Heat Index Calculator

Category: Physics

- April 02, 2025

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The heat index (or "apparent temperature") combines air temperature and relative humidity to determine how hot it actually feels to the human body.

This measurement is especially important during hot weather as it helps predict heat-related health risks.

Air Temperature:

Relative Humidity:

Decimal Places:

Display Result In:

Heat Index Results

Heat Index

0.0

°F

Danger Level

Not Available

Calculation Method

Heat Index Equation

Heat Index Risk Levels

Heat Index	Risk Level	Effects
80-90°F (27-32°C)	Caution	Fatigue possible with prolonged exposure and physical activity
90-103°F (32-39°C)	Extreme Caution	Heat stroke, heat cramps, or heat exhaustion possible
103-125°F (39-52°C)	Danger	Heat cramps and heat exhaustion likely; heat stroke possible
Over 125°F (52°C)	Extreme Danger	Heat stroke highly likely with continued exposure

Heat Index Chart

This chart shows how heat index changes with humidity at your selected temperature. Higher humidity makes hot temperatures feel even hotter.

About the Heat Index

The heat index, also known as the apparent temperature, is a measure of how hot it really feels when relative humidity is factored in with the actual air temperature.

Scientific Basis:

When the body gets too hot, it cools itself by sweating. This cooling process works when the sweat evaporates from the skin. High humidity reduces the rate of evaporation, making it harder for the body to cool itself.

The heat index was developed by R.G. Steadman in 1979 and is based on physiological studies of evaporative skin cooling for various combinations of ambient temperature and humidity.

Calculation Details:

The heat index is calculated from a complex formula that includes:

Air temperature (in Fahrenheit)
Relative humidity (percentage)
Multiple regression terms to account for non-linear effects

The National Weather Service (NWS) equation is only valid for:

Temperatures ≥ 80°F (27°C)
Relative humidity ≥ 40%

For conditions outside these ranges, this calculator uses appropriate approximations or direct temperature values.

Important Notes:

The heat index is calculated for shady areas. Direct sunlight can increase heat index values by up to 15°F.
Wind can affect the heat index. The formula assumes a wind speed of 5.8 mph (9.3 km/h).
The heat index was designed for humans and may not apply to animals or plants.
Individual factors such as age, health, body type, and acclimatisation also affect how one perceives heat.

Health Implications:

High heat index values can lead to various heat-related illnesses:

Heat Cramps: Painful muscle spasms, usually in the legs and abdomen
Heat Exhaustion: Heavy sweating, weakness, cold/pale/clammy skin, fast/weak pulse, nausea, fainting
Heat Stroke: Body temperature above 103°F, hot/red/dry skin, rapid/strong pulse, possible unconsciousness