The "science" of warmth

(found in the Internet)

No doubt, most of us are at least familiar with the story of Goldilocks and the Three Bears. After trial and error, she finds a porridge that is neither too hot nor too cold, but just right. Ditto for the comfy bed. Yet, while the outdoor industry has reams of knowledge to share on layering, numerous insulation types, and even temperature ratings for sleeping bags, more often than not, working with a salesperson to find the right combination of garments or the right sleeping bag to stay warm comes down to trial and error. But it doesn't have to.

There is a science to keeping warm.

Let's start with a bit of Biology 101
When the temperatures dip and the body is trying to stay warm, heat is conserved first by constriction of blood vessels near the body surface. When that fails to work sufficiently, the body triggers shivering (essentially rapid muscle contractions) to increase metabolism. Studies have shown that shivering can result in a five-times increase in metabolism. But even with all the body's best efforts, when it is naked, it cannot sufficiently increase metabolic rate to replace heat loss to the environment once the temperature dips below about 40 degrees F or 4 degrees C. While the body also generates goose bumps (something known in scientific circles as piloerection), it's really an attempt to raise the body hairs which doesn't work too well since most humans are quite hairless.

Our only alternative is to insulate artificially with clothing, sleeping bags and more.

How does a person lose heat?
People lose heat four ways:

1. Conduction -- Meaning surface-to-surface contact, conduction allows us lose heat through the soles of the feet or our body when lying or sitting on colder ground, for example.
2. Convection -- Convection is heat lost by air circulating around our body or via respiration when we breathe.
3. Evaporation -- Evaporation of moisture from our skin chills our body, which is great when we are too warm, but not so good when we are trying to stay warm.
4. Radiation -- Although a very small part of heat loss, the body does lose heat via radiation of that heat outward.

The body is the engine and only means of generating heat
When you strip away all the technical data and discussion of variable metabolisms, etc., you arrive right at the heart of staying warm. The body will feel warm only if it is producing heat at a level that is equal to or greater than the heat it is losing to the environment around it.

Layers of clothing, sleeping bag insulation or any other artificial means of retaining heat around the body is only as good as the heat being generated by that body. And here are where the variables come into play -- psychological, physiological and environmental.

Psychological -- There can be little debate that an experienced outdoors person is more likely to be able to tolerate colder temperature, simply because of experience and preparation in knowing how to conserve heat.

Physiological -- Metabolism, otherwise known as heat output as the body burns fuel from food, is variable. A person who is fatter is not processing all the food the body is taking in, while a slimmer person is burning the food. However, a fatter person has fat reserves which provide more insulation than a thinner person experiences. Gender also plays into how warm or cold a person feels, with women generally, though not always, feeling colder than men. Age also affects a body's ability to stay warm, as older people generate less heat and, as a consequence, tend to feel colder more quickly and at warmer temperatures than younger people. Fitness also plays a part as a person who is more exhausted is unable to generate sufficient heat to stay warm, whereas a person who is more fit, is, as a result, less tired during outdoor activity, and feels warmer.

Environmental -- Humidity, wind, rain, snow, ice -- they all affect how a body responds to the cold. If it is less humid, cold weather feels less piercing whereas in higher humidity, it feels damp and raw. Wind too affects a body's ability to stay warm as it increases convective and evaporative heat loss. And then there is extreme cold or extreme altitude, both of which affect mountaineers and polar explorers who discover no matter how many calories they take in to generate heat and energy, it is just not enough.

So what is a person to do?
Insulate. Layer up. Dress for success. And your goal will be to work with a good specialty outdoor salesperson to decide what the best clothing system is (combinations of base layers, middle layers, outer layers and shells) and best sleeping bag system (bag, sleeping pad, maybe a bivvy sack, certainly a tent) that will help you stay comfortable. And by comfortable we mean, neither too hot, nor too cold, but just right.

What the heck is a CLO?
You may have read or heard something about CLO, and even some buying guides and product literature may reference CLO or CLO value. Developed in 1941, it was the first real attempt to explain the insulation value of clothing so people would know how much clothing they might need to stay warmer or cooler in a given temperature environment. A CLO value of 1 is defined as the amount of clothing required by a resting human (in other words, sitting, lying down or standing, but not moving) to be comfortable at a room temperature of 21 degrees C, or approximately 71 degrees F. That single value of CLO is equivalent to a typical business suit worn by a man -- shirt, undershirt, trousers and suit jacket. The higher the CLO number, the more insulating value is provided.

Yeah, but who hikes in a business suit?
Not many folks. But it is a good point of reference. Think of it like this: If something has a CLO value of 2, it means it provides the same amount of insulation as two suits. All garments and even sleeping bags carry a CLO value. As a point of comparison, the Department of Performance Textiles at Leeds University in the United Kingdom developed a comfort model to relate the thermal resistance of sleeping bags to temperature ratings. The study found that a CLO value of 2.5 was needed to keep a sleeping person comfortable at an air temperature of 59 degrees F, or 15 degrees C. A CLO value of 4 was required for comfort at 48.2 degrees F, or 9 degrees C. Thirty-two degrees F, or 0 degrees C, you'd need a CLO value of 6.

Just as there is a CLO value for sleeping bag insulation, there is a CLO value for most every garment possible. And, CLO values are cumulative, meaning as you layer, the sum of the CLO values of each component in the layering system will approximate how warm a person might expect to be. The following represents average CLO values for various articles of clothing, according to the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE): T-shirt, .09; Long Underwear Top, .35; Long Underwear Bottoms, .35; Heavy Long Sleeve Shirt, .29; Basic Sweater, .10; Socks, .08; and Long Pants, .32.

So, a person dressed in socks, long underwear, sweater and long pants would have a clothing system with a CLO value of 1.2.

Shedding the elements
However, no matter how many layers you dress in, you're not going to stay warm or comfortable if the wind if howling through the fabric weaves or freezing rain is pounding down so hard it soaks through every possible fiber. And this is where it becomes important for you to work with your salesperson, to understand what garments do what in their store's arsenal. Some are more air permeable, some are less. Some are more waterproof, but less breathable. Others are more breathable, but less waterproof. Some shed light mist, but stop a wind blast dead. Others let a little wind through, but still shed misty air.

At all times, think about what you will be doing, and how you can best manage your heat-generating engine. Are you looking to stay warm while running? Likely you'll need less layers and more streamlined ones than a person seeking to stay warm while simply walking. You want to keep the engine as dry as possible when it is cold so it will not chill. You also want to be sure that you are not piling so many layers on that the engine overheats.

By applying a bit of science to the dressing equation, you'll be warmer and much happier in the long and short term.