The Physics and Physiology of Staying Warm

By Rob Loveman

With about twenty-five years of winter skiing, hiking, and climbing, I thought I had a pretty good handle on how to stay warm.  Yet, the first few years I dogsledded distances longer than a couple of miles, I’d come back shivering.  As I became a more experienced musher, the runs got longer.  With each season, I also relaxed more on the runners.  These just made matters worse.  

When I’m mushing a large team long distances, the dogs do the work.   As the musher, I stand relatively still, but face an ever present headwind.    A fast trot of ten miles an hour effectively changes 0 F to about -16 F.  A downhill skier certainly sees higher effective headwinds than a musher, but even downhill skiers work pretty hard.  Additionally, distance races like the ones I was training for run 24/7 and in any weather.  One of the things running dogs has taught me is how to operate at night.  This includes both darkness and cold.  Finally, I’ve been standing relatively still on the runners for eight hour runs, or longer.  In all of my other winter adventures, I was either moving or in my sleeping bag.  I never had an extended period of low activity other than in camp and very well insulated.

It took a few years, but eventually I figured out mushing was just much colder than skiing or even doing chores in a snow camp.  And as time has gone on, I’ve become pretty adept at staying warm, even on the runners.

The Physics and Physiology of the Body

Our bodies are designed to maintain a constant temperature.  If we get too hot, we sweat and evaporation cools us off.  If we get too cold, we shiver and produce heat to keep our bodies warm.  Generally, we feel most comfortable when our skin temperature is between 80 F and 90 F.

There are several mechanisms that the body uses to maintain temperature.  The two that are most important to understand for winter are perspiration and how the body regulates blood flow. 

When we get hot, we perspire.  This is called sensible perspiration.  The amounts vary, but can easily be a couple of pints in times as short as an hour.  The water in this perspiration has to go somewhere.  Two critical goals of clothing systems are to minimize the amount of sensible perspiration produced and to provide paths for the water to escape without interfering with the clothing’s ability to insulate. 

In addition to sensible perspiration, there is senseless perspiration.  Essentially, our skin produces enough moisture to keep the surface humid.   Though the quantities are smaller, a few pints a day or so, this moisture has to be dealt with as well.  The issue of providing a path or paths for the water to escape without interfering with clothing’s ability to insulate is the same.

What is clear from a physics perspective is we do not understand everything that is going on regarding moisture transport in clothing.  The cause of the complexity of the problem is that the moisture near our skin is warmer and denser than the moisture in the atmosphere.  With this, the moisture near our skin is “out of equilibrium” with that around us.  The water will evaporate, probably condense, maybe freeze, and maybe evaporate again as it passes through our clothes and on its way to freedom.  The point of this is not to teach complicated thermo-physics, but to underscore the complexity of moisture transport.  The good news is there are several clothing systems that work well, even if we don’t know all the details of what is going on.

The physics and physiology of blood flow is much more straightforward.  The body will constrict or dilate blood vessels in order to maintain a constant temperature in the core and head.  If we’re too hot, the vessels dilate to radiate more heat.  If we’re cold, the vessels constrict.  The key is that the vessels the body primarily uses to do this are in the extremities, particularly the hands and feet.  Blood flow to the head remains constant.

There are three critical consequences to this.  First the old saw, if your feet are cold, put on a hat, is absolutely correct.  Because there is a lot of blood flow to the head and because it has a lot of surface area, a lot of heat can be lost if a person doesn’t wear good headgear.  The standard number is up to fifty percent of a person’s heat loss is from the head (Note:  50% comes from many references.  I suspect that under most circumstances, it’s 30% or less.  I use the word “standard,” without mentioning accuracy.  Twenty-five plus years of proposal writing trained me to always cover myself either by wording or caveats.  This does come through in the strangest places).  Second, while good mittens and boots are indispensable, it is at least as important for the hands and feet to keep the core and head warm.  If the core starts to cool, the blood flow to the hands and feet is restricted, and these extremities get cold regardless of what is worn.  Finally, if hands or feet do become cold, it can take a long time for them to warm back up.  This is because cold blood vessels tend to stay constricted.   Using hand and/or footwarmers really helps with this.  In particular, these can keep hands or feet warm when the body would otherwise be reducing the blood/heat flow to them. 

Four other things to note are the following: 

First, we are basically a big vat of water.  Water is a great medium with which to store heat.  Just like a vat of water, it takes time for us to cool off.   There have been many nights snow camping when I was toasty warm in my sleeping bag, stepped outside to relieve myself with little, if any, clothing on and made my way back into the bag, all without getting uncomfortable.  Similarly, the first forty-five minutes after I’ve hooked up my dogs, I rarely feel cold.  I’m still warm from handling the dogs.

Second, activity is as important as any other variable in keeping warm.   In his publication “Man in a Cold Environment” (“Man in a Cold Environment,” Alan C. Burton and Otto T. Edholm, Williams and Wilkins Co. ,1955),  Burton delineated how much insulation is necessary to stay warm.  The insulation required varied by a factor of roughly eight depending on activity level.  Sleeping has the lowest level of activity.  Running, or some equivalent, has the highest.   Hard shivering is also very effective at producing heat.

A clear consequence of these last two factors is an outdoorsman working hard---even walking with a full pack does this---doesn’t have to worry about getting cold.  Overheating, wetting clothing by sensible perspiration, and then cooling too much after hard exercise are of far greater concern. 

Third, the amount of heat lost is dependent on the surface area.   Hands, head, arms, legs and feet all lose heat quickly because they have a lot of surface area, particularly compared to their blood flow and volume.  Conversely, our main bodies themselves have much less surface area compared to their volume. 

Finally, acclimatization is not quite as important as activity level, but it makes a big difference in how warm you will be.  I’ve been stuck on a mountain through the night only twice in my life.  The second time was on Mt. Darwin in the Sierra Nevada Mountains.  The peak is just under 14,000’ in altitude.  We had summited late and had a choice of spending the night on the summit, a flat football field sized area, or on the side of the mountain.  We chose the football field.  I spent a lot of the night doing laps around the summit.  By morning, the speed walking I was doing no longer stopped my shivering.  Sunrise was a very welcome sight.  The next night we were in our down sleeping bags camped at about 11,000’.  Our bodies, however, were kicking out heat like we were still on top of the mountain with just mid weight jackets.

Most of the time, acclimatization takes longer than this, at least a couple of weeks.  Still, I find that I wear a lot less clothing at the end of the season that I do at the start. 

The Physics of Insulation

The purpose of winter clothing is to insulate the body from the cold.  Our body’s produce heat and maintain a core temperature near 100 F.   Temperatures outside during the winter are significantly colder than this and heat flows from warm to cold.  The purpose of insulation is to slow down the heat flow from our bodies to the great outdoors. 

There are several basic concepts for doing this.  First, the heat loss through a “standard” insulator is proportional to the temperature difference.  If our skin is at 85 F, it takes roughly twice as much insulation when it’s 5 F as it does at 45 F.  The difference in temperature is 40 F when it’s 45 outside, 80 F when it’s 5 F.  This assumes the same level of activity and, other than temperature, the same conditions---wind, sun, relative humidity, etc.  Second, the heat loss through a standard insulator is inversely proportional to its thickness.  The heat lost through a standard insulator ½” thick is twice as much as through the same insulator 1” thick.   Third, a “standard” insulator is an approximation and really doesn’t exist.   The concepts end up being pretty close, but not perfect.  You will need about twice the thickness at 5 F as you do at 45.  Finally, not all insulators are equal.  There are better and worse insulating materials. 

All clothing is based, at some level, on using dead air as an insulator.  Air which is absolutely still, like in most insulating clothing, is a very good insulator.  Additionally, lots of things deaden air.  It has been written that a thickness of steel wool works as well as the same thickness of goose down.  This is a nice concept.  However, it is incorrect.  While both do a good job of deadening the air, the ability of the steel in steel wool to conduct heat is still significant.  The down itself in goose down is a relatively poor conductor of heat.  The range of dead air based insulators in their insulative ability is roughly a factor of two.   An inch of steel wool is about half as good as an inch of polyurethane foam at insulating.  Most insulators used in clothing are much closer to the polyurethane foam than they are to the steel wool, but they still vary in their ability to insulate.

Finally, there are dynamic processes that are very important.  Some can help.  Some can hurt.  A fur ruff is not an insulator at all, and yet it clearly provides protection from the cold.   I was quite surprised at how much warmer my face was whenever I wore a ruff.  The ruff works as follows.   The ruff is colder than the face, but warmer than the outside temperature.  As cold air comes in from outside the ruff, it is heated by the ruff before it strikes the face.  As warm air from the face moves out, it in turn warms the ruff.  In this process, the air moving away cools.  In addition to this, the ruff changes the airflow around the face and this is also significant at keeping our faces warm.

Cotton is an example of where it can hurt.   Cotton fabric makes an excellent wick.  Once a cotton garment gets wet, this wicking action becomes problematic.   Moisture will tend to evaporate faster on the surface of cotton nearest the skin.  Unfortunately, the wicking action of cotton then moves moisture from other parts of the clothing to fill in where moisture has evaporated.  This means that as a cotton garment dries, there will always be a wet part of it near the skin, and the body will have to work hard to produce the heat to evaporate this water.

The consequence of these, and particularly the unknowns, is that it is wise to have a tested clothing system and one probably shouldn’t deviate much from what other folks use. 

Clothing Systems Concept

It is important to look at outdoor winter clothing as a system.  It isn’t a single piece of gear that’s going to keep somebody warm, it’s everything together.  Even in the simplest systems, there are several pieces of clothing.  The key is having all the clothing work together both to keep us warm and allow us to snowshoe or ski or mush dogs.

Generally, clothing systems consist of a base layer to help handle perspiration, an insulative layer---a layer could refer to one or several garments, and an outer layer to protect the other layers from wind, snow, and/or rain.   The base layer also serves to keep insulative layers cleaner so they have to be laundered less frequently.  Head, hands, and feet have variations on this.  Often, there simply isn’t a real base layer.  However, all have an insulative layer and an outer layer.

Base Layer

The layer of clothing next to the skin in pretty much any system is going to be a set of synthetic long johns.  Synthetics absorb very little moisture and, consequently, tend to move sweat away from the skin.  In general, modern long johns are also loosely knit.   This is to facilitate moisture transport.  While comfortable in warm weather and indoors, cotton has no place in a good winter clothing system.   Cotton is the material with the poorest ability to handle moisture, bar none.  Synthetic underwear may stink after a few days, but it is warmer and much safer.  Obviously, the issue of smell becomes much less of a concern if clean thermal underwear is available all the time.  Silk and wool or combinations of these with synthetics are superior to cotton and will not smell as much as synthetics.  However, they will absorb moisture that synthetics will simply pass.  Synthetics will be warmer.  The combinations may be more comfortable for some people.

Ultimately, the thickness of the base layer is a matter of preference.  The main reason for thicker long johns is wearing them in camp or cabin where walking around with only these on is the norm.  If this isn’t going to happen or camp or cabin are reasonably heated, mid-weight thermal underwear is quite adequate and still breathes very well. 

One other “base layer” that has been used, particularly with feet, is a “vapor barrier” (VB).  This is a waterproof fabric.  It simply keeps the moisture from escaping at all and in that way keeps the insulation dry.  While these have been manufactured and used for thermal tops and bottoms, the most common usage is for the feet.  Most people do not tolerate well the “slimy” feeling that comes with using these.  This slimy feel is because they collect a thin layer of moisture on their surface.   That said, VB socks are popular and definitely keep insulating layers for the feet dry.  They do, however, keep your feet a little wet, and it is important to dry your feet on a daily basis for health reasons.  I happily used VB socks when I skied across the Sierras, but we could easily set up camps where I would hang them to dry as I wandered around in booties, my feet also being allowed to dry.  I should add, the socks were not pleasant smelling.  On the other hand, they did dry quickly, could have been cleaned relatively easily, and did not smell near as bad as my more standard socks when I came off of Iditarod.  Those drew complaints from the next room.

Insulation

Down

Goose down remains the standard for light weight insulation.  Garments made with down may look funky, but the compressibility of down allows for very easy movement.  High quality down is incredibly durable.  Good down lasts for decades.  Though down is expensive, generally down garments are not much more expensive than any other well manufactured garments. 

Down’s big disadvantage is it handles moisture poorly.  If you sweat hard into a down jacket, it can effectively drop its ability to insulate to near zero.  Moreover, once down is wet, it doesn’t dry easily.  Down garments are excellent in very dry climates or when access to dry heated areas is available. 

This may be changing very dramatically.  “Hydrophobic down” a.k.a. “waterproof down” garments are now available.  Several manufacturers have figured out how to treat down so that its ability to handle moisture has improved greatly.  The book isn’t closed, but my tests of hydrophobic down have really impressed me.  At the very least, it’s not clumping when it gets damp, particularly by perspiration, and that by itself looks to be enough to allow hydrophobic-down to be much more usable than non-hydrophobic versions. 

Down can be laundered, but generally this should be done with care and not as frequently as synthetic insulators.  Down garments have one other disadvantage.  That is that the materials used to hold the down in place, typically nylon, breathe only moderately well.   For the same reason, many down garments can be used both as an insulating layer and an outer layer depending on conditions, design of the garment, and the weather itself.

Wool

Before there was fleece, there was wool.  Good wool wears very well, but is very expensive.  Adding a little nylon to wool fabric increases the durability a lot and allows a lower grade of wool to be used for a garment.  In the days before fleece and pile, wool was the standard warm when wet material.   Except for feet, wool has largely been replaced by fleece.  Most high end socks still have a significant wool component.    As well, some manufacturers of ultra cold weather footwear use wool in their insulation.  This is because feet produce a large amount of perspiration and the ability of most footwear to let that escape is limited.  If the moisture has to be absorbed, nothing beats wool.  Wool does not wick like cotton, and consequently “feels warm when wet.”  The insulatitve ability of wool does drop significantly when wet, however, the drop is much less than other insulators.  Wool is capable of absorbing a lot of moisture and dries slowly once wet.  Most wool outdoor garments are easy to launder, though not all are.  Some may need dry cleaning.

Fleece and Pile

Fleece and Pile are generally inexpensive, durable, handle moisture very well, and make nearly perfect layering material.  All synthetics have the advantage of being very poor at absorbing water.  Under many conditions, a fleece or pile jacket may be wrung out and then worn dry.  Fleece and pile also transport moisture easily and this gives garments made from these “range,” that is a fleece jacket may be comfortable for a wider range of “variables” than other insulators.  While fleece and pile are durable, they will wear out after a couple of seasons of hard use.  There are now relatively windproof fleeces available and these can work as outer-layer insulation combos.   Fleece and pile garments are easy to launder.  There are lots of low end fleece garments that are well manufactured and quite usable in a layering system.  The high end garments tend to have a much greater ability to be used in a “stand alone” mode and cost about as much as well made garments of any other type.

Synthetic Batting

This includes everything from Qualofill to Polarguard to just plain polyester batting.  Most are, in fact, polyester.  Differences are in how they are spun, fiber dimensions, surface coatings, and so forth.  They all have similar advantages and disadvantages, though some are clearly superior to others.  In general, better insulators come in better jackets and pants and are more expensive.

Modern synthetic batting has almost all the advantages of goose down and handles moisture very well.  Like pile and fleece, garments based on synthetic batting can often be wrung out then worn dry.  Synthetic batting is slightly heavier and noticeably less compressible than down.   This means it stores poorer and is not as easy to move in.  In clothing, the disadvantages relative to normal down are not that significant.  The advantage of handling moisture well is particularly important.   Hydrophobic down seems to have solved most, if not all of this problem. Usually, a given thickness of synthetic insulation has a slightly greater ability to insulate than the same thickness of goose down.  Synthetic batting wears well, though not quite as well as down.   Clothing made with synthetic batting is easy to launder.

Generally, jackets, pants, and suits using synthetic batting have a similar basic design to down garments.    Nylon or some other relatively windproof fabric is used outside of the insulation to protect it and hold it in place.  As with down, this means it breathes less well than fleece or pile, but the garment can often be used without an additional outer layer. 

Polyurethane Foam

Many years ago, Boys Life Magazine, the magazine for the Boy Scouts of America, ran a story about a winter hiker who went off into the Sangre de Cristo Mountains in New Mexico with all of his gear made out of polyurethane foam.  I made my first sleeping bag out of poly foam based on a design I found in a couple of books.  That was 1970.  Polyurethane foam has been used as insulation for many years. 

Polyurethane foam handles moisture better than any other available insulation.  It, too, may be wrung out then worn dry.  It is about as durable as other synthetics and has significantly better ability to insulate for a given thickness than any other insulation.   Its biggest disadvantage is it compresses very poorly.  This means moving in it is generally more difficult than moving in other insulated clothing.  It also means it packs poorly.   One other critical note on polyurethane, in order to get the most benefit from a piece of foam clothing, it should not be layered with anything other than another piece of foam clothing.  The reasons for this are likely related to complicated dynamic processes involving moisture transport, but knowing why really isn’t critical.  The important thing is that if polyurethane gear is used, it is best to have a thin synthetic base layer, the polyurethane, and then a breathable wind and snow proof outer layer.

Outer Weatherproof Layer

Arguably, the most important layer in a clothing system is the outer layer.  If the wind, rain, or snow get into insulation, that insulation simply doesn’t work well, if at all.   Conversely, a good outer layer with any base layer is a usable system with a significant range.  Unfortunately, high quality outer garments are expensive.  However, more than any other layer, this is the one for which it is worth coughing up the cash.

Non-Waterproof Synthetic

Empirically, the most effective outer layer for extreme cold is a wind resistant water repellent fabric such as nylon or polyester.  The water repellency should be only from a “Durable Water Repellent,” DWR.   Even wind-proof fabrics which incorporate a membrane which is nominally “very breathable” appear to be inferior at keeping somebody warm when compared to the simple DWR wind resistant fabrics. The physics that drives this is probably not simple moisture transport, but a combination of dust and moisture.  The membranes or coatings used in waterproof-breathable garments simply do not let dust through them (to be published).

Breathable Waterproof Fabrics

For cold wet weather ranging from 20 F to 50 F, waterproof breathable fabrics make the best outer layers.  Modern versions of these fabrics are reasonably breathable as long as the DWR is working.  Any well made garment will do a good job of keeping a person dry from rain or snow.  The problem is once the DWR is gone, water no longer beads up on the fabric, and the water on the surface of the fabric keeps the fabric from breathing.  Maintaining the DWR isn’t that difficult.  Typically, it means just washing and drying the garment regularly during the season.  Drying in a tumble dryer is generally important as it lets the DWR “flow” over the surface of the garment.  Eventually, the DWR does wear off and needs to be “revived.”  At least for me, I will revive the DWR on all of my clothing once a year.   Several products to revive DWR’s are widely available.

Design and Manufacturing Considerations

Nothing annoys me more than a failure of equipment in the field.  Winter outdoor clothing is equipment. 

There are several keys to avoiding failure in clothing.  First, either get clothing from a reputable manufacturer or inspect it very carefully for workmanship.  Stitches are a telltale sign of good or poor quality work.  They should be even, preferably small, and the lines should be straight.  The garment should “feel” and look well made.  My preference is to buy from high end manufacturers.  I generally loath buying name brand gear but, particularly with outdoor clothing, I’ve found that the name brand item that costs twice as much lasts four times as long as it’s cheaper counterpart.  Second, try garments in less critical situations before relying on them.  A few overnighters where failures aren’t that critical will flush out any “infant failures” as well as many design flaws.  Do this before heading out to climb Denali or do the Iditarod.  Third, maintain the clothing.  This includes regular cleaning whenever possible.   It also includes fixing rips and tears early.  My analysis shows a stitch in time saves 7.2 +/-2.5.  Finally, don’t count on broken gear, particularly clothing, away from home and car.  You can extend the usable life of these near a nice warm shelter, but counting on them when this is not available is too risky.

Outdoor clothing has to be designed to let you work.  Reaching, bending, looking around, etc are all necessary to enjoy what you’re doing.   Along with this, it is certainly preferable to have some ability to quickly ventilate a system.   Part of handling moisture well is opening these vents both to let it out and keep the body cooler while working.  Keeping cool means we are not producing as much sensible perspiration in the first place.

Finally, particularly for hands and feet, it is critical that the clothing not be tight or restrictive.  Feet should easily move in boots.  Snug gloves can mean very cold hands.  These may be used for short periods when necessary for tasks requiring high dexterity, but should not be kept on for long periods.

Head Hands and Feet

Clothing for head, hands, and feet have almost as many variations as there are people.  These end up being very much a matter of personal preference.   There are a few key points to keep in mind when figuring out what works for you. 

First, it is critical to insulate your head well.  Happily, the new trend toward “hoodies” has made hoods available on most jackets.  A good hood not only insulates the head directly, it allows warm air from the body to come up around the neck and surround the head.  All arctic peoples had hoods on their cold weather clothing----they work. 

Second, mittens are much warmer than gloves.  On the other hand, gloves are necessary for many chores.  For Iditarod, I settled on a pair of very warm mittens and a “fleet” of different gloves.  I’d add that two very nice features in mittens are a nose wipe and a thumb big enough for its own hand warmer. 

Third, for feet, it depends on what you’re doing and what you like.  Cold weather boots vary from mukluks to plastic skiing and mountaineering boots.   The first consideration is what you’re doing.  Downhill ski boots make no sense for somebody who’s dogsledding.  Second, particularly for extended trips, having footwear that stays dry is critical.  Snow, rain, and water from the outside and sweat from the inside both matter.   Beyond handling moisture, movement matters.  Standing on sled runners does nothing to stimulate blood circulation in feet.  By comparison, cross country skiing with the weight shifting back and forth between feet will keep feet much warmer simply because that cycle forces blood to flow.  The other consideration, of course, is that a pound on your feet tires you out as much as five in your pack.  More than for any other clothing, the weight of shoes matters. 

Basic Clothing Systems: Putting It All Together

Layering

The most flexible inexpensive system is one based on layers.   With modern clothing, this will generally mean a base layer of synthetic long johns, a bunch of fleece or pile layers, and a weatherproof outer layer.  Layering can handle any temperature or condition.  As it gets colder, just add another layer.  As the layers add up, it is important to have layers sized so some can fit over others.  This includes the outer weatherproof layer.  The two biggest difficulties in layering are it is not easy to move around with a bunch of layers on and it is inconvenient to make quick adjustments while wearing them.   Most other systems have some sort of venting that can be opened and closed and are effective at cooling a person when they are open.  This really isn’t available in layered systems.   Layered systems are also slightly heavier than some of the other systems and pack a little less well than most other systems, though these differences are not that significant.

Body Suits

It’s hard to beat the convenience of a one piece suit.  One of the other things that took me a while to learn was how long it can take to put on clothing for the winter.  A hooded one piece body suit largely eliminates this problem.  These usually incorporate a weatherproof outer layer and a synthetic or down insulating layer.  Depending on the suit’s design and the temperature, you may have to add layers of insulation underneath.  In general, the suits are designed to accommodate this.  If the suit is to be used for long periods outdoors where a squatting elimination may be required, it should include a potty hole.  As with all synthetic based garments, synthetic based body suits are easy to launder.

Bib and Jacket

A bib and jacket combination is almost as flexible as a layered system and almost as convenient as a body suit.  These systems are available with down, synthetic batting, and foam insulation.  For practical purposes, a pants and jacket combination is the same as long as there is sufficient overlap between the bottom of the jacket or parka and the top of the pants.  At least 6 inches of overlap is needed so that nothing gets exposed, even while reaching up.  Depending on the insulation, these are often easy to launder.   A big advantage of un-insulated outer pants and parkas is they can be laundered easily.  This works particularly well as the outer layer gets the dirtiest.  If the insulation in the system is synthetic or foam, it too can easily be laundered.  Unfortunately, there is a dearth of outer-layer clothing that doesn’t have some sort of wind or rain membrane.  As noted earlier, the best extreme cold weather gear has no membrane. 

Tricks of the Trade

Work with your clothing.  Actually work with any gear before you go out on something serious.  You should know where every zipper, piece of velcro, and so forth is on any piece of clothing before you start a major expedition.  Working with your clothing will also tell you what does and does not work for you.

Do everything to stay dry.  Here, the most critical thing is to avoid breaking a sweat, sensible perspiration.  Slow down, if possible.  Open vents as necessary.  Most cases of hypothermia, a.k.a. exposure, involve the victim getting wet, often from perspiration.

When I started mushing, I figured ruffs weren’t necessary.  Climbers had given up their fur ruffs by the end of the sixties, so why not mushers?  I now own three ruffs.  They may not be politically correct, but a good fur ruff makes a huge difference in staying warm, particularly as the temperature or windchill drop below zero.   

Stay hydrated.  All the mechanisms our bodies have to stay warm work better when we are hydrated.  This includes bloodflow which decreases with dehydration.  Dehydration is also a significant contributor to hypothermia.

Either rest with a heavy coat on, or keep moving.   On my Iditarod leg from Takotna to Ophir, the temperature dropped from about 20 F to -15 F.  I had started the leg with a system built for 0 F.  Half way, I changed the jacket to a -30 F model.  The pants were still the 0 F pair.  It was passable, but I wanted to be comfortable when I arrived in Ophir.  I started running on anything that looked like a hill.  And when I arrived, I was toasty warm.

Conclusion

One of the most satisfying things about outdoor play in the winter is when you make the gear work for you in severe conditions.  When the wind is blowing hard and all you want to do is look at the trees sway then up at the stars rather than contemplate whether you’ll ever by warm again, well, that’s a great night.  And with the right clothing, it will be the sights and sounds you will enjoy, not the cold cutting through you.