Heat Loss in a Sleeping Bag
When venturing out into your latest outdoor adventure
in cooler temperatures, a sleeping bag is an essential piece of equipment,
allowing you to replenish yourself with a good night’s rest even amidst
climate conditions that would normally make sleep impossible.
The sleeping bag accomplishes this in a number of ways, such as by
providing a soft area for comfortable slumber, but its crucial function is
to maintain the warmth of your body so that you don’t end up shivering the
whole night, or worse, succumb to hypothermia.
To better appreciate how the contemporary sleeping bag achieves this,
it may be helpful to understand the basics of how the body loses heat, how
your sleeping bag prevents it from happening, and what else you can do to
stay warm.
The body loses heat in one of four ways:
Conduction, Convection, Radiation and Evaporation.
To compensate, your body burns calories and devotes energy towards
keeping you at the critical 98.6 F regular body temperature.
- Conduction
– this is the process of heat transfer through direct contact, such
as when your warm hand touches a cold rock.
The greater the difference in temperatures between the two
different objects, the faster the heat will transfer away from the hot
object and into the cold object.
The sleeping bag acts as a first line of defense, shielding your body
from contact with the chilly mountain side or forest floor.
Moreover, good bottom insulation on the bag is filled with many
pockets of dead air that trap heat, keeping it from being carried away
by conduction into the surrounding area.
- Convection
– heat transfers in this manner when two objects pass each other,
the simplest example being a cold wind against a warm body.
Heat passes quickly from the warm object to the cold object,
similarly to conduction. The
greater the velocity and the volume of the moving cold object, the more
rapidly the heat gets displaced.
The sleeping bag acts as a wind shield, keeping cold air from carrying
warmth away from you as it hits your skin.
For this reason, mummy bags with hoods and draft collars, which
envelop the entire body and wrap it snugly, are excellent against the
cold.
- Radiation
– this is the direct emanation of heat by an object to its
surroundings, which are of a lower temperature and thus absorb that
heat. The human body is generally
warmer than the surrounding area, and releases its heat through the skin
into the air around. 50 to 70
percent of body heat is actually lost this way through the head and
neck. A sleeping bag, preferably
one with a hood, or some sensible clothing that includes a hat and some
neck insulation, help prevent heat loss through radiation.
- Evaporation
– heat is lost in this manner when your body gets wet, and the water
absorbs heat from your body to turn into a gaseous state.
It also occurs when you breathe in cold air, which is warmed by
your body, and when exhaled, it takes warmth and moisture away.
A sleeping bag made of a waterproof shell is thus essential, and
it would also be good to have a draft collar or wear a scarf around your
mouth so the air is pre-heated before it enters your lungs.
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