Estimate apparent temperature from air temperature and relative humidity using the NWS Rothfusz heat index formula.
Last updated
Result
Feels like
Actual: 95°F
Heat cramps or exhaustion likely; heat stroke possible with prolonged exposure.
Heat index (°C)
50.3 °C
Heat index (°F)
122.6 °F
Temperature (°C)
35 °C
Humidity
70%
Science — Physics
The heat index — also called apparent temperature or "feels like" temperature — combines air temperature and relative humidity to estimate how hot the air feels on the human body. The NWS Rothfusz regression equation is the standard US method for calculating heat index.
The NWS formula is a multivariate polynomial regression across temperature (°F) and relative humidity (%). For combinations above 80 °F and 40% RH, the full Rothfusz equation is applied with two NWS adjustments for edge cases with very low humidity or high humidity at lower temperatures.
For temperatures below 80 °F, a simpler equation is used: HI = 0.5 × (T + 61 + (T − 68) × 1.2 + RH × 0.094). When this value is below 80 °F, the simple formula result is reported.
The NWS defines four heat danger categories: Caution (27–32 °C), Extreme Caution (32–41 °C), Danger (41–54 °C), and Extreme Danger (>54 °C). At Extreme Caution and above, prolonged outdoor activity poses real health risks.
Heat exhaustion and heat stroke are genuine medical emergencies. Key symptoms of heat stroke include core temperature above 40 °C, confusion, and cessation of sweating. This requires immediate cooling and emergency medical care.
A worked example helps translate the heat index calculator maths into a realistic scenario so the user can compare the headline result with a concrete set of inputs.
That matters because a result is easier to trust when the page shows how the same logic behaves in a practical case instead of leaving the formula abstract.
Frequently asked questions
The body cools itself through sweat evaporation. At high relative humidity, the air is already nearly saturated with water vapour, reducing the evaporation rate. Less evaporation means less cooling, causing body temperature to rise faster.
The NWS recommends caution above 32 °C (90 °F) apparent temperature, and advises against strenuous outdoor activity above 41 °C (105 °F). Individual fitness, acclimatisation, and hydration all affect safe thresholds.
The heat index is an apparent-temperature measure that combines air temperature and humidity to estimate how hot conditions feel on the body. It is a better planning guide than temperature alone when humidity is high.
Yes. The heat index formula assumes shade. Direct sun can make the air feel significantly hotter than the calculated value, so outdoor work or exercise in full sun can be more stressful than the number suggests.
No. Temperature is the measured air temperature. Heat index is an estimate of how hot the conditions feel once humidity is included, which is why the heat index can be much higher than the thermometer reading.
Guides
Step-by-step guides that use this calculator to solve real problems.
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