Royal Army College

Lesson Overview

Everything in this course rests on understanding what the cold does to a person. A member who grasps that will prevent and recognise cold injury almost by instinct; one who does not will be caught out, in themselves or in someone they are trying to help. This first lesson explains how the body holds and loses heat, how wind and wet multiply the danger, the path by which a chilled person slides into hypothermia, what frostbite and the other cold injuries are, and, crucially for the winter welfare operation, who is most at risk.

The cold is not beaten by toughness but by routine: a handful of small habits done every time without fail. You cannot keep a discipline whose reason you do not understand, which is why the understanding comes first. A member who knows exactly why a wet sock or a missed meal is dangerous will change the sock and eat the meal on the cold night when it matters most.

By the end you will be able to explain how the body loses heat and why wet and wind matter so much, describe the progression of hypothermia and the warning signs at each stage, describe frostbite and non-freezing cold injury, and identify the people most vulnerable to the cold.

This is the knowledge layer. The treatment of hypothermia and cold injury is taught in full in the Combat First Aid course and is certified in person; this lesson teaches the understanding and the recognition on which that treatment depends. The clinical detail here follows current accepted practice and the College's medical staff. The Royal Kaharagian Army's cold-weather method follows the Commonwealth pattern, and the Canadian Forces basic cold-weather teaching in particular, adapted to a small, lightly armed, humanitarian home-defence force rather than adopted whole.

Key Terms

Core temperature: the temperature of the body's vital inner organs, which the body works hard to hold near normal; cold injury is what happens when it cannot.
Heat balance: the running contest between the heat a body makes and the heat it loses; when loss outruns production, the core temperature falls.
Hypothermia: the dangerous fall of the core temperature below normal, which clouds the mind and, untreated, kills.
Frostbite: the freezing of the skin and the tissue beneath it, most often at the extremities.
Frostnip: the earliest, still-reversible stage of freezing of the skin, a warning that frostbite is beginning and the part must be protected at once.
Non-freezing cold injury: damage to the feet or hands from being cold and wet for a long time, without the tissue actually freezing; also called trench foot or immersion foot.
Windchill: the increase in heat loss, and so in the felt cold, caused by wind moving across exposed skin.
Vasoconstriction: the body's narrowing of the blood vessels in the skin and limbs to keep warm blood in the core; it is what makes a cold person pale and their hands clumsy.
The umbles: the cluster of early warning signs of hypothermia, when a person stumbles, mumbles, fumbles, and grumbles.

The body's heat balance

The body works constantly to hold its core temperature near normal, producing heat by burning food and by movement, and losing it to the colder world around it. In the warmth this balance keeps itself with ease. In the cold it becomes a contest the body can lose, because heat flows out faster than the body can make it, and the core temperature begins to fall. Cold injury, in all its forms, is the result of that contest going the wrong way. Picture it as a simple sum, heat in against heat out: everything a member does in the cold is an attempt to push that sum back to the safe side.

On the making side there are only two real sources of warmth. The first is metabolism, the slow background heat of simply being alive and digesting food, which is why food is literally fuel in the cold and why a hungry member runs cold. The second is work: muscles burning fuel as a member moves, marches, or labours throw off far more heat than rest, and it is the most powerful heat source a member controls. When that is not enough the body adds shivering, involuntary work, the muscles firing to make heat. Shivering can roughly double the resting heat the body makes, but it burns fuel hard, cannot be sustained, and stops once the fuel runs out, a turning point we return to below.

The whole craft of cold-weather soldiering is keeping this sum positive: making heat by working and eating, and closing the routes of loss so that what is made is not thrown away. Let the sum go negative and stay there, and the core temperature falls and the slide towards hypothermia begins.

   THE HEAT BALANCE

   HEAT IN  ........................  HEAT OUT
   metabolism (being alive)          radiation
   work (moving, marching)           conduction
   shivering (involuntary work)      convection
                                     evaporation
                                     respiration

   Safe   : heat in   >  heat out   (warm, or warming)
   Danger : heat out  >  heat in    (cooling; the core falls)

   You raise HEAT IN  by working and eating.
   You cut  HEAT OUT  by closing the doors below.

How the body loses heat: the five routes

The body loses heat by five routes, and a member should know every one by name, because keeping warm is the closing of these doors. The first four carry heat away by physics; the fifth, the breath, is easy to forget and matters more in hard cold than most expect.

Radiation is the steady loss of heat from any uncovered warm surface to the colder air, greatest from the head, the neck, and the hands. An uncovered head on a cold night pours heat away for no purpose. The countermeasure is plain: cover bare skin, and put a hat or hood on first, because the head is hard to insulate any other way.
Conduction is heat lost by direct contact with something colder, and it is far greater than most people expect when that something is cold ground or, worst of all, cold water. Water draws heat from the body roughly twenty to twenty-five times faster than air at the same temperature, which is why a soaking, or a fall through ice into cold water, is a true emergency in minutes. The countermeasures run through this course: get off the cold ground onto an insulating mat or a bed of branches, never sit straight on stone, metal, or snow, and keep dry, because dry clothing conducts slowly and wet clothing conducts fast.
Convection is heat carried away by moving air or water. Still air trapped against the skin insulates; wind strips that warm layer away and replaces it with cold, which is why wind is so dangerous and why windchill matters. The countermeasure is to break the wind, with a windproof outer layer, a hood up, and a wall, a bank, or a treeline between the body and the weather; getting out of the wind is half the work of staying warm.
Evaporation is heat lost as moisture turns to vapour, from sweat and from wet clothing drying against the body, because turning water to vapour takes a great deal of heat and takes it from the body. This is why sweating in the cold and then stopping is a trap, the sweat-soaked clothing now chilling the body as it dries. The countermeasure is to work at a pace that does not soak the clothing and to shed a layer before sweating rather than after, a discipline taught in full in the next lesson.
Respiration is heat, and water, lost with every breath out. Air drawn in cold is warmed and moistened deep in the body and breathed straight back out, carrying that warmth and moisture away; in hard, dry cold the loss is real and never stops, because breathing never stops, and it is one reason a member grows quietly dehydrated without feeling thirsty. The countermeasure is modest but worth it: breathing through a scarf, a face covering, or a cupped hand pre-warms the air a little, recovers some moisture, and shields the face from the wind.

Two of these dominate, and they are the two a member must guard against above all: the wind, which multiplies every other loss, and getting wet, which turns clothing from insulation into a heat drain and exposes the body to the fierce conduction of water. Together they can bring on hypothermia even in air that is merely cool rather than freezing, and each is taken up in the two sections that follow.

Almost everything a member does against the cold attacks one or more of these routes at once: a windproof shell breaks convection and slows evaporation, an insulating mat kills conduction to the ground, a scarf over the mouth softens respiration, radiation, and convection from the face together. Knowing which door a measure closes is what lets a member improvise when the issued kit is not to hand, because they are solving a problem they understand rather than copying a drill.

   THE FIVE ROUTES OF HEAT LOSS, AND HOW TO CLOSE EACH DOOR

   "Radiation"   : from bare skin, most of all the head, neck, hands
                   -> cover skin; hat or hood on first
   "Conduction"  : by contact with cold ground or, far worse, water
                   -> mat off the ground; stay dry; out of the water
   "Convection"  : warmed air stripped away by wind and moving air
                   -> windproof shell; hood up; shelter behind cover
   "Evaporation" : from sweat and from wet clothing drying on the body
                   -> do not overheat; shed a layer before you sweat
   "Respiration" : warmth and water carried out on every breath
                   -> breathe through a scarf or cupped hand in hard cold

   Guard above all against the two that dominate:
        WET   turns clothing from insulation into a heat drain
        WIND  multiplies every other loss

Windchill: why moving air strips heat

Wind deserves its own word, because it changes everything. A still, cold day is survivable in good clothing; the same temperature with a hard wind can be deadly, because the wind constantly tears away the thin layer of warmed air the body builds against the skin and clothing, replacing it with fresh cold air until the body cannot keep up. The windchill is the measure of this: the temperature the body actually feels, which can be far below what a thermometer reads, so that air a few degrees below freezing in a strong wind robs heat as though it were far colder, well into the range where exposed flesh is genuinely at risk.

Exposed skin suffers most, because the wind acts on it directly. In a strong wind frostbite can take hold on a face or a hand in minutes at temperatures that would be merely uncomfortable in still air, which is why the face, the ears, and the wrists between glove and sleeve need watching hardest when the wind is up. The practical lesson is constant: get out of the wind and keep it off the skin. Find or make a wind shadow behind a bank, a wall, a vehicle, or a treeline, turn the back to the wind rather than the face, and cover every patch of skin it can reach.

The deadly multiplier: wet

If the wind is the multiplier of cold, wet is its master, and it is the single most common way a member, or a vulnerable national, slides into danger in the kind of winter this course is built for. There are two ways to get dangerously wet: from the outside, by rain, sleet, wet snow, spray, or a fall into water; and from the inside, by the member's own sweat, the more treacherous because nothing feels wrong until the work stops and the chilling begins. Wet is lethal for two reasons. First, water conducts heat many times faster than air, so wet skin and a fall into cold water bleed heat at a rate dry cold cannot match. Second, wet clothing stops insulating: the warmth of clothing lives in the still, dry air it traps, and soaking drives that air out and replaces it with water, so the garment that kept a member warm now draws heat out of them. Wet wool keeps a little of its warmth and wet cotton almost none, but no wet layer insulates as a dry one does, which is why staying dry is treated throughout this course as a survival discipline and not a comfort.

This is the hard, counter-intuitive truth a member must take to heart: a wet, windy day a degree or two above freezing can be far more dangerous than a dry, hard frost well below it. The dry frost can be shut out with good dry clothing and a broken wind; the wet, windy thaw soaks the clothing, strips the warmth from it, and presses the cold through to the skin no matter how many layers are worn. New members expect the danger to track the thermometer, the colder the worse, and correcting that expectation before it gets someone hurt is one job of this lesson. It is exactly the "merely cool, but wet and windy" night, the kind a maritime winter serves up again and again, that fills the cold-injury cases the welfare operation meets.

The path to hypothermia

Hypothermia is the falling of the core temperature, and it comes on in a progression a member must know by heart, because catching it early is everything and the most dangerous sign is the one that looks like improvement. It is best understood not as a single event but as a slide, in stages, each one a chance to act that the one after it has lost.

It begins with the body fighting back. To make heat it starts to shiver; to keep the heat it has, it draws blood away from the skin and the limbs and holds it in the core, the narrowing of the vessels called vasoconstriction, which is why a cold person turns pale and their hands grow clumsy and slow. This is the mild stage, and the body is still winning if it is helped now. Alongside the shivering come the early warning signs, the umbles: a person who stumbles in their footing, mumbles or slurs their words, fumbles with their hands at a zip or a buckle, and grumbles or grows withdrawn and quiet is telling you, without words, that the cold is winning. The umbles are the most useful single thing in this lesson, because they show on the outside, in another person, before that person knows anything is wrong, and they are how a member reads the cold in a stranger across a doorway or in a buddy on a long task.

As the core falls further the signs grow more serious, and one of them is dangerously misleading: the shivering stops. A person who was shivering and then ceases has not recovered; far more often they have got worse, because the body has burned through the fuel to shiver and can no longer do it. With it come deepening confusion, drowsiness, slurred and rambling speech, stumbling and falling, and a person who is hard to rouse, behaves strangely, withdraws, or wanders. Beyond this lies collapse, unconsciousness, a heartbeat too faint to find, and death; and because the chilled brain stops judging well, a failing person will rarely call for help and may insist they are fine. The single most important thing to take from this lesson is that a person who stops shivering, grows confused or sleepy, or cannot be roused is a medical emergency, and that the slide can be quiet and quick and pass almost unnoticed. The treatment, including the great care needed in moving and warming such a person, is taught in the Combat First Aid course; the recognition is taught here, and it is what buys the time to act.

   THE SLIDE INTO HYPOTHERMIA  (recognition, not treatment)

   MILD   shivering begins; skin pale; hands clumsy and slow
          the UMBLES: Stumbles, Mumbles, Fumbles, Grumbles
          |  still alert but not themselves  -> ACT NOW, while easy
          v
   WORSE  shivering STOPS (this is worse, not better)
          confusion, drowsiness, slurred and rambling speech
          stumbling, withdrawing, poor decisions, hard to rouse
          |  a medical emergency
          v
   SEVERE collapse; unconsciousness; faint or absent pulse
          handle very gently; treatment per Combat First Aid

   The danger sign that looks like recovery: SHIVERING STOPS.
   Treat the quiet, drowsy, "I'm fine" person as the worst, not the best.

Frostbite and non-freezing cold injury

Where hypothermia is the cooling of the whole core, cold injury can also strike locally, in a part of the body, and it takes two forms a member must tell apart, because they come from different conditions and both are prevented by the methods of this course.

Frostbite is the actual freezing of the skin and the tissue beneath it, most often at the parts furthest from the core and most exposed: the fingers, the toes, the ears, the nose, the cheeks, and the chin. It happens because the body, defending the core, has already pulled warm blood back from these extremities, leaving them cold and poorly supplied, and the cold then freezes the tissue. Frozen tissue looks white, grey, yellowish, or waxy, feels hard, stiff, or numb, and loses its normal colour, warmth, and feeling; a deeply frostbitten part may feel like wood. The early, milder, still-reversible stage is frostnip: the skin goes cold, then numb, often with a pale or whitish patch, a warning that the part is beginning to freeze and must be protected and rewarmed gently at once, before true frostbite sets in. Frostbite is made far more likely by wind, by wet, by tight clothing or boots or over-tight laces that cut off the blood, by handling cold metal or fuel with bare skin, and by anything else that reduces the flow of warm blood to the extremities, including dehydration, exhaustion, and the body fighting hypothermia elsewhere. The watch against it is simple and constant: keep the extremities covered, dry, and unconstricted, wiggle fingers and toes to keep blood moving, and check exposed skin, your own and your buddy's, again and again when the wind is up.

Non-freezing cold injury, often called trench foot or immersion foot, comes from feet, or hands, being cold and wet for a long time without the tissue ever freezing. It does not need frost at all; it needs only cold, wet, and time, often many hours, in conditions a member would not even think of as dangerous. It damages the tissue all the same, leaving the foot pale, numb, wrinkled, cold, and later painful and swollen, and at its worst it is disabling. It is a real and serious danger in exactly the wet, mild winter this course is built around, where the temperature rarely plunges but feet stay soaked for hours, and it is the cold injury a member is most likely to meet, and to suffer, in this kind of weather. It is one of the strongest reasons for the discipline, returned to throughout this course, of keeping the feet dry and changing wet socks at every chance. A member who looks after their feet stays in the fight; one who does not can be crippled by the cold without a single degree of frost.

The special weakness of the extremities and the head

It is worth pausing on why the body keeps injuring itself at the same few places, because it explains both frostbite and frostnip and tells a member where to keep watch. When the body is cold it defends the core first, pulling warm blood inward and away from the hands, feet, ears, and nose to protect the heart, lungs, and brain. This is sound survival logic, but it leaves the extremities cold, thinly supplied, and first to freeze, which is why frostbite strikes the fingertips and toes and the edges of the face long before anywhere else. They are also the parts most exposed, furthest from the warm trunk, easiest to leave uncovered, and most readily strangled by a tight glove, boot, or lace. The lesson is to insulate the extremities generously, keep them dry, and keep nothing constricting them.

The head is a special case for the opposite reason. It is richly supplied with blood that the body does not pull back the way it does from the hands and feet, so a bare head and neck on a cold night radiate a great and steady stream of heat that undoes much of the warmth the rest of the clothing is trying to keep. The old saying that most heat is lost through the head overstates the case, but the practical point stands: the head is hard to insulate any other way, so a hat or hood is among the cheapest and most effective warmth a member has, first on and last off.

Who is most at risk

The cold does not strike everyone equally, and knowing who is most vulnerable is at the heart of the winter welfare operation. The body's defences are weakened by anything that lowers heat production, speeds heat loss, or dulls the judgement that would otherwise seek warmth. The most at risk are:

the very young and the old, whose bodies regulate heat poorly, the young losing heat fast and unable to help themselves, the old with both their heat-making and their sense of cold faded by age;
the sick, the injured, and the exhausted, whose reserves are already spent (a casualty bleeding or in shock is losing heat fast, which is why guarding a casualty against the cold is part of first aid itself);
the wet and the under-dressed, for the reasons set out above, the commonest cause of all;
the still and the immobile, who generate little heat because they are not working their muscles, including a person trapped, collapsed, injured, asleep, or simply sitting out a long cold night;
the under-nourished and the dehydrated, who lack the fuel to make heat and the fluid to carry it; and
those under the influence of alcohol or drugs, which cloud the judgement that would seek warmth, suppress shivering, and, by opening the blood vessels at the skin, actually speed heat loss while making a person feel deceptively, dangerously warm.

It is no accident that this list describes, almost exactly, the people the Army goes out to help in winter: the rough-sleeper who is cold, wet, still, often unwell, often under-nourished, and sometimes drinking against the cold is a person in whom every risk factor is stacked together, each one making the others worse. Recognising that is the beginning of the welfare work this whole course is built to support, and it is taken up in full in Lesson 08: Cold-Weather Welfare and Rescue and, alongside it, in the humanitarian-outreach course Caring for Those in Need.

In Practice: Reading the Cold in a Person

On a winter welfare round through a town centre on a raw, wet night, your section comes upon a man sheltering in a doorway, and what you have learned in this lesson tells you how worried to be. He is wet through and sitting still on cold stone, in a cutting wind that funnels down the street, every one of the five doors of heat loss standing wide open at once: conduction into the stone, convection from the wind, evaporation from his soaked coat, radiation from his bare head, and his breath fogging away into the dark. He fumbles and drops the cup you hand him, and answers you slowly and a little strangely, the umbles, the cold already reaching his mind. You realise he is no longer shivering, though the night is bitter, and a chill of your own goes through you, because you know now that this is not a sign he is warm enough but a sign he has slid past the mild stage into the dangerous one. You run the risk list in your head and find almost every factor against him: old, wet, still, under-fed, the warmth on his breath telling you he has been drinking against the cold. You do not need a thermometer to know this man is in real danger and that time is short, because you can read the cold in him. What you do next belongs to the welfare and first-aid lessons; that you knew to be alarmed, and quickly, and exactly why, belongs to this one.

Check Your Understanding

Describe the five routes by which the body loses heat, and give one practical way a member closes each door. Explain why getting wet and being in the wind are the two dangers a member must guard against most, and why a wet, windy day just above freezing can be more dangerous than a dry, hard frost.
Trace the progression of hypothermia from the first signs to collapse, naming the umbles and what each one looks like. Why is the stopping of shivering a dangerous sign rather than a reassuring one, and why will a failing person often insist they are fine?
Explain the difference between frostbite (including frostnip) and non-freezing cold injury, including what conditions cause each and which one a member is more likely to meet in a mild, wet winter. Then list the kinds of person most at risk from the cold, and explain why this list describes so closely the people the winter welfare operation goes out to help.

Reflection (write a short paragraph): This lesson argues that a member who truly understands what cold does to the body will recognise its danger almost by instinct, in themselves and in others, and that the cold is beaten by routine and discipline rather than by toughness. Think about the people you may meet on the winter operation, and about a long, cold task of your own. Which warning signs would you want fixed so firmly in your mind that you would notice them without having to think, and why might noticing early, while the person is only at the umbles and still easy to help, be the difference between a problem and a tragedy?

Summary

The body holds its core temperature by balancing heat made (by metabolism, by work, and by shivering) against heat lost; cold injury is what happens when heat is lost faster than it is made and the core temperature falls. You raise heat in by working and eating; you cut heat out by closing the routes of loss.
Heat is lost by five routes: radiation (from bare skin, above all the head), conduction (greatest through cold ground and, far worse, water), convection (the wind stripping away warmed air), evaporation (sweat and wet clothing drying on the body), and respiration (warmth and water carried out on every breath). Each has a practical countermeasure.
Getting wet and being in the wind are the two greatest dangers: wet conducts heat fast and destroys the insulation of clothing, and wind multiplies every other loss, so that a merely cool but wet and windy night can cause hypothermia and can be more dangerous than a dry, hard frost.
Hypothermia progresses from shivering, pale clumsy hands, and the umbles (stumbles, mumbles, fumbles, grumbles) to a dangerous stage where shivering stops and the person grows confused, drowsy, and hard to rouse; that stage is a medical emergency, the slide can be quiet, and the treatment is taught in the Combat First Aid course.
Frostbite is the freezing of exposed extremities (with frostnip as the early warning) because the body pulls warm blood back to the core; non-freezing cold injury (trench foot) comes from feet being cold and wet for a long time without freezing and is the likeliest cold injury in a mild, wet winter, which is why keeping the feet dry matters so much. The head, kept well supplied with blood, must be covered because it pours heat away when bare.
Most at risk are the very young and old, the sick, injured, and exhausted, the wet and under-dressed, the still, the under-nourished and dehydrated, and those affected by alcohol or drugs; this list describes almost exactly the vulnerable people the winter welfare operation exists to help. The defences against every one of these dangers are the subject of the lessons that follow, beginning with Lesson 02: Keeping Warm: Clothing, Insulation, and Staying Dry, and turned outward to others in Lesson 08: Cold-Weather Welfare and Rescue.

Cold and the Body