Awareness of the risk of hypothermia is pretty good these days but still many people succumb to it.  What’s often lacking is an understanding of the basic rules that govern the heating and cooling of the human body.  This knowledge could make all the difference between an enjoyable trip and a survival situation.

Fundamentals: Body Temperature And The Environment

Your body needs to maintain a core temperature of approximately 37^oC (98.6^oF).

If the ambient temperature is between 28^oC and 31^oC (between 82^oF and 89^oF), you can be naked and motionless and your body is able to comfortably maintain its core temperature.

When the ambient temperature is less than 28^oC (82^oF), unless your body is protected, you will lose heat to the environment around you.

It’s worth remembering that your body has to generate its own heat internally through the metabolism of food or utilising energy stored within it.  If you are in a wilderness survival situation, or living off the land by choice, this energy is unlikely to be easy to come by.  It’ll certainly be a lot harder than picking food off the shelves of your local store. 

Even on a multi-day trip or expedition, you often only have access to the food you carry with you. Wasting energy through unnecessary heat loss means you either have to carry more food – which is heavy – or go hungry.

If heat loss to your environment continues for long enough, and if you don’t replenish the energy you have expended, you will become exhausted.  Your body will lose heat faster than it can generate it.  This will lead to a lowering of your body core temperature and the onset of hypothermia.  This may seem like an extreme scenario, applicable only to intrepid explorers but there are plenty of cases of fit hikers dying from hypothermia after only a few hours out on the trail in cool, wet conditions.

In cold conditions you can lose heat through a number of different processes – radiation, convection, conduction, evaporation and respiration (breathing).  There’s not much you can do about breathing.  Holding your breath to retain heat is a very short-term strategy!  The other four processes, however, are the apocalyptic four horsemen of heat loss that you want to avoid.  In a cold, wet or windy environment, you should do everything you can to keep them at bay.

Radiation

When you sit by a good campfire and feel its warmth, you are benefiting from heat radiated by the fire.  The other way to look at this is that while you are getting warmer, the fire is losing heat to its environment.  It’s the same for you when everything around you is cold – you will radiate warmth and lose heat to your environment.

Many people think that heat loss from radiation occurs only from exposed skin.  But even if you are clothed, heat radiates from your body to your clothes, then from your clothes to your surrounding environment.  It’s somewhat counter-intuitive but the colder the environment, the more heat you will radiate – even if you are wearing the ‘correct’ clothing.  In fact there is little you can do about this radiant heat loss.  It’s still really important to wear the ‘correct’ clothing as it minimises heat loss via the other processes, particularly convection.

The good news is that radiant heat loss isn’t a huge problem unless you are in a very cold environment, below -30^oC to -35^oC (below -20^oF to -30^oF).  Even then, if you dress correctly, keep active and eat well, radiant heat loss can be compensated for.

Convection

Warm air is lighter than cold air.  Warm air rises, cold air sinks.  We were all told this in science class at school.  You maybe didn’t realise it at the time but you can now put this knowledge to valuable use in the wilderness to help you stay warm on the coldest of days.

Convective heat loss occurs between a surface and a moving fluid or gas in contact with it.  The air nearest your skin is warmed by the body.  If this warm air is allowed to move away from your body, colder air will take its place and you will lose more heat in warming this cold air.  Putting on insulating layers of clothing helps to trap warm air near to your body.  It’s the air trapped in your clothes that keeps you warm, not the clothes.

Particular attention should be paid to insulating your core body area – head, neck, trunk and groin.  There is always a good blood supply to your brain and there is always a lot of warm blood passing close to the surface of your neck.  If you are losing heat from all over your body, 20-40% of your heat loss is typically from the head and neck.  If you are wrapped up really warm on your body but not wearing a hat and scarf, 70-80% of your heat loss can be from the head and neck area.  Even though you have reduced the overall heat-loss by wearing more clothes, a greater proportion is being lost via the head and neck.  This will still be a significant amount of heat, particularly in a very cold environment.

Tip: It is always good to have a warm hat with you, even in summer. It doesn’t need to be bulky.  A merino wool beanie is excellent.  In colder conditions, you should also include a head-over or scarf to prevent heat loss from your neck.  These items, although small, can make a massive difference to how warm you are on a cold day, if the weather turns bad, or if you have to spend an unplanned night outdoors.  In seriously cold conditions you should also have a balaclava and a larger, more insulating hat.

Now there are four very important general points you need to understand about convective heat loss:

1.  Strong winds and low temperatures can create the potential for massive convective heat loss.  Unless your outer layer is windproof, cold air passing over your body will disturb the warm air and take heat away from you.  

2.  Even in the absence of wind, the effectiveness of your warm layers will be greatly increased by taking extra care to keep the warm air in your clothes.  Wearing a windproof outer layer will prevent hot air rising away from your body and cold air replacing it.  

3.  As you move around, even on a still day, you will force cold air into your clothing as you ‘crash’ into it.  A windproof jacket will prevent cold air entering your clothing as you move around, even while walking.  You don’t have to be speeding down a black-run to feel the benefit. 

4.  A windproof layer also gives you the added benefit of trapping another layer of air – between itself and the warm layer directly underneath.  This means the coldest air is on the outside of your windproof, not in contact with your top insulating layer.  

Tip: You can also extend the above principle to your sleeping kit.  You’ll also be warmer in your favourite sleeping bag if you slip it into a breathable bivvy bag, particularly if you are sleeping under a tarp rather than in a tent.  Bivvy bags aren’t just about staying dry. 

Tip: When you seek shelter – looking for a lunch stop on a ski tour, deciding where to put your tarp at the end of a day, or considering where to build a shelter from natural materials, evaluate the natural wind protection of the site itself.  Anyone who has been walking in the open hills, even in summer, knows that on some days you don’t have to move far around a hillside to escape a cooling wind, into a sheltered haven with hardly any breeze.  

Tip: If you are staying in an area for any length of time, think about the direction of the prevailing wind and make sure your camp has some protection from it.  If you are building a shelter you will want to make sure it is well draught-proofed.  This is particularly the case if you are without any sleeping equipment.  When you think you have put enough thatching materials on your shelter put on at least 20% more! 

Conduction

Heat loss through conduction comes through direct contact with cold surfaces or objects.

How quickly heat is transmitted to an object depends on the conductivity of the materials from which the object is made.  How cold does an aluminium snow-shovel handle or canoe paddle shaft feel in your hand compared to the wooden equivalents?  Aluminium is 2,325 times more conductive than wood.  Aluminium is an extremely good conductor, water is a good conductor of heat, air is a poor conductor of heat (hence why it’s so good to have trapped in your clothes).

So, insulate your body from cold surfaces with a poor conductor.  A common example of this is the use of a sleeping mat when camping out.  If you’ve ever tried sleeping out on the ground (if you haven’t, I’m happy for you to learn from my experience here), you lose an awful lot of heat into the earth.  If you are building a shelter, make sure you build a good bed, with adequate insulation. 

Avoid handling cold objects, particularly metal, with your bare hands.  Do not touch metal objects with your bare hands in temperatures much below 0^oC (32^oF) as they can cause near-instant freezing of skin.  Even if you are using gloves, limit how much you handle metal objects in sub-zero (Celsius) temperatures. 

The amount of heat you lose to an object depends on how much energy it would take to heat up the object.  This, in turn, depends to a large extent on how cold the object is and how large the object is.  On a cold day you could warm a small pebble in your hand but you will never warm a large boulder up to your body temperature by getting naked and hugging it! 

Note that as you try to heat up the boulder, it will also lose heat to the (colder) environment.  The physicists and chemists amongst you can nod sagely when I mention the Second Law of Thermodynamics.  The rest of you can laugh and point at the nerds. 

Tip: It’s good to use an unbreakable metal insulated flask for hot drinks in winter.  Rather than having to handle the bare metal in cold conditions, tape around the outside of your flask with a fabric tape like the type used on ice-hockey handles or the sort sold as ‘sniper tape’ in army surplus stores.  You’ll feel the benefit of lower conductivity straight away, even if you keep your gloves on.  You can also apply this idea to a snow-shovel handle.

Tip: Rather than letting your feet get cold on snow or frozen ground, put some spruce boughs or similar down under your feet.  It makes a massive difference. 

Tip: Even in summer, slip a rectangle of an old closed-cell foam sleeping mat down the back of your day-pack as a ‘sit-mat’ for when you are eating your lunch or having a break.  It stops your backside getting cold (and damp!) when you sit down. 

Evaporation

Back to science class again.  Only for a moment though.

To convert a liquid to a vapour or a gas requires energy. 

For the nerds, we can go one step further and state it takes 0.58 kcal, or ‘calories’, to turn one gram of water on the skin to a gas.

OK that’s it; school’s out.

What this means is when sweat on your skin or moisture in your clothing evaporates, this evaporation draws heat away from your body.  This is called evaporative heat loss. 

Evaporation is a very effective mechanism for getting rid of heat.  It explains why sweating works so well at keeping you cool on a hot day or while you’re working hard physically.  The flip side is that if you get wet in a cold environment, you can get very cold, very quickly. 

Even when you are not obviously sweating, your body is always moistening your skin through what is known as ‘insensible perspiration’.  This occurs even in cold climates.

Water conducts heat away from your body about 25 times faster than air.  This is one reason why you will become hypothermic much more quickly in water even if it’s the same temperature as the air.  Another reason, and this is mainly for the science nerds, is that the volumetric heat capacity of water is much higher than that of air.  The practical take-away from this is that if your clothes are wet, you will lose heat into them much more quickly than if they were dry.

Also, because water is a better conductor than air, if you fill all the airspaces in your clothing with water, your clothes won’t insulate anywhere near as effectively.

So, moisture in your clothing has several effects:

1. there is increased conductive heat loss;
2. there is a decrease in insulation;
3. there is increased evaporative heat loss. 

Hence, it is critical that you do everything you can to stay dry whilst in the wilderness.

Your first line of defence is effective waterproof clothing to stop rain, and other water, getting into your clothing.

Photo credit: Hamish Morton

But it’s not just about stopping water getting in; you’ve also got to minimise the amount of sweat in your clothing.

When you exert yourself, getting wet from the inside can be a bigger problem than water coming in from the outside of your clothing.  Breathable fabrics that allow perspiration to escape will help to an extent but if you are working hard, you can still get sweaty and damp.  Below about -20^oC (-4^oF) all fabrics cease to be breathable anyway (the water vapour freezes on the inside surface or within insulating layers). 

Tip: If you take account of the environmental temperature and your level of exertion you can help to maintain an even body temperature and minimise sweat by: 

1. Adjusting layers:  Before exertion, take layers off and start cold.  Be bold, start cold! The exertion will soon warm you up.  After exercise, put layers back on before you start to feel cold.
2.  Ventilation: Open up front zips, expose the neck, use ventilation holes (e.g. some clothing has zipped ventilation holes in the armpits), loosen sleeves or even roll them up.  Again, try to do this before you exert yourself.  Adjust as necessary during exercise. 
3. Regulation: if you are still getting hot and sweaty despite the above measures then you should probably slow down a bit and pace yourself.  Apart from pumping moisture into your clothing, you are possibly working at a rate that will leave you exhausted in a relatively short period of time.  Low-blood sugar (hypoglycaemia) is a significant factor in susceptibility to hypothermia.

Tip: Don’t get complacent – In wet weather, even when wearing effective waterproof clothing and the correct level of warm layers, you can become hypoglycaemic in approximately one-third of the time you would on a warm sunny day.  Even if you are just out for a day’s hiking, take plenty of snacks and keep energy going into your system.
 

Respiration

As you breathe in cold air it is warmed before it gets to your lungs.  You then breathe out warm air and you are effectively losing heat every time you breathe.  As mentioned above, there’s nothing we can do about breathing.  We can’t stop breathing to conserve heat! In comparison with the other mechanisms of heat loss, however, heat loss through breathing is relatively insignificant. 

One thing that is worth pointing out is that cold air is often very dry.  As air cools, its ability to hold moisture decreases.  You can witness this in the boreal forest in winter – you can watch ice crystals form in the air and fall toward the ground.  The air is literally freeze-dried.  When this cold air is re-warmed it has the capacity to take on more moisture again.  When you breathe it in, this moisture comes from your mouth, nose, throat and trachea.  This evaporation takes some heat away from your body but it also takes away a significant amount of moisture.  

Tip:  A common misconception is that when sleeping outside in sub-zero (Celsius) temperatures, that you should put your head inside your sleeping bag and keep your hot breath inside your bag.  The problem with this is all the moisture your breath holds.  In arctic temperatures, this moisture will freeze inside your bag.  It will reduce the thermal efficiency of your bag and be impossible to remove unless you are able to air your bag in a warm cabin or heated tent.  So, wear a balaclava and breathe outside of your bag! 

Tip: Because you lose a significant amount of moisture in re-humidifying cold, dry air, you need to drink more water than you do in a temperate climate.  It’s easier to remember to drink more water in a hot climate but in a cold environment you will probably have to make a conscious effort to do so.

Key Points For Staying Warm Outdoors

 – In many environments you will be naturally losing heat to the environment and you must dress correctly. 

 –  The importance of wearing clothing in the correct combination, with appropriate wind-proofing cannot be overstressed.

 –  Protection of the head and neck is very important – you’ll always lose a lot of heat from this area.

 –  If you have a good understanding of effective clothing for particular environments, you can take only what you will need, rather than items just in case.

 –  Your clothing can only do so much – YOU are the one who has to adjust layers, ventilate and regulate activity.

 –  It is critical that you do everything you can to stay dry whilst in the wilderness.

 –  The colder the temperature, the more important it becomes to insulate your body from cold surfaces that will quickly draw heat away from your body.

 –  All of the above principles apply not only to clothing but also extend to finding and building shelter in the wilderness.

– If you are in a survival situation, living from the land or carrying limited supplies (this includes day-hikes), a solid understanding of the mechanisms of heat loss should at the very least mean you will be more comfortable and effective.  This knowledge will prevent you from expending calories unnecessarily and thus reduce the liklihood of becoming exhaused.  Having a practical understanding of how to apply it will mean you are much less likely to succumb to hypothermia, one of the biggest risks in the great outdoors.

Related Articles On Paul Kirtley’s Blog:

Creeping Death – Hypothermia and How to Avoid it

Take Care When You Enter the Blue Zone

How to Dress for Cold Weather

Recommended Books for Further Reading: