Heat index and wind chill are both “feels like” temperatures published by the National Weather Service, but they answer opposite seasonal questions. The heat index combines air temperature with humidity to estimate how hot it feels in the shade during summer. Wind chill combines air temperature with wind speed to estimate how cold it feels on exposed skin during winter. Neither measures the actual air temperature, and neither is a personal risk assessment.
What each calculator does
The heat index calculator takes air temperature and either relative humidity or dew point (a dew point is first converted to relative humidity with the separate NWS approximation). It computes the NWS simple formula, averages that with air temperature, and applies the nine-term Rothfusz regression only when the average is 80°F or above. It subtracts a low-humidity adjustment only below 13% humidity between 80°F and 112°F, and adds a high-humidity adjustment only above 85% humidity between 80°F and 87°F. The result is rounded to the nearest whole degree Fahrenheit and reflects shaded, light-wind conditions.
The wind chill calculator takes air temperature and wind speed in any supported units, converts them exactly to Fahrenheit and miles per hour, and applies the NWS equation:
It produces a value only when the converted temperature is at or below 50°F and the converted wind speed is above 3 mph; a value at exactly 3 mph is outside NWS scope. The index models the adult human face with no sunshine effect, using wind speed adjusted to an average height of 5 feet.
Side-by-side
| Heat index calculator | Wind chill calculator | |
|---|---|---|
| Inputs | Air temperature and relative humidity (or dew point) | Air temperature and wind speed |
| Seasonal scope | Hot side: full regression only when the simple-formula average is 80°F or above | Cold side: values only at 50°F or below with wind above 3 mph |
| Primary output | Apparent temperature in whole degrees Fahrenheit | Wind-chill temperature in whole degrees |
| Formula basis | NWS simple formula plus nine-term Rothfusz regression with humidity adjustments | NWS wind-chill equation with temperature and wind-speed terms |
| Built-in assumptions | Shaded, light-wind conditions; full sunshine raises the value | Adult human face; no sunshine effect; 5-foot wind-height adjustment |
| What it rejects | Humidity outside 0%–100%, impossible dew point, temperatures below absolute zero | Converted temperatures above 50°F, wind speeds at or below 3 mph, negative wind |
| Best for | Hot-weather comfort context | Cold-weather comfort context |
When to use which
Use the heat index calculator when it is hot and you want the humidity-included apparent temperature — a dew point can be supplied instead of relative humidity if that is what you have. Use the wind chill calculator when it is cold and windy and you want the NWS wind-chill temperature for your measured wind speed.
The tools are complementary across seasons rather than rivals. The one whose scope matches your conditions is the one that applies: the heat index’s regression does not run below its 80°F decision threshold, and the wind chill equation does not produce values above 50°F or at 3 mph or less. Neither page assigns a risk category or frostbite time, so both are best used as context alongside current NWS alerts.
Limits and disclaimer
Both pages are educational only and are not medical advice. Each index is an environmental model for standard conditions — shade and light wind for heat index, an adult face without sun for wind chill — not a measurement of your body. The NWS warns the Rothfusz regression is not valid for extreme temperature and humidity combinations outside its underlying data, and wind chill does not mean an inanimate object will cool below the actual air temperature. Neither calculator estimates heat-illness risk or frostbite time. Seek professional guidance when exposure, symptoms, or chronic conditions are involved.