It is necessary to understand that clothing do not provide
warmth; they help regulate the heat that the body produces.
Body heat is lost in many ways; evaporation, respiration,
conduction, convection, and radiation. By understanding the above mechanisms, one can
better regulate or utilize their insulating equipment.
Evaporative Heat Loss occurs when the body
perspires. When the body is warm, it sends blood to the surface to cool. As the blood
rushes to the surface, the pores of the skin open emitting water. Water evaporating in the
surrounding air cools the skin. Mountaineers for years have understood that this form of
heat loss is one of the leading causes of deaths in the backcountry. You may have heard of
the importance of wearing many layers of clothing in order to enhance you ability to
regulate body heat. The reason for this is that the human body loses its heat to moisture
25 times faster than to dry air. So, it is very important not to allow the body to
perspire too much in the backcountry. If this happens, it is also important not to let the
perspiration sit in contact with your skin. Hence the importance of wearing hydrophobic
clothing in the outdoors, such as Polypropylene Thermax, Polartec, Capilene, etc.
Respiration Heat Loss occurs when cool outside
air enters the lungs, is warmed, and then is exhaled. In the wintertime many people,
without any prior knowledge of this form of heat loss, wear a scarf in order to stay warm.
Wearing a scarf controls heat loss through respiration. To minimize this form of heat
loss, avoid over taxing yourself and breathing hard.
Heat always travels from a warm surface to a cooler one.
Conductive Heat Loss happens when the body contacts a cooler surface, allowing heat
to be lost to the cooler surface. To limit conductive heat loss, avoid contact with cooler
surfaces by putting an insulating pad between you and the ground. To bring home this
point, it is safe to say that on a cold night an insulating pad is almost as important as
the sleeping bag you sleep in. Without an insulating pad, all the heat trapped by your
sleeping bag will be lost to the ground.
Radiative Heat Loss is the escape of infrared
radiation from the body. This form of heat loss is minimized by simply wearing insulating
fabrics. Also, you may have seen that most long-distance runners are wrapped in a shiny
aluminum foil after finishing their race. Aluminized fabrics do a very good job of
reflecting the escaping radiation back to the body.
Convection Heat Loss occurs when
body heat warms the air around the body. The warm air then rises away from the body. One
can prevent this form of heat loss by trapping this warm air around the body and stopping
its escape. Unfortunately, unlike for some animals, this is easier said than done.
In warm-blooded animals, cold air causes blood vessels near the skin to constrict, keeping
the warm blood away from the chilled exterior in order to help retain the heat of the
body's central core. In animals covered with hair, the constricting of the blood vessels
triggers a muscular reaction, causing their hair to stand up, trapping air within it. The
natural heat emitting from the body will then warm this trapped air translating it into an
insulation layer of warm air around the animal.
The human body no longer has this thick layer of hair. As the blood
vessels near the surface constrict, we get goose bumps. If the surface is not warmed
then we shiver, which is a natural muscular reflex to get warm. To really warm up, we must
put on an artificial layer of hair, which is clothing.