Understanding Wind Chill

Wind Chill Temperature

UNDERSTANDING WIND CHILL...

(Wind Chill Equivalent Temperature)

During the winter months many decisions are made which depend upon the air temperature and the extremes of temperature. Not only do we dress according to the thermometer, but we use it to anticipate the feed requirements and care of livestock. If we have to be out-of doors, as is the case with children who have to stand waiting for a school bus, we soon find that the temperature alone gives too little information. Should the temperature be zero with a light wind, we would not be as cold as with a strong wind. Thus, if we have an idea of how much wind increases the chilling effect of temperature on the human body, we can get a better estimate of how "COLD" it really is.
Considerable study has been made of the chilling effect of wind and temperature, much of it by the U.S. Army. The relationship is well documented and can be expressed as a mathematical function of wind and temperature. An easy way to present these relationships is in the form of a graph, shown below. The acceptance of the wind chill factor by the military and others indicates it does have a definite usefulness in planning outdoor activities where proper clothing and wind protection are essential.
Wind chill is based on the rate of heat loss from exposed skin caused by combined effects of wind and cold. As the wind increases, heat is carried away from the body at an accelerated rate, driving down the body temperature.

Wind Chill Chart
Wind Temperature(F) ->
(MPH) 35 30 25 20 15 10 5 0 -5 -10 -15 -20 -25 -30
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5: 32 27 22 16 11 6 0 -5 -10 -15 -21 -26 -31 -36
10: 22 16 10 3 -3 -9 -15 -22 -27 -34 -40 -46 -52 -58
15: 16 9 2 -5 -11 -18 -25 -31 -38 -45 -51 -58 -65 -72
20: 12 4 -3 -10 -17 -24 -31 -39 -46 -53 -60 -67 -74 -81
25: 8 1 -7 -15 -22 -29 -36 -44 -51 -59 -66 -74 -81 -88
30: 6 -2 -10 -18 -25 -33 -41 -49 -56 -64 -71 -79 -86 -93
35: 4 -4 -12 -20 -27 -35 -43 -52 -58 -67 -74 -82 -89 -97
40: 3 -5 -13 -21 -29 -37 -45 -53 -60 -69 -76 -84 -92 -100

15°F to 30°F COLD. Unpleasant.
0°F to 15°F VERY COLD. Very unpleasant.
-20°F to 0°F BITTER COLD. Frostbite possible.
-20°F to -60°F EXTREMELY COLD. Frostbite likely. Outdoor activity becomes dangerous.
* Wind speeds greater than 40 MPH have little additional cooling effect.

Wind chill factor (equivalent temperatures) can also be useful in determining the influence of weather on livestock. The response of livestock to cold varies considerably, depending upon the species, breed, condition, and the ration being fed. In general, lower temperatures increase the amount of feed required to maintain a given production level. The animal may sense that the weather is colder either by a drop in the air temperature or an increase in wind speed.
When determining air temperature with a typical thermometer, the location and exposure of the device generally has much more effect on accuracy than the inherent instrument error. For proper measurement of air temperature with a thermometer, it should be sheltered from precipitation and direct sunlight but be open to free air movement.
If you have no instrument to measure wind speed, the Table below can be useful in making an estimate.

Wind Speed (mph) Description

-------------- ---------------------------------------------

Less than 1 Calm. Smoke rises vertically.

1 - 3 Direction of wind shown by smoke drift, not by

wind vanes.

4 - 7 Wind felt on face; leaves rustle; ordinary vane

moved by wind.

8 - 12 Leaves and small twigs in constant motion; wind

extends light flag.

13 - 18 Raises dust and loose paper; small branches are

moved.

19 - 24 Small trees in leaf begin to sway; crested wavelets

form in inland waters.

25 - 31 Large branches in motion; whistling heard in

telephone wires; umbrellas used with dificulty.

32 - 38 Whole trees in motion; inconvenience felt in

walking against wind.

The research conducted to relate the effects of wind chill to the response of humans assumes that there is no heat being received directly from the sun. This condition would be typical of an overcast day or of the nighttime hours. Sunshine received on a clear day can effectively moderate the wind chill more than one might expect. From mid-morning to mid-afternoon on a sunny day, the amount of solar energy received can raise the temperature by 25 degees (F) in a light wind and 10 degrees (F) in a strong wind.
Those wishing to reduce the wind chill factor on themselves or livestock or just reduce the heating requirements for their home can achieve the best results by using some type of windbreak. This doesn't have to be a solid wall but can be a simple row of trees or shrubery. However, the denser it is, the more effective it will be.

WIND CHILL FORMULA(S): If you have units of wind in mph and temp in degrees F...try this: T(wc) = 0.0817(3.71V**0.5 + 5.81 -0.25V)(T - 91.4) + 91.4 T(wc) is the wind chill, V is in the wind speed in statute miles per hour and T is the temperature in degrees Fahrenheit. --- Here's another formula with temp in Kelvin and wind in meters per sec. Wind Chill T (in K) = 306.15 - (0.453843 * SQRT(Speed) + 0.464255 - 0.0453843 * Speed) * (306.15 - Temp [in K]) As you can see it is a bit complicated but this is the "official" NWS formula. I hope I typed it out correctly! Note: Speed (above) is in meters per second and Temps are in degrees Kelvin.

Developed by Siple and Passel (1945): H = (SQRT(100V) + 10.45 - V) x (33 - T) where H = windchill in kilocalories per square metre of exposed flesh per hour V = wind speed in metres per second and T = air temperature in degrees C
For more information concerning wind chill, contact your local county extension agent or Tom Priddy in the UK Ag Weather Center.