, health, and acclimatization to heat.

## FAQs

**Q:** How does humidity affect the heat index? 


**A:** Higher humidity levels reduce the body's ability to cool itself through evaporation of sweat, leading to a higher heat index value, which indicates it feels hotter than the actual air temperature.


**Q:** At what temperature does the heat index become a health risk? 


**A:** The National Weather Service indicates that a heat index above 80°F can pose risks, with higher values increasing the likelihood of heat-related illnesses, especially above 90°F.


**Q:** Why is the heat index formula not linear? 


**A:** The relationship between temperature and humidity in the heat index formula is non-linear due to the complex physiological responses of the human body to varying levels of heat and moisture, necessitating a multi-variable approach to accurately calculate perceived temperature.


**Q:** Can the heat index be negative? 


**A:** No, the heat index cannot be negative, as it is based on a combination of air temperature and humidity, both of which are non-negative values.

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*Generated from [complete.tools/heat-index-calculator](https://complete.tools/heat-index-calculator)*