About the Heat Index
The Heat Index Calculator estimates how hot the air actually feels to the human body by combining air temperature with relative humidity, commonly referred to as the "feels like" temperature in summer weather reports. It is useful for judging real heat risk on a humid day, when the thermometer reading alone understates how oppressive conditions actually are.
How It Works
You enter the current air temperature and the relative humidity percentage. Below 80°F, the calculator simply returns the air temperature unchanged, since the standard heat index formula isn't considered meaningful at cooler temperatures. At or above 80°F, it runs both values through the National Weather Service's Rothfusz regression, a formula built from several temperature and humidity terms, to output a single "feels like" temperature.
Formula & Methodology
The Rothfusz regression is a polynomial fit developed by the National Weather Service to approximate the more complex physiological heat-balance equations used to model how the human body perceives combined heat and humidity. Each term captures how temperature and humidity interact, including squared and cross-multiplied terms, because the "feels like" effect of humidity isn't a simple straight-line adjustment to temperature; it compounds as both values rise together. The formula assumes shaded conditions, so full sun exposure is a separate factor not built into the math.
Examples
Hot and humid afternoon
At 95°F with 60% relative humidity, the Rothfusz formula works out to a heat index of roughly 113°F, meaning conditions feel over 18 degrees hotter than the thermometer reading alone.
Milder, moderate humidity day
At 85°F with 50% relative humidity, the calculated heat index comes out to approximately 86°F, only slightly above the actual air temperature, since both the temperature and humidity are more moderate.
Advantages
- Uses the same Rothfusz regression formula that the National Weather Service applies in its own official heat advisories, rather than a simplified approximation.
- Automatically falls back to plain air temperature below 80°F, avoiding a nonsensical result in a range where the formula wasn't designed to apply.
- Combines two separate readings, temperature and humidity, into one single number that's easier to judge heat risk against than tracking both values separately.
Common Mistakes
- Judging heat risk from air temperature alone and ignoring humidity, which can make genuinely dangerous conditions look mild on a simple thermometer.
- Forgetting that the result assumes shade, and treating the output as accurate for someone standing in direct, unshaded sun.
- Entering humidity as a decimal, like 0.6, instead of a whole-number percentage, like 60, which throws off the regression's terms substantially.
Edge Cases to Watch For
- Below 80°F, the formula is skipped entirely and the calculator just echoes the entered temperature, since the regression was not fit to be accurate at cooler, lower-humidity conditions.
- The formula assumes shading; direct sun exposure can add another 15°F or more to how hot conditions actually feel, which isn't captured in the numeric output.
- Because this is a statistical regression rather than a physical model, extreme combinations of very high temperature with very high or very low humidity can produce results with reduced accuracy compared to the ranges the formula was originally fitted against.
Common Use Cases
- Outdoor workers or event organizers deciding whether conditions warrant extra hydration breaks or rescheduling activity.
- Athletes, coaches, or parents checking whether practice or game-day conditions cross into a higher heat risk range.
- Anyone comparing how a humid location will feel compared to a drier one at the same air temperature.