The NWS algorithm
The National Weather Service first computes a simple apparent temperature consistent with Steadman (1979),
HI = 0.5 × [T + 61.0 + (T − 68) × 1.2 + RH × 0.094]
with T in °F and RH in percent, then averages that result with the air temperature. If the average is below 80 °F, that simple value is the heat index. Otherwise the Rothfusz (NWS Technical Attachment SR 90-23, 1990) multiple-regression equation is used, with two published adjustments: a subtraction when RH is under 13% and T is between 80 and 112 °F, and an addition when RH is over 85% and T is between 80 and 87 °F. A temperature of 90 °F and 60% humidity gives about 99.7 °F, which the NWS chart rounds to 100 °F.
Categories and limits
The NWS heat-index chart groups apparent temperature as Caution (80–90 °F), Extreme Caution (90–103 °F), Danger (103–125 °F), and Extreme Danger (125 °F and above). The regression is for shade and a light wind; full sun can raise the apparent temperature by about 15 °F, which this page does not add. It is a weather index, not a runway, hangar, or medical tool. For dewpoint and humidity from a METAR-style pair, use the dewpoint / carburetor-icing calculator.
Sources
National Weather Service, The Heat Index Equation; Rothfusz, NWS Technical Attachment SR 90-23 (1990); Steadman, Journal of Applied Meteorology (1979). Category breakpoints follow the NWS heat-index chart.
Frequently asked questions
What is the heat index at 90 °F and 60% relative humidity?
About 99.7 °F with the official Rothfusz equation, which the NWS chart lists as 100 °F. That is in the Extreme Caution category (90–103 °F).
Why is the result not always the regression equation?
The Rothfusz regression is not used below about 80 °F. The NWS first computes a Steadman-consistent formula, averages it with the air temperature, and keeps that simple value when the average is under 80 °F.
Does this include direct sunlight?
No. The NWS equation is for shade and a light wind. The Service notes that full sun can increase the apparent temperature by around 15 °F; that increment is not added here.