Heat Index Calculator

How Heat Index Is Calculated

Heat index describes how hot the air feels to the human body when relative humidity is factored in alongside actual air temperature. The body cools itself mainly by evaporating sweat, and high humidity slows that evaporation — so humid air feels hotter than dry air at the same temperature. The National Weather Service uses the Rothfusz regression, a polynomial fit developed by Lans P. Rothfusz in 1990 against a more complex underlying heat-balance model, as the practical formula behind official heat index readings:

$$HI = -42.379 + 2.04901523\,T + 10.14333127\,R - 0.22475541\,T R - 0.00683783\,T^2 - 0.05481717\,R^2 + 0.00122874\,T^2 R + 0.00085282\,T R^2 - 0.00000199\,T^2 R^2$$

The regression is most reliable for air temperatures at or above 80 degrees Fahrenheit and relative humidity at or above 40 percent — the range it was fitted against. The NWS also applies two small correction terms at the extremes (very low humidity with temperatures between 80 and 112 degrees, and very high humidity with temperatures between 80 and 87 degrees) that this calculator's core regression does not add, since they shift the result only slightly and the main formula is standard practice for a general-purpose heat index estimate.

Worked Example

At an air temperature of 90 degrees Fahrenheit and 70 percent relative humidity: plugging T = 90 and R = 70 into the regression above gives a heat index of approximately 106 degrees Fahrenheit — noticeably higher than the actual air temperature. That falls in the 103-124 degree band, so the risk category reads Danger: heat exhaustion is likely and heat stroke is possible with prolonged exposure or physical activity at that heat index.

Why Humidity Matters More Than It Seems

Sweat only cools the body as it evaporates — the phase change from liquid to vapor is what carries heat away from the skin. When the surrounding air is already close to saturated with moisture, sweat evaporates much more slowly, so the body's main cooling mechanism becomes far less effective even though the air temperature hasn't changed. This is why a humid 90-degree day can feel more dangerous than a dry 100-degree day, and why the heat index — not the plain thermometer reading — is the number used for heat-advisory thresholds.

Common Heat Index Calculation Mistakes

Applying the regression well outside its fitted range (cool, dry conditions where heat index has no real meaning) is the most common misuse — the formula was built for hot, humid weather and can produce unreliable numbers outside that zone, which is why this calculator flags temperatures below 80°F or humidity below 40% as outside the reliable range. Confusing heat index with the actual air temperature is another mistake: heat index is an equivalent-feel value, not what a thermometer measures, so it shouldn't be reported as the measured temperature. Ignoring direct sunlight is a third — official heat index values assume shade; full sun exposure can add another 15 degrees Fahrenheit or more to the effective heat felt.

Heat Index Terms You Should Know

Heat Index — the equivalent-feel temperature accounting for both air temperature and relative humidity, also called the "apparent temperature."

Rothfusz Regression — the polynomial formula the National Weather Service uses to compute heat index from temperature and humidity, developed in 1990.

Heat Exhaustion — a heat-related illness marked by heavy sweating, weakness, and dizziness, which can progress to heat stroke if not treated.

Heat Stroke — a life-threatening condition where the body's core temperature rises to dangerous levels and sweating may stop, requiring emergency medical treatment.