Re: How is wind chill calculated?

Message ID Number: 861154108.Es

Subject: How is wind chill calculated?

Mike:

Wind chill is generally calculated based on just two measurements: wind speed and temperature. I thought you might want a few variations of the wind-chill equivalent temperature (WET) formula so I have provided you with several. This variety is a product of the uncertainty in estimating WET.

The formulas are as follows:

Variation 1)

WET = 91.4 - [(0.47 + 0.3(sqrt(v) - 0.02v) (91.4 -t)]

where: v = wind velocity in mph, t = actual air temperature in degrees F (sqrt = square root)

with baseline conditions of: v >= 4 mph, T < 91.4 °F, 91.4 = baseline temperature (F) for exposed skin

_____________________________________________________________________

For example: Using the first equation with a wind speed of 20 mph and an ambient air temperature of 45 degrees F:

WET = 91.4 - [(0.47 + 1.34 - 0.4) (46.4)] = 26.0 degrees F _____________________________________________________________________

Variation 2)

WET = 0.0817 (3.71 v**0.5 + 5.81 - v/4) (t - 91.4) + 91.4

_____________________________________________________________________

Variation 3)

WET = 91.4 - (0.474677 - 0.020425 * v + 0.303107 * sqrt(v)) * (91.4- t)

_____________________________________________________________________

Variation 4)

For degrees C and meters per second (m/s)

WET = 33 + ( t - 33 ) ( .550 + .417 sqrt(v) - .0454 v )

or:

WET = 33 + ( t - 33 ) ( .474 + .454 sqrt(v) - .0454 v )

where:

v >= 1.79 m/s

t < 33 degrees C

_____________________________________________________________________

Please note that while actual cooling is somewhat dependent on relative humidity, by definition wind chill is NOT. In fact, humidity generally has very little effect on cooling, particularly at low temperatures.

The National Center for Atmospheric Research, Atmospheric Technology Division has an excellent web page for learning about wind chill at the following URL:

http://www.atd.ucar.edu/homes/rilling/wc_formula.html

Several of the formulas above, as well as other information presented here, are from that page.

Clay Harris