Heat Index Calculation is Incorrect

[Whobeu]

The Heat Index calculation function is not using the latest algorithm as documented at http://www.wpc.ncep.noaa.gov/html/heatindex_equation.shtml. I use the following implementation to calculate the heat index. The formula is designed for Fahrenheit so in order to work with Celsius the temperature must be converted to Fahrenheit in the function before calculating the heat index and then converted back to Celsius.

function calculateHeatIndexCelsius(temperature_C, humidity) {
  const t = temperature_C * 1.8 + 32;
  let hi = -42.379 + 2.04901523 * t + 10.14333127 *
    humidity - 0.22475541 * t * humidity -
    0.00683783 * t * t - 0.05481717 *
    humidity * humidity + 0.00122874 *
    t * t * humidity + 0.00085282 *
    t * humidity * humidity - 0.00000199 *
    t * t * humidity * humidity;

  if (t < 80) {
    hi = 0.5 * (t + 61.0 + (t - 68.0) * 1.2 + humidity * 0.094);
  } else {
    if (humidity < 13 && (t >= 80 && t <= 112)) {
      hi -= (13 - humidity) / 4 * Math.sqrt((17 - Math.abs(t - 95.0)) / 17);
    }

    if (humidity > 85 && (t >= 80 && t <= 87)) {
      hi += (humidity - 85) / 10 * ((87 - t) / 5);
    }
  }

  return (hi - 32) / 1.8;
}