Winter is coming…
I have always struggled with the cold. I was that kid who would still be wearing jumpers in July. When I first got into bushcraft, I particularly struggled with sleeping in colder temperatures. I’d see films where the protagonist would sleep directly on a freezing rock face with just a thin wool blanket, and I’d wonder if it was just me who suffered in these conditions, or if the films had got it wrong.
I’m not a scientist, but I've spent most of my working life in science communication, trying to understand complex processes with an aim to make them more digestible. I decided to take the same approach to understanding how warmth actually works. Maybe if I grasped it from a scientific perspective, I’d have the upper hand on those cold nights in the woods. It turned out to be a rich and complicated topic, leading me to research everything from Newton’s Law to the way wool works - with so much ground to cover that I’ve broken this blog into two parts. If you’ve ever dealt with a freezing night yourself, hopefully you’ll find it useful too.
Thermodynamic Principles
To understand what keeps you warm or makes you cold in the wilderness, it helps to first go over some basic principles of thermodynamics.
Second Law of Thermodynamics – Heat Moves Towards Equilibrium
When two things have different temperatures, heat transfers from the hotter one to the colder one until both reach the same temperature. For instance, if two people are lying next to each other, the colder person will take warmth from the warmer person.
In the wilderness, your body is often the warmest thing around, and your environment is trying to bring you to its temperature. This isn’t great news for you, as the human body typically functions at a core temperature of around 37°C, while in a UK winter the ground can drop below 0°C.
But even as heat is constantly being drawn away from you, your body is generating more from the food you eat. It’s a constant tug of war between your metabolism and the elements.
Newton's Law of Cooling – Bigger Differences Mean Faster Heat Loss
The bigger the temperature difference between two things, the faster heat transfers between them. As their temperatures get closer, the transfer slows. So, the colder the night, the faster you lose heat.
How Your Body Loses Heat
Physics recognises three main ways heat moves: conduction, convection and radiation. When looking at how to stay warm outdoors, it's also worth considering evaporation.
Understanding these four mechanisms can make a huge difference for keeping warm in the wilderness.
Conduction
Occurs when two surfaces are in direct contact. Heat moves from warmer molecules to cooler ones, with the transfer rate depending on the materials involved. Metal, stone and wet ground conduct heat quickly, while still air conducts it very slowly. Water is a particularly good conductor, moving heat around 25 times faster than air.
Imagine lying on the ground on a night when both the air and the ground are 0°C. The ground will pull heat from you far faster than the air, because the soil conducts heat much more efficiently than air does.
This is why getting off the ground is so important. To reduce the ground’s strong pull on your heat, you need distance between you and it.
A thick sleeping mat, a raised bed, or a deep bed of dry leaves can give you that. A sleeping bag on its own does very little here, as your body weight squashes it into a thin layer.
Clothing and blankets keep you warm by trapping pockets of still air. But if moisture gets into the fabric, it pushes some of that air out and replaces it with water. Wet fibres can also clump together, bringing them closer and creating more direct pathways for heat to conduct through the fabric.
There's a lot more to say about fabrics and moisture, so I’ll be covering the topic in detail in part 2 of this blog.
Convection
Convection is heat being carried away by moving air or water.
Air and water are made of countless tiny molecules that are free to move. When cold air molecules touch your skin, they absorb some of your heat through conduction, then drift away, carrying that heat with them. Fresh, cold molecules move in to take their place. This keeps the temperature difference high, and as we saw with Newton's Law, a bigger difference means quicker heat loss.
In a way, convection is just conduction with extra steps: your heat passes into the air by conduction, and the moving air then carries it away, bringing fresh cold air in to take its place.
It doesn't have to be windy. Even in still air, temperature differences can cause air to move: warmer air tends to rise while cooler air sinks to replace it.
Cold air molecules sit against your skin. Because they’re much colder than you, heat flows into them quickly.
The air molecules warm up as they absorb your heat through conduction.
The warmed air rises or is blown away, carrying your heat with it, and fresh cold air moves in to take its place.
The fresh air absorbs more of your heat, and the cycle repeats. Because the air touching your skin is always being replaced, it stays cold, and you keep losing heat quickly.
Picture two nights at the same temperature, one still and one windy. On the still night, the warm air around your body drifts away slowly, so your heat loss stays fairly gentle. On the windy night, that warm air is constantly stripped away and replaced with fresh, cold air. This keeps the temperature difference between your skin and the air high, and as Newton’s Law tells us, a bigger difference means you lose heat faster.
This makes shelter from the wind important. The aim is to put a barrier between you and the wind, whether it's a cliff face, a tarp, tent or natural shelter.
Even when you’re sleeping on a raised bed, convection can still catch you out, since the wind blowing underneath you steals your heat. Make sure to block the wind and insulate the area below the bed to keep warm.
Radiation
Radiation is heat that travels as infrared light, a type of light our eyes can’t see. Everything gives off heat this way, including you. It’s the warmth you feel from the sun or a campfire at a distance. You’re constantly swapping radiated heat with everything around you, and because you’re usually the warmest thing in your environment, you give off more heat than you receive.
Imagine two nights with the same air temperature, one clear and one cloudy. On the clear night, much of the infrared radiation you emit upwards passes through the atmosphere and out into space, while relatively little radiation comes back down from the sky. On the cloudy night, the clouds emit some infrared radiation back towards you, so you receive more heat than you would under the clear one. You lose less heat on the cloudy night, which is one reason clear nights tend to get colder than overcast ones.
This is why hard frosts in the UK often form on clear, still nights. With less cloud cover to radiate heat back towards the ground, the surface can cool enough to form frost. It’s also why deserts can get so cold after dark, clear skies and dry air allow heat to escape more easily, with less moisture in the atmosphere to trap the heat.
Due, in part, to radiation, sleeping under a tarp, shelter or tree canopy can help you stay warmer than sleeping in the open. Like clouds, these materials absorb and emit infrared radiation, reducing the heat your body loses to the night sky. You can sometimes see this effect in the morning. Things left exposed to a clear sky are more likely to collect dew or frost than stuff kept under cover.
While most materials absorb radiated heat, shiny surfaces can reflect it. Some products are designed around this characteristic. I'll go into more detail on synthetic materials of this kind in part 2 of this blog.
You can use shiny surfaces to reflect heat from a campfire. Just make sure to keep those made from synthetics, like space blankets, well back from flames and sparks.
You might have heard that a wall of stones or logs will also help to reflect your heat back at you. They do help you to stay warm, but not by reflecting. They instead absorb heat from the fire, warm up, and radiate that heat back at you.
When the fire dies out, the space blanket instantly stops reflecting heat, whereas the wall continues to radiate the stored heat outwards. It’s worth noting that stone holds onto heat far better than wood. Wood is a poor conductor and doesn't stay warm for long. A log wall can still be worth building though, as it makes a good wind barrier.
Evaporation
Turning water into vapour takes a lot of energy. When water evaporates from your skin, your clothes or your lungs, it draws that energy from your body as heat. This happens in three ways:
- Sweating – your body’s deliberate way of cooling down, releasing water from sweat glands.
- Insensible perspiration – even when you’re not actively sweating, water constantly seeps through your skin and evaporates.
- Respiratory loss – every breath you take carries warm, moist air out of your lungs, taking heat with it.
Let’s say you had a hard day of physical work. You sit down to rest as the evening cools. Your clothes are damp from sweat. Even if you are sheltered from the wind, that moisture keeps evaporating and rapidly pulls heat away from you.
It’s good practice to try and minimise sweating. Take layers off before you work hard, slow down if you start to overheat, and most importantly, change into dry clothes before going to bed. This is especially important if you are sleeping in a sleeping bag. Sleeping bags trap moisture, whether from your sweat or your breath. This is why you should avoid breathing into a sleeping bag. It may feel nice and warm for a few moments, but your breath adds moisture, which can condense inside the insulation, replacing trapped air with water and reducing its ability to keep you warm. The effect can be noticeable in cold, damp conditions, and it can build up over several nights, steadily making your bag less effective.
Knowing more about the science of warmth has led me to far more comfortable nights in the woods, and I hope it saves at least one other person from a sleepless night - or at least gives you something to think about while you’re shivering in the forest at 4am. If you spot something I’ve got wrong, I’d genuinely like to know, so get in touch! And keep an eye out for Understanding Warmth in the Wilderness, Part 2, where I’ll be exploring more about materials and best practice.
References that may interest you:
Conduction - how well different materials conduct heat
- Thermal conductivity of common materials (The Engineering ToolBox): https://engineeringtoolbox.com/thermal-conductivity-d_429.html
- Thermal conductivity of common materials, including soils (Neutrium): https://neutrium.net/articles/heat-transfer/thermal-conductivity-of-common-materials/
Radiation - how clouds radiate heat back to the ground
- Clouds do not “act like blankets” (Penn State University): https://courses.ems.psu.edu/meteo3/book/export/html/2263
Evaporation - moisture building up in sleeping bags
- Moisture accumulation in sleeping bags in the cold (Havenith et al., 2004, Ergonomics): https://repository.lboro.ac.uk/articles/journal_contribution/Moisture_accumulation_in_sleeping_bags_at-7_degrees_C_and-20_degrees_C_in_relation_to_cover_material_and_method_of_use/9349577