Heat Index Calculator

heat_index.js

// Returns a Celcius temperature that transforms the input temperature based on the relative humidity
function calculateHeatIndex(airTempC, relativeHumidity){
    relativeHumidity = Math.min(100, Math.max(0, relativeHumidity));
    var hitemp = 61.0+((airTempC-68.0)*1.2)+(relativeHumidity*0.094);
    var hifinal = 0.5*(airTempC+hitemp);

    if(hifinal > 79.0){
        var hi = -42.379+2.04901523*airTempC+10.14333127*relativeHumidity-0.22475541*airTempC*relativeHumidity-6.83783*(Math.pow(10, -3))*(Math.pow(airTempC, 2))-5.481717*(Math.pow(10, -2))*(Math.pow(relativeHumidity, 2))+1.22874*(Math.pow(10, -3))*(Math.pow(airTempC, 2))*relativeHumidity+8.5282*(Math.pow(10, -4))*airTempC*(Math.pow(relativeHumidity, 2))-1.99*(Math.pow(10, -6))*(Math.pow(airTempC, 2))*(Math.pow(relativeHumidity,2));
        if((relativeHumidity <= 13) && (airTempC >= 80.0) && (airTempC <= 112.0)) {
            var adj1 = (13.0-relativeHumidity)/4.0;
            var adj2 = Math.sqrt((17.0-Math.abs(airTempC-95.0))/17.0);
            var adj = adj1 * adj2;
            var hi = hi - adj;
        }
        else if ((relativeHumidity > 85.0) && (airTempC >= 80.0) && (airTempC <= 87.0)) {
            var adj1 = (relativeHumidity-85.0)/10.0;
            var adj2 = (87.0-airTempC)/5.0;
            var adj = adj1 * adj2;
            var hi = hi + adj;
        }
    }
    else{
        var hi = hifinal;
    }

    twoplaces_rh = hi.toFixed(1);
    celsius_temp_rh = (hi - 32) * .556;
    return celsius_temp_rh;
}