Re: Which types of clothing best protect from windchill?
Date: Sat Jan 29 18:37:08 2005
Posted By: Joseph Weeks, Engineer
Area of science: Physics
ID: 1106495651.Ph Message:
When you are outside on a cold day, there are two main methods of losing
heat. The first is conduction, where heat is transferred from a higher
temperature to a lower temperature by physical contact with a solid,
liquid, or gas. The second mechanism of heat loss is by convection.
Convection is the process of transferring heat to a moving fluid, whether
the fluid is liquid or gas.
Conductive heat transfer depends upon the thermal conductivity of the
material that you are in contact with. For example, steel is a pretty good
conductor (certainly compared with air). So, just imagine how you would
feel if you sat on a steel chair while not wearing any clothes. Even if
the chair is at room temperature, it still feels cold because it transfers
more heat away from you than air. Because steel is such a good conductor,
if you touch your tongue against a steel pipe when the temperature is low
enough, the moisture on the surface of your tongue will freeze against the
Air, on the other hand, is a pretty poor thermal conductor. So most of the
clothes we wear to keep warm are designed to keep air trapped against our
bodies in order to take advantage of the insulating properties of air.
More trapped air keeps you warmer; less, cooler.
Convective heat transfer is involved in the process we call windchill. In
convective heat transfer, let's say that there is a little bit of air
around your body that you have heated up. Because of movement of the air,
that warm air is removed from being close to your body, replaced instead
with air that has yet to be warmed. So you can see that the more often
that warm air layer next to your body is replaced, the more heat is going
to be lost from your body.
Windchill comes from the idea that when you expose bare skin to
temperatures below freezing, sooner or later you will get frostbite. If
the air is still, there is not a lot of convective heat transfer taking
place. If the wind is blowing, there is a lot more convective heat
transfer, so your skin will freeze faster. There is a windchill chart at
this link (http://www.nws.noaa.gov/om/windchill/index.shtml
) which shows
the relationship between temperature, windspeed, and windchill. It is
pretty easy to understand a windchill graph. For example, in calm air, you
will start to get frostbite after 30 minutes when the temperature is about
-13F. In a 30 mile per hour wind, you will get frostbite in 30 minutes if
the outside temperature is 5F.
Your question is which material is the best for protecting against
windchill. Since windchill is caused by the movement of air, replacing the
warm air layer next to your skin with cold air, the best material for
preventing windchill is a material that prevents outside air movement from
disturbing the warm layer of air next to your skin. From that point of
view, a material that doesn't let air move through it would work to prevent
windchill. A plastic bag, for example, provides good protection from air
movement. So, if you are in a survival situation, a plastic trashbag that
you can put over your clothes may help you trap enough heat to help save
your life. Space blankets are sheets of aluminized mylar that also prevent
air movement through them keep you warm by limiting convective heat transfer.
In your question, you suggest cotton, denim, and felt as three materials
under investigation. Denim is one type of fabric made from cotton; the
only thing different is how tightly the cotton is woven and how much fiber
is included per square yard. Felt is often made from wool, although
felting is a process, not a specific material. Yes, you can get cotton
felt. So, let's concentrate on specific fibers rather than differences in
the structure of the fabric.
When designing clothes for cold weather wear, however, you also have to
keep in mind that your body regulates it's heat by releasing moisture,
which when if evaporates, produces localized cooling. Dealing with
perspiration is a significant problem. Water, whether in the form of
perspiration or whether it comes from rain or snow, has different effects
on different fibers. Cotton, for example, adsorbs quite a bit of water
(about 7%). As soon as cotton adsorbs moisture, it's thermal conductivity
goes higher (since the air pockets within the cotton fiber are replaced
with water pockets). You can read a lot about the structure and properties
of cotton here: http://www.engr.utk.edu/mse/pages/Te...n%20fibers.htm
Wool has long been known as a fiber that retains much of it's insulating
properties, even when wet. The surface of wool fibers consists of an outer
layer of overlapping scales, called cuticles, which are water repellant.
The interior of the wool fiber is similar to cotton so that it adsorbs
moisture. Also wool fibers seem to have more natural bending than cotton.
As best as I understand it, because the outerlayers of wool fibers are not
hydrophillic or wetted by water, there is less tendency for water to be
trapped between fibers. Instead, the gaps between fibers will tend to hold
some air between them. Cotton, on the otherhand, has a hydrophillic
surface, so water is trapped between fibers, which will push out the
insulating air between fibers.
Synthetic fibers, such as polypropylene, have a number of interesting
features which are making them more popular for cold weather ware.
Polypropylene does not adsorb moisture, it has low thermal conductivity
, about half way down the page for a chart that shows thermal conductivity
of different fibers). And you can get different surface treatments on
polypropylene so that it can be either hydrophillic (loves water) or
hydrophobic (hates water).
So, in the process of trying to stay warm, you want a fabric that will
prevent the movement of air from the outside in, yet you want a fabric that
can handle perspiration that your body generates and resists moisture from
the outside. The W. L. Gore company has made a science out of trying to
balance the properties of fabrics for cold weather protection. For
example, their GORE-TEX® Fabrics use a special membrane that has holes that
are too small for air movement to penetrate, is naturally hydrophobic so
water won't get through, yet have small enouth pores that water molecules
can get through. The have other fabrics such as WINDSTOPPER® Fabrics that
attempt to further reduce windchill, along with AIRVANTAGE™ Adjustable
Insulation, where the wearer can vary the amount of trapped air retained
within the structure, allowing you to change how much heat you retain. See
3M has also worked to improve the insulating properties of clothes by using
much finer fibers than normal textile fibers. Trapping a large number of
very small air pockets seems to make their THINSULATE® insulation work
better than most other insulating fibers. You might want to check out: http://cms.3m.com/cms/US/en/2-147/crzuRFW/view.jhtml
So, as you might guess, there is a lot of technology involved in keeping
you warm on a cold, windy day. And the process is one of optimization,
answering questions such as how heavy should the clothes be, how thick and
bulky, how concerned should we be about water, and how much will a consumer
pay for a technologically superior solution. I hope this gives you a bit
of direction for your science fair project.