This was one of my first, original posts way back when I decided to start throwing my thoughts out to the world, originally published Jan 9, 2017. Given that I’m ramping back up after a hiatus, and it’s another early January, I thought it appropriate to refresh this a bit. Enjoy!
Starting a blog in the winter, with the objective of getting people to spend more time outdoors, is a bit of a challenge. Dealing with winter weather is a bit of a varsity endeavor if you’re new to camping.
That said, winter is a beautiful time of year, and while nothing beats curling up next to a warm fire while the blizzard rages outside, it’s a real shame to spend the whole season inside and miss out on the simple beauty of a stark winter landscape. It can certainly be harsh, but a little gear, planning and understanding go a long way to staying comfortable at night when the thermometer drops.
Knowing how to stay warm starts with knowing the enemy, and understanding why you get cold in the first place. So bear with me a bit, as we summarize the physics, and the four primary enemies you need to defeat if you want to sleep warm this winter.
The mechanisms of heat transfer are all inter-related, to a degree (hah! get it?), but conduction is probably the most straightforward. Heat, thermal energy, equates to motion and vibration at the molecular level. High energy molecules transfer their vibration to neighboring low energy molecules, and slow themselves down in the process, until a balance is achieved. That is, hot objects will give up their heat to cold objects until both reach the same temperature. Touch something cold, and it will chill you. The speed at which this happens is a function of thermal conductivity, and thermal mass. Metals, bare stone, and water have high conductivity, and conduct heat quickly. Air (relatively speaking) has low conductivity, and is an insulator.
Thermal mass is really an indicator of how much time it will take to reach equilibrium, and which side “wins”. A small object, like a cold tent stake, can be warmed in your hand fairly quickly without losing a lot of body heat. If you lay yourself down on a cold slab of granite, though, you’re effectively trying to warm the Earth with your puny little body – and the Earth will keep sucking heat out of you until you’re the same temperature it is. That’s not really a winning proposition for you.
So, how do you defeat Conduction? Insulation!
First of all, understand you have biochemistry on your side. As long as you’re well-fed, your body generates its own heat to maintain a steady body temperature, and so you are likely one of the warmest things around. You can use this to your advantage, by letting your body warm a protective bubble around you. Clothes and layering are your first defense, as is a good sleeping bag. The various materials – down, and several synthetics – all represent various methods to trap air, usually with other materials that are themselves good insulators, and slow the spread of heat energy from your toasty warm body to the cold environment outside. This is how fur works!
Insulating properties are expressed as an “R” value (R is for Resistance, and a higher value is better insulation), and in the case of your sleeping bag, translated into a temperature rating – the minimum outside temperature at which you can expect to stay comfortable in a given bag.
A good bag is a great investment, but the more critical consideration is that big, cold Earth. An insulating ground pad is essential for winter warmth, and high R-values are your friend. You’ll pay more for those R-5 systems, but you need to look beyond simple pad thickness – When shopping for ground pads, insulating properties are far more important than duplicating the cushy feel of a home mattress.
(TIP: depending on your situation, loose leaves and even snow under the pad, or under your tent floor, add even MORE insulation. Despite its surface temperature, snow is an excellent insulator as it traps a lot of air. Just limit direct contact – think “igloo”.)
Finally, you can use conduction as a positive contributor to warmth, by sleeping with something warmer than you are. A bottle full of water heated on the fire before bed is a good tool to slow the chill, particularly when put near the neck, under arms, or between your legs where blood flow through major arteries can be warmed rather than lose heat. It likely won’t last all night, but anything that delays the point where heat starts flowing OUT of your body is worth the effort.
Now, all you hammock-users are laughing at this point, because you avoid the whole contact-with-Earth problem. If you’re thinking you get a free pass because you’re not touching any cold objects, think again, because:
Convection has to do with the transfer of heat energy in a fluid (like air). Though it starts with the same energy transfer of hot-to-cold, it also sets up convection currents in the fluid where temperature changes cause changes in density and pressure. Basically, equilibrium is harder to achieve, because the fluid starts moving and keeps moving, and the warmer air keeps getting replaced by colder stuff.
Convection isn’t as easy to visualize, but it’s powerful. Convection in the atmosphere due to uneven heating of the Earth’s surface is what causes weather – it generates thunderstorms, causes wind, develops regions of high and low pressure. Even on a smaller scale, convection causes motion in the air, and you can feel heat getting ripped away from your body as moving air flows over you – what we know as “wind chill”.
The solution? Stay out of the wind, and keep the air volume around you small so that you can minimize currents and loss. This means some form of shelter and wind-break is useful even in moderate temperatures. This also plays an important role in site selection. Sleeping on the lee side of a hill, or among vegetation, out of the wind, is only part of the solution. You also need to remember that air will move, even on “still” nights, as dense cold air settles into hollows and depressions overnight. An elevation difference of only a few feet is enough to keep a good sleeping area sleeping area a few degrees warmer than in the low spots.
Hammock users, convection is a big deal for you, as you are essentially immersed in a cold fluid without much of a way to trap any still warm air around you, particularly underneath. Block what you can, and insulate! Even though you’re not on the ground, a ground-pad or under-quilt is still a must-have!
Radiation is just what it sounds like. It’s the emission of long-wave electromagnetic energy, in the form of infrared light, from hot objects. This is the energy you see with infrared or “thermal” cameras and some low-light optics as a glow, and the reason you can feel warmth coming off a hot pot or another person without actually touching them. Electromagnetic waves don’t need air, or any other medium, to propagate. They can travel through the cold vacuum of space, and good thing too, because that’s the only way solar energy can heat up the planet.
As shown with those infrared cameras, energy does indeed radiate out of your body, but compared to conduction and convection, this loss is pretty small. Radiation can be a consideration in site selection and planning for a warm night, though. If you’ve spent any significant time outdoors in the winter, you know that the coldest nights are crystal clear, those that allow the heat energy of the Earth to radiate unabated into interstellar space. A “blanket” of clouds leads to a much warmer night.
It’s not really correct to think of the clouds as a blanket – the effect is better described by remembering that the clouds, and in fact all water vapor, and other gases in the atmosphere are warmer than space, and radiate some of their energy in all directions, including back at the ground. The net effect is small but noticeable – cloudy nights will be warmer than clear ones. Even a few degrees helps.
The same effect is present with vegetation. Tree cover, however sparse and cold it seems, provides an added bit of heat. Some of the benefit of tree cover comes from providing a wind break and trapping still air. But you’ll note the radiation effect as well. Radiation fog and frost are most likely to occur in open areas, out from under the trees, where the temperature drops a little lower.
Radiation can be your friend too, of course. It’s what makes a fire feel warm, even at a distance. This warmth can be reflected off a stone hearth, a cliff face, a wooden barrier. If your site is arranged properly, you can magnify the effect of radiation from a fire, and even if your fire goes out, the rocks heated by proximity to fire will continue to radiate energy at you long after the embers die.
4) Phase Transition (Evaporation)
The final tool in nature’s arsenal discussed here is that of evaporation. Liquid water absorbs energy as it changes states and converts to a gas. If this moisture is on your body, the energy required to dry it is taken, in part, from you. This is, in fact, why sweating works to keep you cool – your body’s temperature is reduced in the process of helping evaporate the water in your sweat.
The counter to this should be obvious – don’t get wet! Water itself is a good conductor, and it tends to create pathways through insulation, rendering it ineffective. In addition to these conduction effects, the evaporation of moisture as you dry provides a double whammy and chills you even more. In fact there’s almost nothing so dangerous in cold weather as moisture. It’s often easier to stay comfortable in a nice dry 15 deg F snowfall than in much warmer 30 deg F conditions, because the warmer snow is that much closer to melting, and does so readily when it gets close to your body.
The combination of wet and cold is what leads to the often used quip that “cotton kills”. It’s not that cotton is somehow a malevolent fiber, the problem is that saturated cotton is as almost as good a conductor of heat as if you were immersed in water, and pulls even more heat from you as it dries. In the winter, this combination can lead to hypothermia, quickly. So, seriously, avoid cotton, and if you do get wet, it’s important to get warm and dry, quickly, even if that means shedding layers to do so.
In practice, given the body’s natural sweat response, staying dry is not so easy, even if the weather is nice – there’s a delicate balance to be struck in being warm, but not being TOO warm. During the day, if you’re hiking or otherwise active, remove a layer, start your activity slightly chilled and let the exercise itself warm you up. Just be ready to add layers during breaks. In winter, particularly, a tight-fitting synthetic wicking layer against the skin will pull moisture away from your skin, and give up ITS heat, rather than yours, in drying. Couple this with synthetics, or fibers like wool that retain their insulating properties even when wet, and you stand a much better chance of staying comfortable.
In preparing for a cold night out, it’s relatively easy to bundle up TOO much, such that you wake up in the middle of the night, sweating. Cold soon follows. The trick here is to go to bed warm. Do a little exercise before bed, have a warm drink (carefully – you don’t really want to set yourself up for any nocturnal trips to the restroom if you can help it), and a full belly to keep those cellular furnaces busy burning carbs. When you feel warm enough to shed a layer or two when you go to bed, chances are you’ll warm up your bag and be comfortable without overheating. If you miss your perfect temperature target, it’s easier to add a layer when needed than to wake up wet, chilled, and already wearing everything you have.
As a side note – you also lose heat every time you breathe. Bringing cool air into your lungs, warming it, and then exhaling it generates a net heat loss, as well as a moisture loss. You can prevent this by not breathing, but that’s a short-term solution. If you plan to breathe, it’s just a loss you have to be prepared to take. Though it might seem like a good idea to release your warm exhalation back into your bag, the moisture you introduce into your cocoon when you do this is a setup for net energy LOSS. Particularly when it’s really cold (below zero deg F), the moisture in your breath will condense and freeze, and the moisture effects far outweigh the perception that you’re breathing warmth back into your bag. Keep your nose and mouth clear of your bag, and let those breaths escape.
So, there you have it! These four mechanisms are the world’s primary means of pulling energy away from your toasty warm-blooded self. If you understand how this happens, you’ll be much better prepared to prevent it, and be able to sleep comfortably, cozy and warm.