(Equivalent Temperature)
ISSUED: 4-76
Tom Priddy, Extension Ag. Meteorologist and George Duncan, Extension Agricultural Engineer

During the winter months many decisions are made which depend upon the temperature and the extremes which might be expected. 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 effects of wind and temperature, much of it by the U.S. Army Quartermaster Corps. The relationships are reasonably well known and can be expressed as a mathematical function of the wind and temperature. An easy way to present these relationships is in the form of a graph, shown here as Figure 1. 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.
A further example of the effect of wind for a temperature of 10 degrees F is shown below:
Windspeed Reactions
5 mph very cold
10 mph bitter cold
15 mph exposed flesh begins freezing
30 mph exposed flesh freezes in less than one minute

Equivalent temperatures can also be useful in determining the influence of weather on livestock. The response of animals to cold varies considerably, depending upon 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 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, Table 1 can be useful in making an estimate.
The research which has been done to relate the effects of wind chili 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 equivalent temperature by about 20 degrees in a light wind and 10 degrees in a strong wind.
Those who recall occasions when they had to be outdoors on a cold, windy day, either waiting on a school bus or doing some type of work, know the benefit of using a fence, a tree, a building, or some type of obstruction to reduce the force of the wind. Anyone wishing to reduce the wind chill factor on their livestock feed lot 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 shrubbery. However, the denser it is, the more effective it will be. The results of research on the influence of different windbreak densities are shown in Table 2 for a low (6 foot) windbreak.

Table 1: Guide for estimating wind speed.
Wind Speed (m.p.h.) Description
Less than 1 Calm. Smoke rises vertically.
1-3 Direction of wind shown by smoke drift, but 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 difficulty.
32-38 Whole trees in motion; inconvenience felt in walking against wind. 

Table 2. Mean Wind speed at a height of 3 feet with windbreaks of various densities: (expressed as a percent of the wind in the open.)
Distance downwind from the windbreak:
Distance from windbreak 
to point of minimum wind
0-15 ft.  15-30 ft. 30-60 ft. 60-90 ft.
loose 49% 41% 63% 82% 37% 18 ft.
medium 45 35 48 72 34 20 ft.
dense 29 33 58 80 26 9 ft.
very dense 22 46 72 87 14 3 ft.
*loose: 55-60% of the windbreak is open.dense: 15-25% of the windbreak is open.
medium: 40-45% of the windbreak is open.very dense: less than 15% of the windbreak is open.