THE PHYSICS AND PHYSIOLOGY OF STAYING WARM

With about twenty-five years of winter skiing, hiking, and climbing, I thought I had a pretty good handle on how to keep warm. Yet, the first few years I dog sledded 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. It took a few years, but eventually I figured out mushing is 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.VariablesMushing a large team long distances, the dogs do the work. The musher stands, hardly working, but facing 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 skiers work pretty hard. Additionally, distance mushing is a 24/7 all weather sport. I’ve certainly run my dogs in temperatures far below what I’ve skied in. Finally, I’ve been out with the dogs 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.Most of the variables associated with getting cold point to a frigid musher. Among the key variables are temperature, wind, precipitation, humidity, and sun. In addition to these external variables are activity level, perspiration levels, exposure time, hydration, and acclimatization. Finally, there’s clothing and external heat sources (read: hand and foot warmers). It takes time to master all of these.Physics & Physiology of the BodyLike our dogs, 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 in the 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 in clothing systems are to minimize the amount of sensible perspiration produced and to provide paths for the water to escape without interfering with 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 complexity of the problem stems from the fact that it is a non-equilibrium condition involving multiple phase transitions. That is, the moisture near our skin is not in equilibrium with the moisture in the atmosphere, and the water will evaporate, probably condense, maybe freeze, and maybe evaporate again on it’s path 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. Second, while good mittens and boots are indispensable, it is at least as important for 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 significant amount of time for them to warm 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 shutting down 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 clothed like a newborn, and made my way back into the bag all without getting uncomfortable. Similarly, the first forty-five minutes or so 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. Insulation required varied by a factor of roughly eight depending on activity level. Sleeping is the lowest level of activity. Running or some equivalent is the highest. Hard shivering is also very effective at producing heat. A clear consequence of these last two factors is a sprint musher who is working hard to control and help his team and is only out for a short time really doesn’t have to worry about getting cold during the run. Overheating, wetting clothing by perspiration, and then cooling too much after a run are of far greater concern. Third, the amount of heat lost is dependent on the surface area. Hands, head, arms, legs and feet all radiate very well because they have a lot of surface area particularly compared to their blood flow and volume. Conversely, our main body’s 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 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, even the speed walking I was doing couldn’t stop the severe 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. Physics of InsulationThe purpose of winter clothing is to insulate the body from the cold. Our bodies produce heat and maintain a temperature near 100 F. The temperatures we run dogs at range from about -40 F to 50 F. Heat flows from warm to cold. Obviously, our bodies are always warmer than the environment in which we run our dogs. The purpose of insulation is to slow down the heat flow from our bodies to the great outdoors. There are several basic concepts to using 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 at 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 same conditions except the temperature (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 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 the insulative ability is roughly a factor of two. A given thickness of steel wool is about half as good as the same thickness 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 consequences of these, and particularly the unknowns, are it is wise to have a tested clothing system and one probably shouldn’t deviate much from what other folks use. Clothing Systems ConceptIt 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 to both keep us warm and allow us to do the things we have to do to run our dogs.Generally, clothing systems consist of a base layer to help handle perspiration, an insulative layer (here, 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 LayerThe layer of clothing next to the skin in pretty much any system is going to be a synthetic set of 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, depending on you and the underwear, 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 synthetic. 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 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, middle weight thermal underwear is generally quite adequate and still breaths very well. One other “base layer” that has been used, particularly with feet, is a “vapor barrier.” 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 sort of “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 something like a daily basis for health reasons. I happily used VB socks when I skied across the Sierras, but we could easily set up dry 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, arguably could have been cleaned, 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:DownGoose 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 should last 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. Down can be laundered, but generally this should be done with care and not as frequently as any other material. Down garments have one other disadvantage. This is that the materials used to hold the down in place, typically nylon, breath 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.WoolBefore 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 classically, “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 PileFleece 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 wind proof 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 certainly 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 BattingThis 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 down are not that significant. The advantage of handling moisture well is particularly important. Usually, a given thickness of synthetic insulation has a slightly greater ability to insulate than the same thickness of goose down. Synthetic batting usually wears well, though not quite as well as down. Clothing made with synthetic batting are 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 FoamMany 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 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 far better to have a thin synthetic base layer, the polyurethane, and then a breathable wind and snow proof outer layer.Outer Weatherproof LayerArguably, 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 layers of clothing are expensive. However, more than any other layer, this is the one for which it is worth coughing up the cash.Non-Waterproof SyntheticEmpirically, the most effective outer layer for extreme cold is a windproof 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 windproof 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 FabricsFor 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 ConsiderationsNothing pisses me off 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 loathe buying name brand items, but particularly with 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 half day runs will bring out any “infant failures” as well as many design flaws. Do this before heading out on 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. While most of the time, a distance musher is standing still, all of the clothing should allow a musher to run. Reaching, bending, looking around, etc are all also necessary to run a dog team. 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 FeetClothing for head, hands, and feet have almost as many variations as there are mushers. 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. My “high end” system is two polyurethane foam hats, one sized to fit over the other (foam CAN be layered with other foam). I don’t know that I’ve ever been cold while wearing this two hat combination, and that includes feeding dogs on the Alcan at -55 F. 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, there are five reasonably common systems people use for extreme cold. These systems include Cabela’s Trans-Alaska boots, Steger Mukluks, Northern Outfitters Boots, Bunny Boots, and a Neos/Lobbens shoe combination. The best bet is to chose one of these. There are also slightly lower cost pack boots that work fine for a lot of circumstances, but the five systems listed aren’t that much more expensive and are used down to the very lowest temperatures. Talk to friends and decide what seems best for you and what you want to do. Basic Clothing Systems: LayeringThe 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 that 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 SuitsIt’s hard to beat the convenience of a one piece suit. One of the other things that took me a while to appreciate 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 insulating layer. In extreme cold, you will 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 wilderness runs where a squatting elimination may be required, it should include a potty hole. As with all synthetic based garments, body suits are easy to launder.Bib and JacketA 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 12 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 gear has no membrane. Tricks of the TradeWork 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 Iditarod.  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. Staying hydrated is critical to staying warm. 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.Have a stash of hand and foot warmers. These can make poor winter clothing quite workable as long as they are kept dry and the altitude isn’t too high (I’ve used these to about 6,500’ without a problem). About twenty miles into the Eagle Cap 200, I discovered I had left my good insulating hat back at the truck. I was not happy. All I had was a simple fleece hat and of course my good parka including its ruff. I made it through the race including some cold night runs by stuffing hand warmers under the hat. Better living through chemistry.Get off and run. The easiest thing you can do if you start feeling cold is run. With a large team, even a tired team moving at only a trot, this does take practice. However, uphills offer an opportunity to both help your team and warm up. On my run from Takotna to Ophir, the temperature dropped from about 20 F to -15 F. I had started the run with a system built for 0 F. Half way, I changed the jacket to a -30 model. The pants were still the 0 F. 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. In lieu of running, pedaling will help some to keep you warm. A good second best and easy to keep up forever.ConclusionOne 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 rather than the cold cutting through you.Rob Loveman has been an outdoorsman for about forty years. His outdoor experiences include backountry skiing, mountaineering and, of course, mushing. Rob studied physics in college, earned his Ph.D. in 1984 and worked as an experimental physicist for the next 22 years. Rob’s interest in outdoor gear, how it worked, and what worked best started during his teens and has not waned.

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