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heat-index.md

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Calculate relative humidity

This expression calculate the heat index.

Use cases

Another factor in firefighter safety is the heat index. A high heat index is an indication of a heightened risk of heatstroke or heat exhaustion.

Workflow

Copy and paste the expression found in the expression template below to the Arcade editor in ArcGIS Online, the relevant location in ArcGIS Pro, or the relevant location in a custom app.

To configure the script to your layer, edit the first two lines to use the fields representing the desired wall height and type.

var dryBulbTemperature = $feature["drybulb_temperature"];
var wetBulbTemperature = $feature["wetbulb_temperature"];

Expression Template

This Arcade expression will return the cubic yards of sand required to construct a sandbag wall of a given type.

// Replace with appropriate fields for dry and wet bulb 
// temperatures
var dryBulbTemperature = $feature["drybulb_temperature"];
var wetBulbTemperature = $feature["wetbulb_temperature"];

var pt = Geometry($feature);
var altitude = pt.Z;

function BarometricPressure(T,h){

    var g = 9.80665; //gravitational acceleration
    var P0 = 101325; // pressure at sea level
    var M = 0.0289644; //molar mass of air
    var R = 8.31432; //universal gas constant
    
    //convert F to Kelvin
    var Tk = (T - 32) * 5/9 + 273.15;

    // calculate barometric pressure
    var P = (P0 * exp((-g * M * h) / (R * Tk))) / 100;
    return P;
}

//Relative Humidity using Wet (Tw) & Dry Bulb (Td) Temps
function RelativeHumidity(Td, Tw, h){
	
    var Tdk = (Td - 32) * 5/9;
    var Twk = (Tw - 32) * 5/9;
    
    var Es = 6.112 * exp((17.67 * Tdk)/(Tdk + 243.5));
    var Ew = 6.112 * exp((17.67 * Twk)/(Twk + 243.5));
    
    //get barometric pressure based on dry temp and altitude
    var Psta = BarometricPressure(Td,h);

    var E = Ew - Psta * (Tdk - Twk) * 0.00066 * 
        (1 + (0.00115 * Twk));
    
    // Relative Humidity
    var RH = (E / Es) * 100;
    
    return RH;
}

// Calculate Heat Index using temperature (T) 
// and relative humidity (RH)
function HeatIndex(T, RH){
    //Rothsfusz
    var heatIndex = -42.379 + 2.04901523 * T + 10.14333127 * RH - 
        0.22475541 * T* RH - 0.00683783 * T * T - 0.05481717 * RH * RH + 
        0.00122874 * T * T * RH + 0.00085282 * T * RH * RH - 
        0.00000199 * T * T * RH * RH;
    
    if (T >= 80 && T <= 112 && RH < 13) {
      var HIadjustment = ((13 - RH) / 4) * 
        Sqrt((17 - Abs(T - 95)) / 17);
      heatIndex += HIadjustment;
    }
    
    if (T >= 80 && T <= 87 && RH > 85) {
      var HIadjustment = ((RH - 85) / 10) * ((87 - T) / 5);
      heatIndex += HIadjustment;
    
    }

    Console('Heat Index: ', round(heatIndex,0));

    if (heatIndex < 80) {
        // Use simple
        var heatIndex = 0.5 * (T + 61.0 + ((T - 68.0) * 1.2) + 
        (RH * 0.094));
}

    return round(heatIndex,0);
}
return HeatIndex(dryBulbTemperature,RelativeHumidity(
    dryBulbTemperature,wetBulbTemperature,altitude));

Example output

See this web map for examples of how to use this expression in ArcGIS Online.

Heat Index