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It’s all about the (H)Air

Now that summer’s coming round with a scorching hot Spring all of my ‘warm’ jackets are being put away, to the back of the wardrobe not to be looked at again for six months.  A question came to mind:  What makes a warm jacket?  Why is it warm?

I know the answer to this question, having thought about a lot over the years, but I was curious as to whether other people did.  Even asking my engineer friends, who pride themselves in knowing how things work, what actually made a jacket warm it did stump them for a few seconds presenting me with a look of bemusement as their minds churned away.

Clothing, be it big, warm jackets in the winter to just a light covering in the summer, is absolutely crucial for our survival.  Think about if clothing was never invented, ignoring the fact that we wouldn’t ever have made it out of Africa and assuming we were all comfortable seeing each other in the nude.  Popping out even on a summer’s day in the buff sends shivers down my spine just thinking about it  and imagine going to a British beach without a wind jacket...brrrr!!

Isn’t it bizarre that despite how crucial clothing is for our survival, for our success in inhabiting large portions of the world, that we rarely give it a second thought about why these materials keep us warm? About why certain materials work better than others?  We just know it.

So why do I know about this topic?  Because I have gotten cold, very cold!  In 2008, during our first attempt to cross the Penny Ice Cap on Canada’s Baffin Island in early spring conditions my feet became so cold they turned blue and were completely numb.  It was apparent that to continue on the expedition was to seriously risk my feet or worse and so with no real choice at hand we had to back off the expedition.  It was a massive blow back then, an expedition a year in the planning, ruined.

 Source:  Antony Jinman

Source:  Antony Jinman

The most curious thing though, was that my team mate, Antony, and I were wearing almost identical clothing.  Everything down to our boots was the same and brand new.   Antony’s feet were fine.  “Warm and toasty” he said.  I had no idea why my feet got cold.

Three years later I figured it out.  It was the simple fact of my boots being too small.  It’s amazing how the best laid plans went awry due to just the simplest of errors.   We went back attempting the crossing again in 2011.  This time, despite the same brand and model of boots, I wore a size bigger.  My feet were fine, more than fine, they were as happy as Larry.

‘Ah, yes of course’ we say to ourselves.  ‘Can’t have boots that are too small.’  But why?  What is the science behind this?  The answer is all around us.  Air.

Air is the best* abundant source of insulating material.  It’s also free, something nature has found out long ago.  Our hair, an animal’s fur and bird’s feathers primary purpose is to trap air.  By trapping a good layer of air between your warm body and the cold harsh world, the smaller the heat transfer, the warmer you will be. 

The polar bear has even gone one step further; each of their hair is actually hollow allowing air to be trapped within each strand as well as between the individual hairs improving the insulating properties.  On top of hollow hairs, the sea otter, with no blubber to keep it warm, relies on the densest fur in the animal kingdom allowing air to be trapped within its fur whilst spending prolonged periods in the water.  It’s all about trapping air.

 Source:  Antony Jinman

Source:  Antony Jinman

That’s why when there is a wind blowing at you; through your hair, or you’ve dunked your head in a bucket of water**, destroying that trapped layer of insulating air; you lose a lot of heat.

That is exactly how our warm jackets work, be it stuffed with goose feathers or high-tech synthetic fibres.  The design of the feathers is optimum to trap as much air in the lightest and smallest structure.  Cover these wonderful feathers/fibres in a windproof or even waterproof shell and you’ve got yourself a jacket to rival nature’s best inventions.

So what happened when my boots were too small?  My toes pressed up against the front of the boot, squashed the fluffy fibres of my socks and destroyed the trapped air that would otherwise have provided a heat barrier.  Instead what I was left with was a direct and solid conduction path to the -35C exterior sucking away my heat.  They had no hope.

Next:  How do wetsuits keep you warm?

 

* - the best actually would be a vacuum such as that you find in your Thermos flask but those are pretty difficult and expensive to create and not very practical to be wearing.

** - water in your hair also creates greater heat transfer by evaporation, but that's something for another day...

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