Science of Staying Warm

(or what you should be wearing when you see this)

It has been the coldest weekend of the year here so far in New york, this was taken in Central Park on sunday morning during my morning walk with Barkley. It was a bitterly cold morning even Central Park was quiet with only a few fellow dog walkers and joggers.

Which is one of the reasons dressing correctly can be advantageous, its was nice to be able to enjoy the parks unusually quiet ambiance.

Staying warm in these temperatures is pretty easy with the kind of clothing available now, and in New York for example the consumer is spoiled for choice. Heres what you should be looking for.

Ways in which a body loses heat are conduction, convection and radiation. Your aim when dressing is to minimize total heat lost and avoid and particularly cold spots – wearing the warmest jacket will not help keep your hands warm (well it will but not in an efficient manner)

Conduction:  Heat is disorderly kinetic energy of molecules and/or vibration of molecules depending on if the matter is solid liquid of gas. Heat moves through solids at varying rates depending on the thermal conductivity of the solid. Copper for example has a very high thermal conductivity 401 which is why you see it being used in high end chefs pans. Glass has a relatively low thermal conductivity of 1.1 . You will likely have verified this also through observations in everyday life.

Of course the lower the thermal conductivity the slower heat will travel. Air has a particularly low thermal conductivity – just 0.025! So air you would think is a superb insulator, and has great potential to keep you warm, and you would be right. However there is one very important catch!

Convection: Is the transfer of heat by the flow of matter with a temperature gradient across the flow. Gases commonly will transport heat fast despite what we were saying above about them being great potential insulators (low thermal conductivity) , as they flow very easily. Also gasses usually change density with temperature giving them a reason to flow. So although gases do not conduct heat well, convection brings a lot of cold gas molecules against you in succession, thereby making gases quite efficient at cooling an object down. Again you can see this effect by blowing on your finger or noticing how much colder it is when the wind blows on a cold day.

Radiation: Most of the heat loss from say a star will be via radiation, there is no atmosphere in space to take heat away from celestial bodies by other means, but its relatively second order to us so we will focus on the first two. This incidentally is the principal on which a vacuum flask works, the air is sucked out from between the 2 layers, so the only way heat can escape is by radiation (and a little conduction around the neck). As radiation is a relatively slow form of heat loss that is why liquids stay hot for so long in them.

We have established that the main ways in which you will lose your body heat are via conduction and convection to the air on cold days. How can these be minimized? Utilizing the great insulative properties of air is the foundation of how most winter clothing works – its not the coat keeping you warm its the air! Because air is a great insulator but its freely moving so convection takes that quality away if you walk around naked. The coats job is to trap as think a layer of air around you that cannot move around, so convection cannot occur. This is why Goose Down is such a great natural insulator, its very light and fluffy, but with a complex internal structure that really constrains airflow. For not much weight a lot of great insulative air is being trapped around your torso.

This forms the insulative layer in your typical winter coat. There are also usually an outer layer and an inner layer (liner). The linear is mostly for comfort and keeping the insulation in (insulators are often made of very fine fibers which can migrate if not properly lined. The outer layer has a more demanding task. Insulators are great at trapping air, but not at forming a good barrier to air. If there is a pressure differential they will often allow air to flow through – another name for a pressure differential is, you guessed it, wind.

So an effective outer layer needs to at least be windproof. Depending on the use it may also be waterproof, although making a fabric waterproof and breathable is a challenge, and usually results in a compromise in the breathability (in a nutshell breathability what stops you getting too sweaty inside your jacket, particularly important for athletic activities in the cold). So if its not going to be wet, or the weather is really cold and the only water you will encounter will be frozen, a windproof outer layer is sufficient.

Outer-layers are usually classed then as windproof or windproof+waterproof, but also by the feel of the fabric. The self-explanatory soft-shell and hard-shell. Soft-shells are usually not fully waterproof, however they are more comfortable and breathable and many keep you dry in all but heavy rain. There are even a few fully waterproof soft-shells around now.

A hard-shell is more of a storm fortress, usually fully waterproof and durable against tears, they are the goto choice for staying dry in any conditions and often used by climbers for their durability.

The consumer then has the choice between soft-shell and hard-shell for the outer fabric. The other main decision is what kind of insulation to go with. Here are the options:

Hard-shell: Jacket is just the outer layer as described above. These need to be layered, so the addition of some insulating mid-layer is required for cold conditions. A shell keeps the wind out but does not trap air effectively. The advantage of shells is that they are incredibly versatile as the choice of layering can be tailored to the conditions. A shell plus a t-shirt can be used on stormy summer days. Hard-shell jackets are usually fully waterproof in addition to being windproof, but not all are breathable. Goretex Pro Shell is a well regarded hard-shell material.

Soft-shell: Made of a softer material, a soft-shell jacket is more pliable and usually windproof, sometimes water resistant but usually not fully waterproof. Soft-shells are a great choice for high output activities in colder environments due to their better breathability. They can be layered to provide excellent warmth and are versatile. Windstopper is a common soft-shell fabric.

Insulated Soft-shell: Used for winter sports in colder conditions where high breathability is desirable and waterproofing is not important as will be unlikely to encounter liquid water. Also great for urban environments as they are typically very comfortable and provide enough waterproofing for city life.

Insulated Hard-shell: Where you will need full waterproofing in colder climates. Insulated hard-shells are usually very warm jackets and so their use is constrained to colder weather. They are very convenient however, and with a good insulated hard-shell you can often throw it on over whatever you are wearing and know you will be well protected from the cold and rain.

If that was not already enough of a selection, the buyer will then need to decide which type of insulation. There are natural insulations for jackets, most commonly down, which is extremely compressible and lightweight, and the best choice for really really cold weather where there is no chance of the insulation getting wet.

Synthetic insulation offers an alternative, and these vary greatly in performance, some can be extremely cheap and perform accordingly. Others such as PrimLoft are high end and offer a spectrum of performance that cannot be matched by natural fibers. In particular the best synthetics retain their insulative properties even when wet and resist water much better then down.

So there in a nutshell is what to consider when buying a winter jacket. I have a soft-shell, a hard-shell and an insulated soft-shell (synthetic). The jacket i use the most in NYC is the insulated soft-shell, it just works great to throw on and provide warmth in a wide temperature range. For skiing i use the hard-shell when its cold and gnarly and the soft-shell in less extreme conditions. For hiking in general will usually use the hard-shell. I also have a soft-shell made of down which i sometimes use under the hard-shell when its REALLY cold (<10F)

If i could only have one jacket for the city it would be the hard-shell, as it can do everything, but i just do not use it that much with the other choices available.

My Gear

I happen to be a fan of Arc’teryx, there are many other brands that offer great quality as well, i hear Patagonia, REI and Eastern Mountain Sports are all good. These are the jackets currently in my activewear wardrobe.

Soft-shell: Arc’teryx Gamma MX (pictured worn this weekend 15F with a base-layer)

Hard-shell: Arc’teryx Alpha SV (my fav jacket)

Insulated Synthetic Soft-shell: Arc’teryx Atom – my first Arc’teryx product bought 4 years ago and wore a lot, finally showing signs of wear due for replacing soon, will probably get another.

Insulated Down Soft-shell: Mountain Hardware Kelvinator, with hood, quite convenient to throw on when taking out the dog in cold weather as the hood on this is great. Also layer under the Alpha SV for a really warm combination.

There is a lot more to keeping warm and the subtleties’ of the different fabrics and constructions. The above is a basic guide which should give you enough to go on to do some more research and know what questions to ask in the shop.

(PM in her Arc’teryx insulated hard-shell, and Isabella in the Snug² Monday morning 16F)

Lastly, don’t forget the legs, feet, hands and head. The principles are the same but the choice of options is lower for these accessories.

We designed our baby bunting, the Snug² to use PrimaLoft One, a top notch synthetic insulator in 200g/m2 weight. Their warmest weight and considered by PrimaLoft to offer an ‘expedition level’ of insulation. The outer-layer is Supplex a windproof and water resistant material. Babies have a higher surface area to mass ratio than we do, and so its especially important to shield them from the colder weather. We put a lot thought into the design of the Snug², keeping warm is important enough for us to have researched the science behind it thoroughly.


5 comments on “Science of Staying Warm

  1. It is great to share this information. This is a good example that the whole family whatever the age of the kids can learn to enjoy an active lifestyle at an early age. It great to start early. Thank you so much for your blog.

    • Thank you for your comment! We hope to inspire other parents to start or continue their active lifestyle for themselves and for their children. We believe this is the best way to promote a healthier future. 🙂

  2. Just wanted to say this article is great, when the winter comes it often challenges parents with the ability to go outside, stay active, and not be freezing or properly insulated from the harsh elements to avoid them and their child from becoming sick.

  3. sorry for late response, but thank you for the comment! It really is easier to keep your baby (or yourself)) warm in the cold with the right clothing than cool when its very hot. Winter should not be view as a time to be trapped indoors now with the modern fabrics and clothing we have available.

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