Download How Your Body Loses Heat

How Your Body Loses Heat
The human body loses heat through the skin in four ways: convection, conduction, evaporation and radiation. Maintaining
proper oversight of these four potential freeze factors is the easiest and best way to keep cold-weather injuries at bay.
Convection is simply heat loss caused by cooler air or water coming into contact with the skin. Just as you cool your cup of
coffee by blowing on it, your body cools as the wind whips past. Minimizing exposed skin is the easiest way to prevent this.
A better way is to ensure that your skin is covered and that you have an adequate amount and type of clothing on to keep
your skin warm. "Wind chill" is a term used to signify how wind multiplies the cooling effect of cold air temperatures. Check a
wind chill chart and you will see how easily a 25 mph wind cools a reasonably chilly winter day of 10 degrees Fahrenheit to
30 below zero. Wind plus cold equals increased danger.
Conduction is the transfer of heat to objects or substances in direct contact. Your tongue sticking to the flagpole in grade
school is a perfect example of how tissue can almost instantly freeze when in contact with cold metal, an excellent heat
conductor. Keep in mind that alcohol is also an excellent conductor and often freezes at much colder temperatures than
water, so if you bring a beer out on your next cold-weather adventure, don’t throw back a few swigs until you’ve checked its
temperature. Extremely cold alcohol can instantly freeze and damage your lips, tongue or other mouth tissue. And if the
alcohol comes in contact with your throat or esophagus, that can turn out to be deadly.
Evaporation, or the changing of liquid to a gas (e.g. sweating), is a very effective means of cooling the body. Simply
sweating out one gram of water extracts almost 600 calories of heat! Unfortunately, this highly efficient means of cooling
doesn’t help when you’re trying to stay warm. In other words, sweating makes you colder. Furthermore, sweating and not
knowing it (e.g. in water) can be dangerous because dehydration can speed along cold weather injuries.
In addition to perspiration, another form of evaporation takes place when the air you breathe is humidified by your body so
your lungs can readily accept and process it. Since cold air is generally very dry, your body loses a lot of heat in simply
moistening your inhaled breath. While there is little you can do about hydrating your breathing (besides staying hydrated by
drinking fluids), wearing layered clothing made of moisture-wicking fabrics will help minimize your skin-level evaporation.
Radiation. The human body constantly radiates heat to lower-temperature objects, especially when not absorbing heat from
the sun or a fire. There is little one can do about loss of heat from radiation, but with proper clothing to minimize other heat
losses, radiation heat loss can generally be compensated for.
1.
In order to maintain temperature, total heat loss must equal total heat gain, if mass is not changed. Heat can be
lost by radiation (R), conduction (K), convection (C), and evaporation (E). Heat can be gained by increasing metabolic rate,
radiation, convection or conduction. The rate of heat loss for convection, conduction and radiation depends upon the
temperature gradient between the individual and the environment, the area through which heat loss can occur and the
thermal conductivity coefficients for each mode of heat loss.
The contributions of each of these mechanisms depends on the external air temperature. The rate of evaporation depends
on the difference between the partial pressures of sweat and air. It is therefore the only mechanism for heat loss when air
temperature exceeds body temperature. The relative contributions of these mechanisms becomes more weighted towards
evaporation and away from radiation, convection and conduction as the external temperature increases .
"Heat will always be conducted from a body of higher temperature to one of lower
temperature."
"If you were to sit in still air [at an air temperature of 23 degrees Celsius], conduction would
account only for about 1% of your heat loss, convection for about 40%, radiation for another
50% and evaporation for about 9%. A breeze of just 15 km/h will increase total heat loss
substantially by increasing convection fivefold. Evaporative cooling is increased greatly by
the production of sweat."
In a hot, dry environment most of the sweat evaporates into the air. This helps cooling
because evaporation consumes energy and hence draws heat from your skin.
"However, evaporation can only occur if the surrounding air is not saturated with water
molecules (that is, the relative humidity is less than 100%). This is the biological basis fot
the common complaint "the heat is not as bad as the humidity."