Heat loss 1


Winter is upon us here in the British Isles and so staying warm and dry when outside becomes particularly important.  When I’m running survival courses I tell students that they should prioritise, and that their priority will always be the thing that will kill them first.  There’s a common rule that can be applied here, often known as the rule of 3s:

You can go 3 minutes without air, 3 hours without shelter, 3 days without water and 3 weeks without food.  OK, a little generalised, but the message is clear.

The biggest killers outdoors are hypothermia (body temperature below 35°C) and hyperthermia (elevated body temperature), so if we couple that with our rule of 3s, shelter is likely to be the first priority.  Of course, shelter starts with your clothing and so it’s imperative that you’re dressed appropriately with insulating layers and water and windproof outer layers.  So let’s think about how our bodies produce and lose heat.

How your body produces heat

Our bodies have 2 main ways of producing heat, either by eating or by exercising.  If I’m camping out in cold weather, I’ll have a big meal right before bedtime.  Add to that some exercise before getting into my sleeping bag (I tend to use big muscle groups to generate maximum heat, so squats is always a good one), and then wriggling around in my sleeping bag to get undressed, and already I’m feeling warm.

But this advice stands at any time; either eat or do something to get warm.  Bear in mind that this is something of a balancing act, as after maintaining your core body temperature, you want to conserve calories.

How your body loses heat

Many years ago, back in school, I was taught about methods of heat transfer.  It’s useful to put this knowledge into context and think about how it affects our body temperature.

Our bodies radiate heat away from them a bit like a radiator at home.  It’s formally known as thermal or infrared radiation, think of the image you get from a thermal imaging camera.  Sometimes, if we’re in a hot place, we’ll encourage this heat loss by taking layers off.  In a cold place we try to prevent heat loss through radiation by wearing many thin insulating layers.

If you watch the weather forecast you might hear the forecaster mention wind chill, something like ”It’ll be 4°C, but with wind chill it will feel like 1°C.”.  This is convection at work.  Put simply, convection is when heat is lost as air flows past your body.  We can reduce the amount of heat lost this way by wearing windproof clothing or simply getting out of the wind; this could be as straightforward as taking cover on the lee of a hill, walking below the ridgeline, heading to tree cover etc.

Conduction is when heat is transferred from something hot to something cooler.  It differs from radiation in that molecules are required.  In general terms a solid is a better conductor of heat than a liquid, which in turn is a better conductor than a gas.  This is because the molecules are packed closer together in a solid than in a liquid or gas.  What this means in practice is that we can lose lots of heat to the ground; just think of sliding off your roll mat in the night and how cold that can feel.  It’s why a blanket underneath is worth two on top and why we teach the importance of making a bed in a debris shelter.

Even though liquids aren’t as good conductors as solids, getting wet is a very bad idea (despite what you might see on some TV shows)

If you’re wet heat can be transferred away from your body up to 30 times faster.

Lastly we have evaporation, which for our purposes comes down to sweating.  Why is this an issue in a cold place?  Because if that sweat is absorbed by your clothes and then freezes, you’ll be in a bad way very quickly.  So use your layering system to make sure that you don’t sweat.

So hopefully from this you can understand why it’s so important to keep out of the wind and rain.  Statistically the people who get themselves into difficulties in the outdoors aren’t people on overnight trips, who are typically experienced and well prepared with waterproofs, tent, sleeping bag and food, but day hikers who might not be as experienced and don’t have those basic items with them.  Even if I’m only on a short hike, I still carry waterproofs, spare clothes, a bothy bag and a mylar blanket.

walking kit


About Gary

Lead Instructor at Jack Raven Bushcraft, teaching bushcraft, wilderness and survival skills to groups and individuals.


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