Sri Lanka Heat Index Calculator — “Feels Like” Temperature

The heat index is the “feels like” temperature — what the air feels like to your body once relative humidity is taken into account. Your body cools itself by evaporating sweat; humid air slows that evaporation, so on a muggy day the same 32°C feels far hotter than it would in dry air. This calculator uses the exact equation published by the US National Weather Service (NWS) and categorises the result against the Sri Lanka Department of Meteorology's advisory bands.

The NWS equation is defined in degrees Fahrenheit, so the tool converts your °C input to °F, computes the index, then converts back to °C for display. Let T be the temperature in °F and RH the relative humidity in percent.

1. Simple check. First a simplified Steadman formula is computed and averaged with the temperature. If that average is below 80°F (about 27°C), humidity barely matters, so the heat index equals that average and the calculation stops.
2. Rothfusz regression. At or above 80°F the full nine-term regression is used:HI = −42.379 + 2.04901523·T + 10.14333127·RH − 0.22475541·T·RH − 0.00683783·T² − 0.05481717·RH² + 0.00122874·T²·RH + 0.00085282·T·RH² − 0.00000199·T²·RH²
3. Adjustments. If humidity is under 13% and T is between 80–112°F, a small amount is subtracted (dry air feels a touch cooler). If humidity is above 85% and T is between 80–87°F, a small amount is added. Both adjustments come straight from the NWS Weather Prediction Center.
4. Categorise.The whole-degree °C result is placed into the Met Department's advisory band — Caution (27–32°C), Extreme Caution (33–41°C), Danger (42–54°C), or Extreme Danger (above 54°C).

Every result is independently cross-checked against the metric-native form of the Rothfusz regression — a version with different constants that works directly in °C without any unit conversion. The two methods agree to within a fraction of a degree, which is the credibility check behind the number you see. This is a point-in-time calculation: it uses the reading you enter, not a live weather feed, so it stays free and works offline.

• NOAA Weather Prediction Center — The Heat Index Equation (Rothfusz regression + adjustments)
• US National Weather Service — Heat Index background and safety categories
• Sri Lanka Department of Meteorology — Heat Index advisory bands (Warnings → Heat Index Advisory)

The heat-index equation and the 4 advisory bands on this page were last cross-checked against the NWS and Sri Lanka Met Department sources on 2026-07-10. The formula is a long-standing NWS standard; the page is reviewed whenever the Met Department revises its advisory thresholds.