by Bill Mauney


There is a chill in the air on this crisp fall day.  An explosion of bright hues rocks your senses as the golden rays of the rising sun start to illuminate the hardwoods at the top of the ridge.  Seeing your breath for the first time this season reminds you that winter is coming.

The new fallen leaves crunch beneath your feet as you trudge back and forth up the switchback.  You are starting to get hot so you stop and shed your jacket.  Finally you crest the top of the ridge.  You swing off your pack, sit down on a large rock and drink in the commanding view of the mountains in all directions.  At first the breeze feels good against your overheated body, but soon you start to shiver.  Hey!  What’s the deal?  Isn’t it only room temperature outside today?  What is going on?

To answer that question you need to have an understanding of how your body works and how it is affected by your environment.  Applying this knowledge will not only keep you comfortable in the outdoors, but also safe from a slippery slope that can lead to death, called hypothermia.

Let’s start with some basics of how your body works.  Our bodies are magnificently created and incredibly sophisticated.  However, in order to KISS (Keep It Simple Stupid) let’s think of our body as an engine, just like in your car.  Our body takes in its daily ration of fuel (carbohydrates, proteins and fats)  (sometimes I take in too much fuel ?).  When this fuel is burned its byproducts are energy and heat.  The faster the engine runs, the more heat produced and the faster we consume our stored fuel.  If we build up too much heat, then we are prone to over heat (hyperthermia) which can lead to heat cramps, heat exhaustion and worst of all, heat stroke.  So, the body has a radiator to dissipate heat and keep your engine running at tolerable temperatures.  As your core temperature (the temperature of the deep organs of the chest and abdomen) rises, the blood running through your heart (water pump) is heated up.  As it circulates to the extremities of your arms and legs as well as through your neck and your head, it runs through vessels that are close to the surface of your skin, The heat dissipates to the cooler air surrounding your body, the blood returns to the core cooler than it left and thus keeps your engine at the optimal temperature of 99 degrees F.  If this circulation alone does not dissipate enough heat, then our bodies begin to sweat.  The evaporative heat loss begins to cool us down.  If the engine is running extra hard, then our heavier breathing throws off extra heat as well.  There can come a point, if conditions are right, that our body is losing heat to the environment faster than it can produce it.  This leads to hypothermia, which if undetected and untreated, can lead to death rather quickly.  An increase or decrease of just a degree or two can tip the scales in a negative direction.  This delicate balance between running too hot or running too cold is called homeostasis.  Your body is marvelously adept at adjusting one way or the other up to a point.


So what is happening to you as you take in the view at the top of the ridge?  When you first reached the top, you were very hot from your hike up from the valley.  Your body and your clothes were wet with sweat.  You stripped down to just a t-shirt.  Tired, you sat down on the large rock to take in the view you had earned.  But soon, you start to shiver.  Where did all of that heat go?  You lost heat via the following means:

  • Radiation – The heat rises from the surface of your skin and dissipates into the air surrounding your body.  This is particularly true where blood vessels run close to the surface of your skin in your extremities, neck and head (your “radiator”).
  • Convection – The breeze carries off the radiated heat just above the surface of your body.  More heat radiates and it is carried off as well, etc., etc.
  • Evaporation – The sweat on your exposed skin and on your wet clothes begins to evaporate with the byproduct of evaporation being a cooling effect.
  • Conduction – Because the rock is colder than your body, heat travels from your warm body to the cooler rock.
  • Respiration – Just as on a cold day you exhale on your cold hands to warm them up, your exhalations remove heat from your body, just like a dog dissipates heat.

As your body core temperature starts to decrease, your body automatically starts to react to maintain the core temperature at 99 degrees F.  What happens?  When the core temperature drops below 99, the core sends a message to the brain to turn up the heat.  The command center in the brain starts to throw some switches to reduce core heat loss.

You will notice the following:

  • Shivering – Your body begins to shiver, thus effectively revving up your engine to produce more heat.  This is your body’s involuntary attempt to exercise.  Heat is produced, but more fuel is burned in the process. Shivering is your first and best sign that your body is losing heat to the environment faster than it is producing it.  It is time to act…NOW!
  • Fingers and toes start to feel cold – Your body begins to constrict the blood vessels and reduce blood flow to the extremities.  This, in effect, keeps the warmer blood in the core area and reduces the amount of heat loss in the extremities as explained in the radiator analogy above.  Since the head and neck are so richly supplied with blood and the brain will not shut down life giving blood to itself, 50 to 70% of your heat loss happens in those areas.  The brain will sacrifice the limbs (the extreme being frostbite) in order to save itself and the core.

These symptoms are the first signs of hypothermia and demand your immediate attention to take corrective action before your body hits the slippery slide of body temperature, the treatment of which is covered in the wilderness first aid literature.

So, knowing all of the above and noticing these first symptoms, what should you do?  The answer, prevent further heat loss, increase heat and add heat as necessary.

Prevent further heat loss:

  • Insulate – By adding clothing layers, insulation is increased and heat loss is reduced.  Putting on an insulated hat and covering your neck stops the heat loss of the head and neck dramatically. Insulation added (ex. a closed cell insulated pad) between you and a cold object, like a rock, prevents heat loss via conduction.
  • Dry your body and replace damp or wet clothing with dry clothing – This reduces the effects of evaporative heat loss.
  • Seek and/or put on shelter – Getting out of the wind and/or rain by seeking shelter from it or putting on a windproof and/or waterproof shell jacket or parka (portable shelter) will reduce heat loss via convection.
  • Put on your hat – The old timers used to say, “If your feet are cold, put on your hat!” Since the majority of your body’s heat loss is through the back of your neck and the top of your head, putting on a hat slows down your heat loss dramatically.  Once the core temperature gets back to normal, the brain sends a message to the blood vessels to dilate. When the warm blood returns to the extremities, voila, your feet get warm.

Increase heat:

  • Exercise – Body movement increases heat production.
  • Add Fuel – The body must have fuel (carbohydrates, proteins and fats) to burn in order to produce more heat via metabolism and exercise.
  • Stay hydrated – A dehydrated body is more prone to slipping into hypothermia.  Don’t wait until you are thirsty.  If you do so, you waited too long.  Drink often.  Water and occasional electrolyte replacement drinks are best.  Avoid diuretics like coffee, tea, colas, etc.

Add heat as necessary:

  • Add warmth – A camp fire, a warm meal, a warm drink, etc. can turn the internal temperature up a notch.

Note:  The above suggestions are the first responses to preventing further heat loss.  As hypothermia advances through its various stages, the first aid treatment required might modify or exclude some of the suggestions above.

And all of the above information applies to any outdoor lifestyle pursuit.  You don’t have to be in the wilderness to experience this.  You have all had this experience at one time or another.  Watching the second half of a football game, sitting on a cool bleacher in the shade of the stadium on a windy fall day can lead to the same symptoms.  The preventative measures are the same.

Let’s take a closer look at your clothing.  Dressing in layers allows for flexibility in adjusting your body’s insulation quickly.  Not too hot.  Not too cool.  But, just right…comfortable.

The layering system of dressing is broken down into three parts.

  • Next-to-the-skin layer – The first layer is a thin top and bottom (long or short underwear) made from a synthetic material that is designed to provide minimal insulation and wick moisture away from your body, through to the surface of the fabric, where it can evaporate or travel through other layers, eventually to the surrounding air.  This base layer, complete with quick dry pants or shorts may be all that is needed in warm to hot conditions.
  • Insulating layer(s) – One or more insulating layers of synthetic fleece traps air, diminishes heat loss and keeps the warm air around the body.  Adding or subtracting these insulating layers adjusts your body’s comfort.  Down is a great insulator, but is not recommended for active sports.  It is best for inactivity on cold days or nights, say around camp or at the football game.  Also, its extra warmth doesn’t allow for the fine tuning of your layering system as does multiple thinner layers. It also does not wick body moisture during active sports, but the down captures the moisture.  Hats, both insulated and non-insulated, prevent heat loss from the top of the head.  Neck warmers, scarves and hoods prevent heat loss from your neck.  Wool or synthetic blend socks insulate the feet and wick moisture away keeping your feet dry and comfortable.
  • Shell layer – A windproof shell over the next-to-the-skin layer or over it and the insulating layer(s) prevents heat loss via convection.  Waterproof shells prevent the first two layers and your body from getting wet.  Wet clothing sucks heat away from your body at an alarming rate.  A totally waterproof shell not only keeps rain and snow off, but also traps body moisture wicking through the first two layers or directly from the skin if you are wearing a wicking t-shirt.  This body moisture then soaks your clothing and produces a chilling effect.  A windproof, waterproof, breathable fabric in a shell that can be ventilated is the ultimate shell garment. A windproof, waterproof hood covers the neck and head.  Quick drying synthetic pants will keep the wind off and rain pants can be added when conditions deteriorate.

A comfortable body temperature can be maintained by controlling the layering mix needed for a given condition. The best way to stay warm is don’t get hot!  Garment adjustments such as unzipping to ventilate built up heat, removing your hat or pulling your sleeves up exposing your forearms and wrists to the air, also help in fine tuning your comfort during activity or changing conditions.  Garments should be made from synthetic materials that wick moisture and dry quickly.  Even if soaked, these layers can be wrung out and they don’t hold much moisture.  ABC…Anything But Cotton.  Once cotton is damp or wet, it is tough to dry out and will suck heat away from your body quickly.  Wool is a great insulator, still providing some insulation if it gets wet, however, it is heavy, uncomfortable and smelly when wet.  Synthetics are the better option.

My personal experiences, both in treating and having hypothermia myself were not on particularly cold days, but in late spring, summer and early fall in North Carolina.  While leading a team fighting a forest fire on Rumbling Bald near Lake Lure, NC on a hot (90’s) August day, I became extremely dehydrated from the hard work, the heat of the day and the heat of the fire.  When a thunderstorm rolled in that afternoon, the air temp dropped quickly and the wind began to blow.  My wet, wind chilled body immediately went on a nose dive.  Fortunately, those with me were well trained in dealing with hypothermia and treated me.

Knowledge is power.  Knowing how your body works, how you lose heat to the environment, the symptoms of the early stage of hypothermia, how to dress in layers, how to prevent heat loss and how to increase your body’s heat will not only keep you comfortable in your outdoor pursuits, but just may save your life or others.

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