Heat Loss Mechanisms

Originally posted on Jan 20, 2016

As I write this right now, New England is getting ready for a wintry mix of snow, sleet and rain. Precipitation will sprinkle drivers with a concoction of winter weather rendering some of their common sense and driving ability useless. Winter is here but knowledge of cold should be considered year round. Even in the desert, nightly cold temperatures can kill. Understanding how the body loses heat helps the outdoorsman preserve it and prolong life. 

Conduction cooling occurs from direct contact with a cold surface. In 30 degree weather, holding your hand out in front of you won’t be of any discomfort but laying down on a 30 degree concrete floor will send chills through the body. A common mistake I’ve witnessed numerous times is failing to insulate from the ground during shelter building classes. Students become preoccupied with protecting from the wind and rain and forget the largest mass below them that will zap the heat from their body. Evergreen boughs, pine needles, bubble wrap, cardboard, anything that creates sufficient dead air space will help protect from conduction cooling.  Try standing on top of a closed foam pad (or a camp chair like the one pictured) and you’ll notice the protection it offers.

Convection cooling results from blowing wind. Anyone who has been a boater, sat in the stands at a football game or who has ridden in the back of a pickup truck knows how cold the wind can feel. Convection cooling is dangerous because even a light wind amplifies the cooling effect of the ambient air temperature. This can go unnoticed, especially if the body is warming itself by activity from the inside out. A common misconception is fleece, minus additional layers such as Gore Windstopper, will protect from the wind. Fleece is a loose fabric and allows wind to come right through the material. A cheap way to protect from the wind is to use a reflective emergency blanket but the key is to keep the blanket off the body. Otherwise, the wind will cool the blanket and conduction cooling (explained above) will take over. 

I still remember coming inside from sledding as a child and my grandmother taking my scarf and wrapping it around neck and mouth. No, she wasn’t trying to snuff me out but she was trying to protect me from getting cold. When we breathe out, we breathe out air that is approximately 98.6 degrees fahrenheit. When we breathe in, we take in the ambient air. If you can see your breath, you are losing heat by breathing. Since you can’t stop breathing, you have to protect your lungs by either staying active or by covering your mouth with a breathable fabric. Note I said breathable, don’t use that space blanket or you’ll end up suffocating. You can fix cold, you can’t fix stupid. 

The human body is an amazing machine. It self-regulates for warmth by releasing moisture in the form of perspiration. This is natural and some folks sweat more than others. Perspiration is one way the body can cool itself dangerously fast in cold weather. The most common form of this comes from working with too many layers on. You see this at trailheads when folks set off with multiple layers on only to shed them when you catch up to them along the way on the trail. If you wear layers that wick sweat away (not cotton in the cold!) bringing it to the outside of your layers, you will find sweat to be less of a concern. It is better to work slightly cooled than too warm. Let your clothing work as intended and mitigate the effects of sweating by wearing only what you need to. 

If you have ever put your hand next to a baseboard heater, you understand radiating heat. Radiation is the amount of heat given off by a mass. The sun, for example, gives off incredible radiating heat as does a person laying next to you in bed who by all accounts “sleeps warm.” Radiating heat can be converted to warmth through good insulation. Wool, Primaloft, goose down, all of these materials can be used to trap radiating heat and keep the body warm. If you are in a pinch, you can stuff cattail fluff, milkweed pod kapok or dried leaves if you have nothing else. As humans, we radiate heat, it’s natural. Take advantage of your natural furnace and maximize the heat you give off by trapping it. This is where emergency blankets come into their own. They do not insulate but reflect heat. If nothing else, if you are cold, a fire can radiate heat toward the body so make sure you are prepared with the means to make one. Add in a fire reflector made of stone and you’ll notice the added heat it provides after the fire dies down.

Of all the heat loss mechanisms, immersion is my least favorite. I’m a kayaker/canoer and a diver and before that a lifeguard for years. After countless dunkings, I wish I could tell you my body has adjusted to cold water exposure but it hasn’t. While I can better manage my cold by layering when possible and rewarming differently (drinking buttered hot cocoa helps!), I've only been able to offset the effects AFTER getting out. When I am in the water, or anyone for that matter, heat escapes the body much more rapidly than any other way in the atmosphere. The exact rate a person loses heat will vary depending on the temperature of the water, what is worn is whether the person is swimming, treading water, or in the H.E.L.P. position. I’ve heard the rate to be 20-40 times that of conduction. In any case, immersion heat loss must be taken seriously. in full-body cases, it leads to extreme exhaustion, unconsciousness and death. In cases where extremities are submerged, extreme cold, numbness and loss of function. The best course of action is removal from the water and if that isn’t possible, positioning your body in a way to conserve heat. 

If you’re able to understand the ways the body loses heat, you can better take action to prevent it. Just as there are ways to lose heat through the body, there are ways to bring heat to the body. Understanding these concepts will help you better survive the wilds of the outdoors in an emergency or from day to day in the winter until those horrible drivers are no longer affected by .

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