What is Heat Index?
The heat index, also called the humidex, measures how hot it actually feels when you account for both air temperature and relative humidity. While a thermometer gives you one number, your body experiences something different because humidity drastically affects how efficiently you cool down through perspiration.
Moist air creates a barrier that prevents sweat from evaporating effectively. Without evaporation, your body cannot shed heat as rapidly, leading to that oppressive, stifling sensation on humid days. A 90°F day at 70% humidity feels substantially worse than the same temperature at 30% humidity.
One critical limitation: the heat index applies to shaded conditions. Direct sunlight adds another 10–15°F to your perceived temperature, meaning the actual heat stress in full sun can be considerably higher than any calculator reading.
The Two Key Inputs: Temperature and Humidity
Calculating heat index requires only two atmospheric measurements:
- Ambient air temperature: Read from a standard thermometer in shade, typically 5 feet above ground level.
- Relative humidity: The percentage of moisture the air holds compared to its maximum capacity at that temperature. High humidity means the air is saturated or near saturation, severely hampering evaporative cooling.
Some calculators also accept dew point as an alternative to relative humidity. The dew point is the temperature at which air becomes fully saturated; if you know the dew point and air temperature, relative humidity can be derived mathematically. Both approaches yield the same heat index result.
Heat Index Calculation Formula
The heat index formula developed by meteorologist R.G. Steadman integrates multiple atmospheric and physiological factors into a single polynomial expression. The most accurate version (used for temperatures above 80°F / 26.7°C) accounts for interaction terms between temperature and humidity:
HI = −42.379 + 2.049 × T + 10.143 × RH − 0.225 × T × RH
− 0.0068 × T² − 0.0548 × RH² + 0.00129 × T² × RH
+ 0.000853 × T × RH² − 0.00000199 × T² × RH²
where T is temperature in degrees Fahrenheit and RH is relative humidity as a percentage (0–100). For simpler approximations or temperatures below 80°F, the heat index defaults to the actual air temperature since humidity effects become negligible at cooler readings.
HI— Heat index in degrees FahrenheitT— Air temperature in degrees FahrenheitRH— Relative humidity as a percentage (0–100%)
Understanding the Limitations
Steadman's original work examined roughly 20 different factors affecting heat perception, including walking speed, body weight, wind velocity, solar angle, and altitude. The simplified two-parameter formula assumes constant conditions for all these variables:
- A person walking at a typical 3 mph pace (1.3 m/s) in calm wind
- Standard adult body composition
- Minimal wind exposure
- Sea level atmospheric pressure
If you walk briskly (faster than 3 mph), hike at elevation, or face strong winds, your actual perceived temperature will diverge from the calculated value. Likewise, differences in fitness level, clothing, and sun exposure introduce individual variation that no single formula can capture.
Practical Considerations When Using Heat Index Data
Heat index values guide safety decisions, but several real-world factors deserve attention.
- Account for direct sunlight — Heat index readings assume shade. Full sun exposure adds approximately 10–15°F of additional heat stress. Outdoor work, sports, or recreation in bright sunlight will feel noticeably hotter than the calculated value, increasing dehydration and heat illness risk.
- Individual physiology varies widely — Athletes, people with higher metabolic rates, and those accustomed to heat tolerate conditions better than sedentary individuals or those unaccustomed to hot climates. Age, medications (including antihistamines and decongestants), and cardiovascular fitness all influence actual heat perception and heat illness susceptibility.
- Humidity spikes faster than temperature — On warm days, relative humidity often climbs rapidly in late afternoon and evening as temperature drops slightly. This can push the heat index higher than mid-day readings despite cooler air temperatures, creating a dangerous mismatch between what you expect and what you experience.
- Wind chill works in reverse — Strong winds actually reduce heat index slightly by accelerating evaporative cooling. Conversely, still, stagnant air in cities (urban heat islands) or sheltered areas will feel significantly hotter than the calculated value, as air circulation helps remove heat from the body.