Keeping Warm in Winter — Part I: How We Lose Heat

I grew up in northern Minnesota, the coldest part of the continental United States. I remember playing outside when school was canceled due to extreme weather — the coldest I specifically remember was when my friends and I built a snow fort at -40°F (-40°C). That was the winter when we hit -60°F (-51°C). I bring this up to establish my bone fides when it comes to cold weather clothing.

The author’s beard and muffler covered in frost after cross-country skiing.

I kept hearing the saying that “there’s no bad weather, just bad clothing,” which I later found out was a Swedish proverb, not an invention of my family: Det finns inget dåligt väder, bara dåliga kläder. Of course everything has exceptions and much has to do with one’s personal experience and tolerances. Indeed, a study shows that people with arctic ancestry have modified mitochondria: this cellular component can convert glucose (sugar) into power for the cell or heat and people living in cold climates for many generations have evolved to produce more body heat at the cellular level. Another study indicated that bodies experiencing seasonal temperature differences increases the amount of adipose (brown, or “good”) fat accumulation; an argument for letting your thermostat go lower in the winter and going with less or no air conditioning in the summer.

What Happens When We Get Cold?

Although your core temperature is around 98.6°F (37.0°C), much of the rest of your body is closer to 72°F (22°C), which is why most prefer heating and cooling our homes to around that level. I used to teach about humans leaving Africa and spreading out over the earth about 1 million to 700,000 years ago. We came from a tropical environment but colonized extremely cold places, such as the arctic. As animals adapted to the tropics, however, we’ve worked to create micro-tropical climates in our homes and clothing. We try to keep our homes above 50°F (10°C) and humid in the winters. Clothing was invented to let us feel like we’re at a tropical party beneath our layers.

As the mercury drops, your body tries to keep its internal temperatures constant. It does that by burning calories. Remember a (small) calorie is the energy used to heat one gram of water by 1°C. If we’re mostly water, that means our inner furnaces try to keep something like 50–70 kg (110-154 lb) of water at 22–37°C (72–99°F). For example, if it is, say 10°C (50°F), the body is having to increase the temperature of 50 kg (110 lb) of water by at least 12°C (22°F), which requires 600 (food) calories to maintain temperature (a food or kilocalorie raises a kilogram or liter of water by 1°C). So when I go winter camping, I budget 5000 calories of food for each day just because my body is working so hard to keep me warm.

When our bodies have trouble keeping up with temperature loss, we shiver and get cold fingers and toes. Shivering is the body activating muscles rapidly to generate heat. When the body cools, it activates a motor center in the hypothalamus, causing us to shake. Goose bumps (or goose pimples) occur when the arrector pili muscles at the base of each hair tense, creating the characteristic bumps on the skin’s surface. This is a common reflex in mammals, known as piloerection: dogs with their hackles raised, porcupines with raised quills, and apes with hair standing on end. This often occurs when we’re scared to increase our apparent body size, but it also traps an insulated cushion of air between our fur and skin when we’re hit with a sudden blast of cold air. As we humans lost our body hair (some more than others), this vestigial reflex has little insulating value.

We have all experienced the cold fingers and toes during the winter, especially when we’re not dressed appropriately for the weather. When going to high school in Germany, I walked to school with a group of boys who didn’t wear hats because the style of the time included heavily gelled hair. They complained of their cold hands and feet and teased me for wearing a hat. I told them repeatedly what I had learned growing up: if your extremities are cold, put on a better hat, and if that isn’t enough, add another sweater. Your body restricts blood flow to the far extremes of the body to conserve heat in the core, where vital organs have less tolerance for temperature fluctuations. This can become extreme, leading to frostnip and, later, frostbite. If you experience numbness, you may already have crystallized liquids in your capillaries — resist the urge to squeeze numb fingers and toes as you may rupture those tiny blood vessels (which have become further restricted when you’re cold) with the sharp ice crystals. It is better to hold your fingers under your armpits and later rub your hands until they are warm before holding the affected toes.

Ways We Lose Heat

The first type of heat loss is the most expected: radiative heat loss. Radiative heat is what you feel near a bonfire or from the sun. It’s the reason your body loses heat when you’re just wearing a T-shirt and step outside in cold weather. This can be fought with increasing insulation.

Convective heat loss is what you experience when the wind “cuts through” your clothing, drawing away heat. This is also the culprit for rising heat escaping a warm area, such as under your clothes. Wearing a hat and keeping junctures between clothing closed help keep this to a minimum.

Similarly, we experience conductive heat loss, which is behind the classic problem of school-age children who stick their tongues to metal poles. Objects or materials with good heat transfer will draw heat away from uninsulated bodies touching them. For example, metal or water will draw heat out of your body quickly, which is why falling in a river or lake in the winter (or sweating, to a lesser extent) is so dangerous.

Another problem of getting wet in cold weather is evaporative cooling, when water is turned into vapor by absorbing heat. It takes a surprising amount of energy to evaporate water, and steam rising off of a body means it’s losing heat.

Other Considerations

Well clad for a bit of exploring in Alberta, Canada.

When getting ready to spend time outside in cold weather, we have to consider many factors, remembering that activity levels and conditions change, so we can’t have a one-size-fits-all solution for winter clothing. The questions to ask before getting ready are:

  1. Will it be raining or snowing?
  2. Will it be windy?
  3. What is the temperature range?
  4. What will be my activity level?
  5. Will I be changing altitude, location, or activity fast enough for a significant change to any of the above factors?

Tomorrow we’ll look at clothing and behaviors we can use to help keep warm and dry.

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