Heat Index Calculator

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Work out how hot it really feels from air temperature and relative humidity using the NWS Rothfusz heat index formula, with caution and danger bands.

°C
%
Heat index (feels like) – °C
Actual temperature –
Feels hotter by –
Risk category –
Uses the NWS Rothfusz regression with its low-humidity and high-humidity adjustments, defined for shady, light-wind conditions. Full sun can add up to 8 °C (15 °F) to how hot it feels.
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Kuhusu Heat Index Calculator

The Heat Index Calculator converts an air temperature and a relative humidity reading into the apparent temperature — how hot it actually feels to the human body. Humid air slows the evaporation of sweat, which is how you shed heat, so a 32 °C day at 70% humidity feels far hotter than the same temperature in dry air.

The tool implements the US National Weather Service Rothfusz regression, including the low-humidity and high-humidity adjustments that the NWS applies at the edges of its range, and falls back to the simpler Steadman average below 27 °C (80 °F) exactly as the official method does.

Use it to decide whether outdoor work, sport or training should be moved, shortened or shaded, and to know which NWS caution band the day falls into. Free, instant and computed in your browser.

Jinsi ya kutumia Heat Index Calculator

  1. Choose whether you are entering temperatures in °C or °F.
  2. Enter the current air temperature.
  3. Enter the relative humidity as a percentage.
  4. Read the heat index — the apparent temperature — and how much hotter it feels.
  5. Check the caution band before planning outdoor activity.

Maswali yanayoulizwa mara kwa mara

The heat index, or apparent temperature, combines air temperature and relative humidity into a single figure describing how hot it feels. It is the summer counterpart to wind chill.

Your body cools itself by evaporating sweat. When the air already holds a lot of moisture, evaporation slows, less heat leaves your body, and your core temperature climbs — so the same air temperature feels much hotter.

The NWS uses four bands: caution from 27 °C (80 °F), extreme caution from 32 °C (90 °F), danger from 39 °C (103 °F) and extreme danger from 52 °C (125 °F). Heat exhaustion becomes likely in the danger band.

No. The Rothfusz regression assumes shady, light-wind conditions. Full sunshine can add up to about 8 °C (15 °F) to how hot it feels, so treat the result as a floor on a cloudless day.

No. The wet bulb temperature is a physical measurement of how far evaporation can cool the air, while the heat index is a human-comfort model derived from temperature and humidity. Both rise together, but they are not interchangeable.
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