Sunday, 30 December 2012

Keeping Warm Part 4: Wind

Staying Warm 99In my summary post on how to keep warm, I mentioned how critical is was to prevent the wind from cooling your insulator. In this post, I will look at this issue in more detail.

To look at the effects of "wind" or "draft", we need to add a bit more of technical analysis. But rather than push another equation at you, I will keep it simple. If you take a look at the heat flux page over on the Engineering Tool Box site, you will see that forced air convection will transfer 10 times the amount of heat that free air convection will. Ten times! That means that you would have to increase the thickness of your insulating layer by a factor of 10 in order to compensate for it!

In Canada, meteorologists will report on wind chill during the weather forecasts because the wind can cool you fast enough that it can be lethal if you are not properly dressed.

Wind chill, when combined with a wet insulator can be a big factor. Once during a ski race when it was about -15 C, I was neck-and-neck with a guy at the end of a mass start scramble and we were tied at the hip pretty much on the entire outward (and down wind) leg of the race. I had a wind proof shell on and he didn't. When we rounded the turn to come back, we were now skiing into the wind and he lasted only a few minutes before his wet insulation, now being rapidly cooled by the wind, started to drop his body temperature to the point where he slowed considerably. He finished the race well behind me, despite being a better skier. While he did not suffer from hypothermia, he did not look very well at the finish.

The point of that story is that wet insulation and wind can be very dangerous. Your body can lose a lot of heat if you are not careful. And you do not have to be a ski racer to be cautious about wet insulation or the wind. Damp socks can cause your feet to be cold. Wet mittens can result in cold hands. These may result in you only being uncomfortable, but they can also result in frostbite or hypothermia if you are not careful.

Fortunately, there is a solution: just don't let the wind get at your insulation layer! A good wind block is really important. If you look at many cross-country ski or winter running clothes, you will usually see a wind-blocking nylon (or other poly) in critical areas such as the chest, crotch, and thighs. In areas like the armpits or back, there usually is no wind-block material. This is to allow the sweat to escape and keep your insulator drier than it would otherwise be.

Wind block on the front:
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A different, non-wind block fabric in the pits to let cool air in and hot sweaty air out:
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You can get your wind protection for your layers in different ways.

One way is to buy jackets that are fleece / wind-block combinations. With this type of garment, there is no need to carry around an extra article of clothing. However, for colder days, you may want a separate wind-block shell to go over a layer or two of insulation.

A wind-block material is typically a really dense fabric, but you may not be able to feel the difference just by touch. An easy test for a fabric's wind-blocking ability it to try and suck air through it. If you can, then chances are you will be disappointed in its wind-blocking abilities. When I am looking for a new jacket or even new gloves, I will always pick up the article of clothing and do this quick little test. You may look odd, but this check will certainly give you quick feedback on how well it blocks wind.

Katya checking the wind-block abilities of a fleece shell:
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As with my point on layers, do not forget the rest of the body when it comes to wind-blocking! For guys, it is really, really important to wear underwear with an appropriate wind-blocking material right in front of you know what. Not much is as painful as frost bite down there! I was working the Canadian National X-C Ski Championships one year when it was -20 C. This is the coldest we are allowed to start a race under FIS/CCC rules. The moms were running around getting socks for the boys who didn't have the right underwear on. You can guess where they were putting the socks!

Men's underwear with windblock fabric in the most important areas:
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Pay attention to your hands, too. A lot of gloves and mittens do not block the wind at all, so while they may look nice and warm in the store, if you get them outside, you may find them to be very cold. Wind could very well be the culprit.

As I mentioned above, my legs do not need a lot of insulation. On really cold days (below - 15 C), I will often wear only long underwear and a pair of rain pants when I run. Even though it is not raining, the incredible wind blocking ability of my rain pants is enough to give me the extra warmth my legs need during a run.

Even if I am only walking to the grocery store at -15 C, I will often just put my rain pants on over my jeans. This is faster and easier than pulling off my jeans, putting on a pair of long underwear, then pulling on my jeans again. By blocking out the wind, my rain pants protect my normal pants' insulating abilities as well as providing a bit of insulation to boot!

For really good warmth, I will wear long underwear, fleece pants, and my rain pants. This gives me a couple of layers and a wind block. I'm usually good in this to -20 C for activities like walking to the store. For higher output activities, I will wear less.

First layer of poly long underwear:
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Second layer, cotton jeans (ugh!):
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Third layer, a windblock, waterproof, Goretex top shell:
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There are other areas where you need to pay attention to the wind.

On gloves or mittens, there are often straps designed to cut down on the amount of loose fabric. Tighten these up so that they are snug, yet do not cut off your circulation.

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Zippers let a lot of wind through, so look for covers to the zippers in your clothing.

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Better jackets that are designed for the cold will also have a strap on the hood. This is meant to reduce the volume of the hood so that there is less cold air around your head. These same jackets often have cuffs or drawstrings in the waist. Use these to block the "chimney effect", which is the warm air exiting through your collar, only to be replaced by colder air coming in at waist level.

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In the case of my jacket above, the wind cuff was a snap belt. In the case of my fleece, it is a drawstring. You will need to look carefully on you winter clothing to see where all of these options are. Here is a shot of the bottom of the fleece.

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The drawstring is barely visible unless you roll the cuff back. Cinch the string tight to cut off cold air from coming up under the fleece.

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As I said before, make sure you do not tighten anything too much so as to restrict your blood flow.

Sometimes you can make "wind" work for you. A really good winter jacket designed for active use will also have "pit zips". By opening the pit zips when you are active, you can get some air flow through your arm pit region, helping to keep you cool. You can then zip them closed to reduce airflow when you are inactive. Here is a shot of my jacket with the pit zips closed.

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Here is a shot of the same jacket with the pit zips opened.

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Finally, a point about shoes, and running shoes in particular: they are a lot like gloves in that they are seldom designed to block wind. In fact, the commonly used mesh siding makes for excellent ventilation to dry out your feet and let the cold air in. So while you might think that some shoes with their nice thick soles would be toasty warm, they are usually pretty cold to wear in the winter.

In summary: like water (wetness), the wind can be deadly. You need to keep your insulation blocked from the cooling effects of drafts and wind, especially if that insulator is damp in any way.

The complete Keeping Warm series is here:

Keeping Warm: Summary
Keeping Warm: Insulation Thickness
Keeping Warm: Dryness
Keeping Warm: Wind

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