Tom Priddy, Extension Ag. Meteorologist and George Duncan, Extension
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
A further example of the effect of
wind for a temperature of 10 degrees F is shown below:
||exposed flesh begins freezing
||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
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,
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.)
|Less than 1
||Calm. Smoke rises vertically.
||Direction of wind shown by smoke drift, but not by wind vanes.
||Wind felt on face; leaves rustle; ordinary vane moved by wind.
||Leaves and small twigs in constant motion; wind extends light flag.
||Raises dust and loose paper; small branches are moved.
||Small trees in leaf begin to sway; crested wavelets form in inland
||Large branches in motion; whistling heard in telephone wires; umbrellas
used with difficulty.
||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.)
*loose: 55-60% of the windbreak is open.dense:
15-25% of the windbreak is open.
Distance downwind from the windbreak:
|Distance from windbreak
to point of minimum wind
medium: 40-45% of the windbreak is open.very
dense: less than 15% of the windbreak is open.