Heat Index & Wind Chill Algorithms

Last Updated: November 09, 2001

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Contents

Heat Index

Calculates the heat index.

Note: A number like 0.38646e-4 is shorthand for 0.38646 * 10-4 and 0.000038646 in many computer languages.

Source Code #1

function calcHeatIndex(t, rh)
{
    var t2=t*t;
    var rh2=rh*rh;

    var result =  -42.379 + 2.04901523 * t + 10.14333127 * rh - 
        0.22475541 * t * rh - 6.83783e-3 * t2 - 5.481717e-2*rh2 + 
        1.22874e-3*t2*rh + 8.5282e-4 * t * rh2 - 1.99e-6 * t2 * rh2;
    return result;
}

Source Code #2

function calcHeatIndex (t, rh)
{
    var t2 = t*t;
    var t3 = t2*t;
    var rh2 = rh*rh;
    var rh3=rh2*rh;

    var result = 16.923 + 0.185212*t + 5.37941*rh - 
        0.100254*t*rh + (0.941695e-2)*t2 + (0.728898e-2)*rh2 + 
        (0.345372e-3)*t2*rh - (0.814971e-3)*t*rh2 + (0.102102e-4)*t2*rh2 - 
        (0.38646e-4)*t3 + (0.291583e-4)*rh3 + (0.142721e-5)*t3*rh + 
        (0.197483e-6)* t*rh3 - (0.218429e-7)*t3*rh2 + (0.843296e-9)*t2*rh3 - 
        (0.481975e-10)*t3*rh3;

    return result;
}

Parameters

t is the temperature in degrees Fahrenheit.

rh is the relative humidity in percent.

Results

Heat index in degrees Fahrenheit.

Examples

t = 95, rh = .4, function #1 returns: 99.0
t = 90, rh = .25, function #1 returns: 87.0
t = 90, rh = .55, function #1 returns: 97.0

t = 95, rh = .4, function #2 returns: 100.4
t = 90, rh = .25, function #2 returns: 87.3
t = 90, rh = .55, function #2 returns: 97.7

Wind Chill In Fahrenheit

Calculates the wind chill in degrees Fahrenheit.

Source Code

function calcWindChillF(t, v)
{
	var vtmp = Math.pow(v, 0.16);
	var wc = 35.74 + 0.6215 * t - 35.75 * vtmp + 0.4275 * vtmp;
	return (wc < t) ? wc : t;
}

Parameters

t is the temperature in degrees Fahrenheit.

v is the wind speed in miles per hour.

Results

Returns the wind chill in degrees Fahrenheit.

Examples

t = 32, v = 10, function returns: 23.7
t = 28, v = 5, function returns: 22.4
t = 24, v = 17, function returns: 10.5

Canadian Wind Chill In Watts

Calculates the wind chill as heat loss in watts per square meter.

Source Code

function calcWindChillWatts(tc, v)
{
	return (12.1452 + 11.6222 * Math.sqrt(v) -1.16222 * v) * (33 - tc)
}

Parameters

t is the temperature in degrees Celsius.

v is the wind speed in meters per second.

Results

Returns the heat loss in watts per second.

Examples

t = 0, v = 5, function returns: 1066.6
t = -2, v = 2, function returns: 919.0
t = 2, v = 8, function returns: 1107.3

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Copyright © 1997-2001 by Mark E. All Rights Reserved.