› Hypothermia - is a condition
in which core temperature
drops below the required
temperature for normal
metabolism and body
functions which is defined
as 95.0 °F.
› Frostbite - is the medical
condition where localized
damage is caused to skin
and other tissues due to
Cold Weather Dangers
› Check the local weather forecast for the time
period you will be in the backcountry – wet,
dry, wind, temperature extremes.
› Consider where you will be going in the
backcountry – water front, mountains, alpine.
In dry weather, the temperature drops about
5.5F per 1000 feet in elevation gain (3.2F for
› Consider how long you will be in the
› Consider your own sensitivity to low
› Consider the extent of your physical exertion at
the end of the day.
Deciding What to Bring
– Combination of
Mechanisms of Heat Loss
› Thermal conductivity, k, is the
property of a material's ability
to conduct heat.
› Heat transfer across materials
of high thermal conductivity
occurs at a higher rate than
across materials of low thermal
› Materials of low thermal
conductivity are used as
Water (vapor) 0.016
Air (sea level) 0.025
Silica Aerogel 0.026
Polyurethane Foam 0.02 - 0.03
Wool 0.03 - 0.04
Polystyrene Foam 0.03 - 0.05
Hollow Fill Fiber Insulation 0.042
Paper 0.04 - 0.09
Wood 0.09 - 0.14
Mineral oil 0.138
Particle Board 0.15
Neoprene 0.15 - 0.45
Cement, Portland 0.29
Water (liquid) 0.561
Thermal grease 0.7 - 3
Concrete, stone 1.7
Water (ice) 2.2
Stainless steel 14
Steel 45 - 65
› Thermal resistivity, R, is the product
of the material thickness and
reciprocal of the thermal
› Good thermal insulators have large
› An insulating material will have a
large R value when the thermal
conductivity of that material is
small and the thickness is wide.
› In layered materials, R-values can
1 tR t
› The thermal insulation of
clothing is proportional to the
thickness of the dead air space
› Dead air is defined as any
enclosed unit of air that is small
enough that natural convection
currents would not arise in it.
› The dead air next to the skin is
heated up by the body and
provides a layer of warmth
around the body.
› The clothing is not what is
keeping you warm it is the dead
› The key to providing this dead air
space is through having a number of
layers of clothing.
› If you have too much clothing on, you
will overheat and start to sweat. You
need to find the proper heat balance
between the number and types of
layers and your activity level.
› Heat loss from a wet surface can be up
to 25 times greater than a dry surface
(due to the higher density of water).
› If you sweat and get soaked, you will
lose heat much more quickly through
evaporation of the water.
› So you want to control your layers so as
to be warm at the activity level you are
in but not sweating profusely.
› Convection may account for the
greatest amount of heat loss under
most conditions. In order to properly
insulate, you need to have an outer
layer that is windproof.
The Layering Principle
› The Base Layer - wick
moisture away from the
body allowing you to
remain dry and warm.
› The Middle Layer - provide
thermal protection from
the wind and the cold.
› The Outer Layer - protect
against rain and wind, but
allowing you to breath.
› Extremities - heads, hands
and feet. Up to 40% of the
bodies heat can be lost
through the extremities.
The 3 Layer System
› Cotton is basically useless in winter time.
› Problems with cotton occur when the cotton gets
› Cotton absorbs this moisture and the water
occupies the space previously occupied by dead
› When water occupies the space previously
occupied by dead air, cotton loses all insulating
› Because cotton holds so much moisture, it can hold
that moisture against your body and sap body heat
from you by high evaporative cooling and
conduction. This can quickly lead to hypothermia.
› A cotton garment is almost impossible to dry out.
› Cotton becomes abrasive when wet.
No Cotton - Cotton Kills!
› Silk loses its insulating properties
when it gets wet and does not
wick like modern hydrophobic
› Wool insulates relatively well
when wet. But while some
weaves do shed water for a
period of time, it will eventually
absorb a great deal more
moisture than comparably
weighted synthetic garments
and become very heavy.
› All of these natural fiber fabrics
take much longer to dry once
wet than comparably weighted
› Light Weight – Capilene 2
› Medium Weight – Capilene 3
› Expedition Weight – Capilene 4
› Fleece, made from polyethylene or other synthetics has
many of the features of wool, but is lighter. It provides
good insulation even when wet, absorbs very little
moisture, and dries quickly.
– 100 Weight
– 200 Weight
– 300 Weight
Mid (Insulating) Layer
› Down Fill has very good warmth to weight ratio, and
can be packed down (squeezed) to take very little
room. It is expensive, makes a thick garment, dries
slowly, loses its insulating properties when wet or
compressed, and stops lofting properly after being
washed several times.
Mid (Insulating) Layer
› Fill power is a measure of the
loft or "fluffiness" of a down
product that is loosely related
to the insulating value of the
› The higher the fill power the
more insulating air pockets the
down has and the better
› Fill power is expressed as cubic
inches per ounce (in³/oz).
› Fill power ranges from about
300 in³/oz for feathers to
around 900 in³/oz for the
highest quality down.
› A lofting power of 400-450 is
considered medium quality,
500-550 is considered good,
550-750 is considered very
good, and 750+ is considered
Mid (Insulating) Layer
Technically speaking fill power is a measurement of the amount of space one
(1) ounce of down (as shown on the right) will occupy in cubic inches when
allowed to reach its maximum loft.
For example, one (1) ounce of 800 fill power goose down will loft to 800 cubic
inches. The higher the fill power the larger the down cluster. Larger down
clusters will loft higher, last longer and sleep warmer.
› Synthetic Fill is polyester fiber (such
as Polarguard, Hollofil, Quallofil) used
similarly to down, but does not have
as good a warmth to weight ratio.
› It is less expensive, provides good
insulation (fairly efficient at providing
dead air space though not nearly as
efficient as down) even when wet,
dries quickly, and absorbs very little
› Over time with repeated
compressions, the fibers become
damaged and become less
effective as an insulator.
› Primaloft - the principal behind super
thin fibers is that by making the fiber
thinner you can increase the amount
of dead air space in a given volume
of material. Other super thin fibers
include Microloft and Thinsulate.
Mid (Insulating) Layer
› Hard Shell – these materials are waterproof and somewhat
breathable. Their essential element is a thin, porous membrane that
blocks liquid water, but lets through water vapor (evaporated
sweat). The more expensive materials are typically more breathable.
The best-known brand is Gore-Tex.
› All rainwear exteriors (also known as face fabrics) are treated with a
durable water repellent (DWR) finish. Even rainwear classified as
water-resistant (which includes soft shells) carries a DWR finish.
› Soft Shell – these are water
resistant materials only partially
block water. On the other
hand they are usually more
breathable and comfortable
than hard shells.
› Soft Shells use sophisticated
stretch woven fabrics
(Schoeller, for example) with
tight layered weaves and
durable water repellent (DWR)
treatments to guard against
wind, rain, and snow in all but
the most severe weather