What Is Heat Index? Definition, Chart, and How It Works

The heat index is a measure of how hot the air feels to the human body when relative humidity is combined with air temperature. It explains why a hot, humid day feels much more oppressive than a dry one at the same temperature. Sometimes called the “feels like” temperature, the heat index is widely used in weather forecasts, safety guidelines, and public health warnings.

The heat index matters because the human body relies on sweating and evaporation to cool itself. When humidity is high, sweat evaporates more slowly, reducing the body’s ability to shed heat. As a result, the body feels hotter than the actual air temperature and the risk of heat-related illness increases.

Key Takeaways: Heat Index

• The heat index combines air temperature and relative humidity to estimate perceived temperature.
• It applies to shaded conditions with light wind.
• High humidity reduces sweat evaporation, making it feel hotter.
• Values above 90°F (32°C) increase the risk of heat-related illness.
• Direct sunlight can increase the heat index by up to 15°F (≈8°C).
• The heat index is most accurate for temperatures ≥80°F (27°C) and humidity ≥40%.

History of the Heat Index

The modern heat index traces back to work by physicist Robert G. Steadman in 1979. Steadman developed a model of human heat balance that accounted for:

• Air temperature
• Humidity
• Wind speed
• Radiation (sunlight)
• Human physiology (sweating, clothing, metabolism)

His work produced what is known as the “apparent temperature.” Later, the U.S. National Weather Service (NWS) simplified Steadman’s equations into a practical regression formula that could be used in forecasts. This simplified version is what we now call the heat index.

Heat Index Definition and Explanation

The heat index is the apparent temperature felt by humans due to the combined effects of air temperature (T) and relative humidity (RH).

Why Humidity Matters

The human body cools itself primarily through evaporation of sweat. When humidity is high:

• The air is already saturated with water vapor
• Sweat evaporates more slowly
• Heat removal from the body decreases
• Core body temperature rises

This is why 90°F (32°C) at 70% humidity feels much hotter than 90°F at 20% humidity.

Effects of Heat Index on the Human Body

As the heat index increases, the risk of heat-related illnesses rises:

• Heat cramps: painful muscle spasms
• Heat exhaustion: heavy sweating, weakness, dizziness
• Heat stroke: life-threatening failure of temperature regulation

Heat Index Risk Chart

Shade vs Full Sun Exposure

Heat index values assume:

• Shade
• Light wind (≈5 mph / 8 km/h)

Effect of Direct Sunlight

Full sun exposure can increase the perceived temperature by:

• Up to 15°F (≈8°C)

This happens because solar radiation adds additional heat load beyond air temperature and humidity.

Heat Index Formula

The National Weather Service uses a regression equation (in °F):

HI = -42.379 
     + 2.04901523T 
     + 10.14333127RH 
     - 0.22475541TRH 
     - 6.83783×10⁻³T² 
     - 5.481717×10⁻²RH² 
     + 1.22874×10⁻³T²RH 
     + 8.5282×10⁻⁴TRH² 
     - 1.99×10⁻⁶T²RH²

Where:

• T = temperature (°F)
• RH = relative humidity (%)

Simplified Approximation

For quick estimates:

HI ≈ T + 0.33RH - 0.70 (wind) - 4.00

(This version is less accurate but useful conceptually.)

Heat Index Calculator

Use the calculator below to estimate the heat index, or how hot it feels when air temperature and relative humidity are combined. Enter the air temperature and relative humidity, then click Calculate to see the apparent temperature. Keep in mind that the result assumes shaded conditions with light wind, as defined by the National Weather Service. In direct sunlight, the perceived temperature can be up to 15°F (8°C) higher than the calculated heat index.

Limitations of the Heat Index

The heat index is useful but has important limitations:

• Assumes shade and light wind
• Not accurate for very low humidity (<40%)
• Less reliable below 80°F (27°C)
• Does not account for:
  • Clothing
  • Physical activity
  • Individual health differences
  • Wind speed (unlike wind chill)
• Overestimates discomfort in some dry environments

Because of these limitations, other indices such as Wet-Bulb Globe Temperature (WBGT) are used in occupational and athletic settings.

Common Misconceptions

• “Heat index is just temperature.”
  It includes humidity and reflects perceived heat, not actual air temperature.
• “It works in all weather conditions.”
  It is only valid within specific temperature and humidity ranges.
• “It applies equally in sun and shade.”
  It assumes shade—sunlight increases perceived temperature.
• “Dry heat isn’t dangerous.”
  Even low humidity can be dangerous at high temperatures.

FAQs

What is the heat index in simple terms?

It is how hot it feels when humidity is included with air temperature.

Why does humidity make it feel hotter?

Humidity slows sweat evaporation, reducing the body’s ability to cool itself.

Is heat index the same as “feels like” temperature?

Often yes, but “feels like” may also include wind chill in cold weather.

What heat index is dangerous?

Values above 103°F (39°C) are considered dangerous.

Can heat index be lower than actual temperature?

Yes, in some dry conditions, but it usually equals or exceeds air temperature.

Heat Index vs Related Concepts

The heat index is just one of several measures meteorologists use to describe how weather affects the human body. While it estimates how hot conditions feel based on temperature and humidity, other indices account for additional factors such as wind speed, solar radiation, or human physiology. The following table compares the heat index with other commonly used weather and heat stress indices, showing what each measures and when it is most useful.

Safety Tips for High Heat Index Days

• Stay hydrated (water, electrolytes)
• Avoid strenuous activity during peak heat
• Wear light, loose clothing
• Take breaks in shade or air conditioning
• Never leave people or pets in parked vehicles

References and Further Reading

• Anderson, G. Brooke; Bell, Michelle L.; Peng, Roger D. (2013). “Methods to Calculate the Heat Index as an Exposure Metric in Environmental Health Research”. Environmental Health Perspectives. 121 (10): 1111–1119. doi:10.1289/ehp.1206273
• Lin, Tzu-Ping (2009). “Thermal perception, adaptation and attendance in a public square in hot and humid regions”. Building and Environment. 44 (10): 2017–2026. doi:10.1016/j.buildenv.2009.02.004
• Steadman, R. G. (1979). “The Assessment of Sultriness. Part II: Effects of Wind, Extra Radiation and Barometric Pressure on Apparent Temperature”. Journal of Applied Meteorology. 18 (7): 874–885. doi:10.1175/1520-0450(1979)018<0874:TAOSPI>2.0.CO;2
• Stull, Richard (2000). Meteorology for Scientists and Engineers (2nd ed.). Brooks/Cole. ISBN 9780534372149.