Royal Army College

Lesson Overview

Clothing keeps a member warm while moving; shelter keeps a member, and others, warm while at rest, and at rest is when the cold most often wins. This lesson makes one point above all: shelter is not only a roof against the weather but a barrier between the body and the cold ground. So much heat is lost downward into cold earth that insulation beneath a person matters as much as cover over them. The lesson covers choosing ground, insulation from the ground, the principles of natural, improvised, issued, and snow shelters, the inside layout that drains cold air below the sleeper, the absolute rule of ventilation whenever any heating is used inside, and the welfare point that getting a cold person out of the wind and wet and off the ground is itself the first shelter.

By the end you will be able to explain why shelter matters and why ground insulation is as important as overhead cover, choose sound ground for a shelter, describe the principle common to all good cold-weather shelters, lay out the inside of a shelter so the cold air drains below the sleeping level, state the rule of ventilation, and apply all of this to the winter welfare operation.

This is the knowledge layer. Building, improving, and warming shelters is practical work, taught and certified in person on the ground under qualified supervision; this lesson teaches the principles those drills rest on.

Key Terms

Conduction: the loss of heat by direct contact with something colder; for a resting person, the cold ground is the great conductor, drawing heat from the body far faster than the air does.
Ground insulation: any dry, non-conducting layer placed between the body and the earth, such as boughs, a mat, or a raised bed, to break the path of conduction into the ground.
Windbreak: anything natural or made that blocks the wind from a position, from a bank or a stand of trees to a built wall or a stretched sheet.
Frost pocket: a low place, hollow, or valley floor where cold, heavy air settles and collects, making it markedly colder than the ground a little above it.
Shelter sheet: a sheet of waterproof material, such as a poncho or tarpaulin, rigged to give cover or a windbreak; the simplest made shelter.
Cold sump: a deliberate low point dug at the entrance or floor of a shelter, below the sleeping level, into which the heavy cold air drains and pools, so that it gathers there rather than around the sleeper. Also called a cold well.
Sleeping level: the raised platform or shelf on which a member rests, set above the floor and the cold sump so that the coldest air settles below the body rather than around it.
Snow shelter: any shelter that uses snow itself as the insulating material, such as a snow trench, a quinzhee, or a snow cave; snow traps so much still air that a well-built snow shelter is far warmer inside than the open air.
Quinzhee: a snow shelter made by heaping loose snow into a large mound, letting it settle and harden, then hollowing it out; suited to ground where the snow is not deep or firm enough to dig into.
Snow cave: a snow shelter dug into a deep, firm bank or drift of consolidated snow, hollowed out and shaped inside; suited to ground where such a bank exists.
Ventilation: the deliberate keeping-open of an air passage from a shelter to the outside, mandatory whenever any flame, stove, or person occupies an enclosed space, so that fresh air enters and the invisible gas carbon monoxide cannot build up.

Why shelter matters

A member at work generates heat and can hold the cold at bay by moving; a member at rest does not, and the cold closes in. Shelter replaces that lost warmth of movement, and it does three things. It gets the body out of the wind, which strips warmth away as Lesson 01 set out; out of the wet, which soaks insulation and drains heat fast; and, the part most often neglected, up off the cold ground.

That last point deserves the most emphasis, because it is the one members forget. When a person lies or sits on frozen earth, snow, or cold stone, heat pours out of them by conduction directly into the ground, and the ground is a far greater thief than the air at the same temperature. A fine roof over a person lying on bare cold earth will not keep them warm; the cold will draw the heat out of them from below all night. The rule is therefore firm: insulation beneath a person matters as much as cover over them. Boughs, a mat, dry leaves, a raised bed, anything dry that breaks the contact between the body and the earth, is not a comfort but a necessity, and on a cold night it is what makes the difference between rest and a casualty.

The reason runs against instinct. Air is a poor conductor; that is the whole reason clothing works, by trapping a layer of still air around the body. The cold ground is the opposite: it touches the body directly, carries heat away by contact, and, because the earth beneath is a vast cold mass, it never warms up under you and never stops drawing. So the first question to ask of any shelter is not "what is over me?" but "what is under me?" That order of priority is the single most useful habit this lesson teaches.

Choosing the ground

Where a shelter is placed decides much of how well it works, and the choosing is governed by a few plain rules that follow from everything already learned.

Get out of the wind. Use what the ground offers, a bank, a hollow on the lee side of a slope, a stand of trees, so that the position is shielded from the prevailing wind; where there is no natural windbreak, one can be made. A forest, where there is one, gives the best of all sites: shelter from the wind, cover overhead, and the very materials, fallen wood and boughs, that a shelter and its ground insulation need. Look at the lee side, the side away from the wind, of any feature: a treeline, a bank, a stand of rock, a deep snow drift. The wind bends and eddies behind such features, so do not site the entrance straight downwind of a gap the wind funnels through; site it where the air is genuinely still.

Avoid the cold hollows. It is tempting to drop into a valley floor or a low hollow to escape the wind, but cold air is heavy and sinks, and it pools in exactly those low places, which become frost pockets, markedly colder than ground a little higher up. On a still, clear night the difference between the valley floor and a slope a few metres above it can be several degrees, and on a marginal night those few degrees are the difference between rest and injury. Better, often, to site on a gentle slope above the valley floor, out of both the wind on the heights and the cold settling in the bottom. The aim is a shelf: low enough to be out of the worst wind, high enough to be above the lake of cold air that forms in the bottom after dark.

Stay off wet ground. Frozen ground is one thing, but a member must avoid sites that are wet underfoot or that will become wet, and must think about drainage and about meltwater if any heat will be used. Ground that drains is the aim: a slight slope, sandy or stony soil, anything that sheds water rather than holding it. Watch for ground that is dry now but lies where water will gather, the foot of a slope, the inside of a bend in a watercourse, a flat below a thawing snowfield; a shelter warm enough to melt the snow or ice beneath it can flood itself by morning, and a member who wakes wet in the cold is in real danger.

And avoid plain hazards. Do not site beneath standing dead trees or unstable branches that could come down in wind or under the weight of snow; a heavy snow-laden bough can fall without warning and kill. In steep country, keep well clear of the run-out of avalanche ground: the foot of an open, steep, snow-loaded slope, a gully that funnels sliding snow, or a slope that has shed before. Keep clear of flood risk, the channels and flats a thaw or a rising watercourse will reach. None of these dangers shows itself on a calm afternoon; the discipline is to picture the site under wind, snow-load, and thaw, and to choose against the worst the ground can do.

In short: out of the wind, above the frost pockets, off the wet and on ground that drains, under good natural cover where it exists, near fuel and water where the choice allows, and clear of what might fall, slide, or flood. The figure below shows the heart of it, the trap of the cold sink in the valley floor and the better shelf above it.

   Siting a shelter: out of the wind, above the cold sink

        wind  >>>>>>
        >>>>>>                          . . . . . . . . (still air,
        >>>>>>   ___                   .   lee side)   .
        >>>>>> /    \  treeline /     .                 .
       _______/      \________ /  ___[ SHELTER ]___      <- shelf above the
                              \    raised, drains          valley floor:
                               \                           out of the wind,
                                \                          above the cold pool
                                 \________________________
                                  \
              valley floor:        \  cold air sinks v v v
              cold air pools here   \_____________________
              (frost pocket, AVOID)   * * cold sink * *
                                      coldest ground at night

Insulation from the ground: the first principle

If a member learns one thing from this whole lesson, it is this. The ground steals heat by conduction far faster than the air does, and a thick insulating layer underneath the body matters as much as, often more than, the roof over it. It has its own section because it is the principle most often skimped, and skimping it costs casualties.

Put method to it. A person at rest needs a continuous layer of dry, trapped air between their body and the earth, built thicker than feels necessary, because lying weight crushes it. Boughs spring back less than they look as if they will; loose leaves pack down; a thin mat under a heavy pack-laden body insulates less than its thickness promises. So:

Build it thick. A bed of cut boughs or brush should be laid up to the depth of a hand or more before it is lain on, knowing it will compress. Lay the boughs butts down and tips up, overlapping like roof tiles so the springy tips face the body, and cross successive layers at angles so there are no gaps. Greenery, bracken, dry grass, heather, reeds all trap air; the drier, the better.
Use what you carry. A foam sleeping mat is made for exactly this and should always go under the body, never wasted as a backrest. An empty rucksack laid flat insulates the torso; spare clothing, a rope coil, even a folded shelter sheet, anything dry, can break the conduction under the parts that bear weight, the shoulders, hips, and feet.
Cover the whole length. Cold drawn into the legs and feet chills the whole body. The insulating layer must run the full length of the sleeper, not just under the torso, and a little wider than the body so a turn in the night does not roll onto bare ground.
Keep it dry. Wet insulation conducts almost as badly as bare ground; the layer beneath you must be kept off wet earth and snow, and protected from sweat and meltwater from above. A waterproof sheet under the insulation, against the ground, keeps the dry layer dry.

The same logic governs sitting as lying. A member on a long static watch should be on a pack, a log, a folded mat, a bundle of brush, never straight on frozen ground or cold stone. Heat lost through the seat over hours is heat that movement cannot easily replace, and a member who lets it happen will be cold long before they understand why.

Kinds of shelter in principle

This course teaches the principles of shelter, not a catalogue of constructions, because the constructions are learned by building them in person. What every good cold-weather shelter has in common is a single idea, and a member who holds that idea can judge or improve any shelter they meet.

That idea is: a small, enclosed, insulated space, well insulated from the ground. Small, because a smaller space holds the warmth of the bodies in it and is quickly warmed, while a large space is hard to keep warm; a low, close shelter beats a roomy one in the cold. Enclosed, so the wind cannot reach in and the warmth cannot pour out. Insulated overhead and at the sides, so the heat is held. And, always, insulated beneath, for the reason laboured above.

Shelters run along a spectrum, and which one is right depends on the time, the materials, and the need.

Natural shelter is the quickest and costs no building: a thick stand of trees, the lee of a bank or a boulder, a hollow under the spreading low branches of a snow-laden conifer, a fold in the ground out of the wind. Its great virtue is speed, and in a sudden need it may be all there is time for. Take it for what it offers, the windbreak and the overhead cover, and add the one thing nature seldom provides ready-made, the thick dry insulation beneath the body. A natural shelter with a good ground layer beats a built one with none.

Improvised shelter is built up from natural materials where there is time and the materials are to hand: a lean-to leaned against a ridge-pole or a fallen trunk and thatched with boughs, a debris pile heaped over a frame, a windbreak wall of brush or stacked deadfall. These suit a longer stay, harder weather, or a site where natural cover is thin. The principle is unchanged: keep it small and low, enclose it against the wind, insulate the roof and walls thickly, and above all insulate the floor. Build only with fallen and plentiful material, and leave the ground as it was found as far as possible, with no lasting scar.

Issued tents and shelter sheets are the mainstay of organised work and should be used in preference to building when they are carried, because they are made for the job and put up fast. At the simplest is a windbreak or a shelter sheet, a wall of whatever is to hand or a waterproof sheet rigged to throw off the wind and the wet; quick to make and, with good ground insulation under the user, a real defence against the cold for a short stay. Pitched low and close to the ground on the lee side, back to the wind, a shelter sheet gives a great deal of protection for very little weight. An issued tent gives more, a fully enclosed space, but the principles still bind: pitch it small side to the wind, keep it as low and close as the design allows, and never burn a flame or run a stove inside one without ventilation.

Snow shelters use snow itself as the insulator, and where the snow is deep and firm enough they are among the warmest shelters of all, because snow is full of trapped air. This surprises members new to it: snow feels cold to the touch, yet a well-built snow shelter, warmed only by the bodies inside and sealed from the wind, can hold an inside temperature far above the bitter air outside, often near freezing point while the open air is far below it. The three forms worth knowing in principle are the snow trench, the quinzhee, and the snow cave, taught here with the safe essentials only, because the building of them is hazardous practical work for the ground.

The snow trench is the simplest and the one a member is most likely to need: a slot dug down into firm snow, just longer and wider than the body, roofed over with blocks of cut snow, a shelter sheet weighted with snow, or boughs and snow, leaving an entrance at one end. It is quick, it needs only firm snow and something to roof it, and with a good insulating layer in the bottom it turns a deadly exposed night into a survivable one. Dig it deep enough to get the body below the level of the wind, roof it close, insulate beneath.

The quinzhee is for ground where the snow lies deep but loose, too soft to dig a cave into. Loose snow is heaped into a large mound and left to settle and harden for a good while, an hour or two, as the disturbed snow bonds; only then is it hollowed out from a low entrance, the walls left thick and even. The settling is what makes it safe and strong; a mound hollowed too soon collapses. The quinzhee suits a longer stay where the snow will not otherwise serve.

The snow cave is dug into a deep, firm bank or drift of consolidated snow, hollowed from a low entrance and shaped inside with the sleeping level raised above the entrance. It is the warmest and most sheltered of the three, but it demands a genuinely deep and firm bank, and digging it is wet, heavy work in which the digger soaks their clothing, which is its own danger in the cold. For all three, two cautions are absolute and are stated here even though Lesson 04 carries them in full. First, ventilation is mandatory: a vent must be kept open to the outside air at all times, and never blocked, because a sealed snow shelter will suffocate its occupants and any flame inside it makes a silent killer of carbon monoxide. Second, mark the shelter so it is not walked on or driven over from above, and keep a digging tool inside in case the entrance drifts shut or the roof comes down.

The inside: cold sump, raised sleeping level, small entrance

Whatever the shelter, the arrangement inside follows directly from one fact: cold air is heavy and sinks. A member who understands that can lay out any shelter so the cold drains away from the body instead of pooling around it, and the difference in a long night is large.

The method is the cold sump and the raised sleeping level. Dig or arrange the floor so that the entrance and a low point sit below the level on which you sleep. The heavy cold air then drains down off the sleeping shelf, gathers in the low sump by the entrance, and stays there, while the sleeper rests in the warmer air the body and any heat have raised. This is most pronounced in a snow shelter, where you can cut the platform a clear step above the entrance, but the same idea improves a tent or an improvised shelter pitched on a gentle slope: sleep with the body uphill of the entrance, so the cold air rolls down and out past the feet rather than settling over the torso. Even a hand's-breadth of difference makes a real change to the air around the body through the night.

Keep the entrance small and out of the wind. An entrance is a hole through which warmth escapes and wind enters, so make it no larger than is needed, set it on the lee side, and offset it from the sleeping level rather than opening straight onto it. In a snow shelter the entrance should be low, ideally below the floor of the living space so it doubles as part of the cold sump; a pack, a snow block, or a sheet can be drawn across it once everyone is in, leaving, always, the air vent open above. A small, low, draught-blocked entrance costs little to make and saves a great deal of heat.

The figure below brings the inside principles together: the dry insulation under the sleeper, the sleeping level raised above the floor, the cold sump into which the cold air drains, the small low entrance on the lee side, and, above all, the vent that must never be closed.

   Cross-section of a sound cold-weather shelter

        VENT (always open)
            |
            v
    ________O______________________  <- roof / cover, insulated
   /                                \
   |                                 |
   |        warmer air here          |
   |     [  s l e e p e r  ]         |   <- SLEEPING LEVEL, raised
   |     ===================        /        above the floor
   |     ^^^ thick dry ^^^         /
   |     ^^ ground       ^^       /   <- GROUND INSULATION
   |     ^^ insulation    ^^     /        (boughs / mat / pack)
   |____________________________/  )___  small, low ENTRANCE
   |                                      on the lee side,
   |   v v cold air drains down v v       draught-blocked
   |________________________________
        * * * COLD SUMP * * *
        cold heavy air pools here,
        below the sleeping level

The absolute rule: ventilation

There is one rule about shelter that admits no exception, and it must be stated before any talk of warming a shelter: whenever any heating is used inside an enclosed space, that space must be ventilated. A shelter that is good at holding warmth is, by the same token, good at holding the invisible, odourless gas that a flame or a stove gives off, carbon monoxide, which is a silent killer. It gives no warning; it does not smell, and it does not choke; it simply dulls and then takes the people breathing it, often while they sleep. The better and tighter the shelter, the greater the danger. An opening for fresh air is therefore mandatory the moment any heat source is lit inside, and if any flame is left burning while people rest, someone must stay awake to watch it. This lesson states the rule; Lesson 04, Fire, Heat, and Light in the Cold, treats heat sources and carbon monoxide in full, because it is the cardinal safety point of this course.

The danger is at its sharpest in a snow shelter, and a member must understand why. A snow shelter is nearly airtight by nature: the snow seals the wind out so completely that it also seals the air in, and as the inside warms, the walls can glaze over with ice and close the last pores. A vent must therefore be opened deliberately and kept open, never allowed to drift or freeze shut, and it is wise to clear it by hand from time to time through the night. The same rule binds a tent and an improvised shelter: any flame, any stove, any vehicle engine, in any enclosed space, demands a clear passage of fresh air, and no exception is ever made for cold or convenience. Carbon monoxide kills people every winter precisely because the shelter that felt safest, the tightest and warmest, was the one that held the gas. Treat the vent as the most important single feature of the shelter, more important than the roof.

The welfare application: shelter is the first need

Everything in this lesson is also the heart of the winter welfare operation, because the people the Army goes out to help in winter are, almost by definition, people without proper shelter: exposed to the wind, soaked by the wet, and very often sitting or lying still on cold stone or frozen ground, losing heat into it hour after hour. For such a person, shelter is the first and most important thing a member can give, and it begins the moment they are reached.

A member does not need to build anything to start. Getting a cold-stricken person out of the wind, under cover from the wet, and above all up off the cold ground onto something dry that breaks the conduction, is itself shelter, and it is often the single most useful act in the first minutes, before any further help arrives. The same knowledge that tells a member to put boughs or a mat beneath their own body on a cold night tells them to get a coat, a board, anything dry, between a rough-sleeper and the freezing pavement. The order is the same as for yourself: get something under them first, then something over them, then out of the wind, then warm.

The welfare member carries the inside principles into a built environment, because they are simply the physics of heat. A person found in a stairwell or a doorway lies in a cold sump of the building's own making, the cold air of the whole structure draining down to where they are; lifting them even onto a bench, a step, a folded blanket, raises them out of the worst of it. A small enclosed space is warmer than a large one, so a person is better moved into a porch, a vehicle, or a side room than left in a hall or a concourse. And the ventilation rule binds here as hard as anywhere: a member who warms a person in a vehicle must never run the engine in an enclosed space, a garage or a closed loading bay, for heat, because the same silent gas that kills in a snow shelter kills in a car, and a well-meaning member can take a life trying to save one. Read this lesson, then, twice over: as how to shelter yourself so you stay effective, and as how to give a vulnerable person the shelter that may keep them alive. This first sheltering is the front of the rhythm taught in Lesson 07, Winter Routine and Looking After the Team, where the discipline of routine turns a single good act into a watch kept well all night.

In Practice: A Long Watch on the Headland

A small party is set to hold an observation point on a windswept stretch of high ground above the coast through a winter night, and the choices they make are this lesson in practice. They do not crouch in the obvious deep hollow out of the wind, knowing the cold air will pool there and chill them worse than the breeze above; they take a spot a little higher, on a shelf in the lee of a low bank, on ground that drains and clear of the snow-laden boughs that hang over the gully to one side. They rig a shelter sheet low and close against the wind and the wet, its back to the weather, its small open side away from the funnel of the gap below. Most of their effort goes not into the cover overhead but into the ground beneath them: they lay down a thick bed of boughs, butts down and tips up, a good hand deep, knowing it will press flat under their weight, and a foam mat over it under the part that bears the most, because without it the cold earth would draw the heat out of them all night whatever they hung over their heads. They scrape the floor so the entrance end sits a touch lower than where they lie, and the cold air drains down and out past their feet rather than settling over them. The space is small and close, enough for the party and no more, so their own warmth fills it. When, late and bitterly cold, one of them lights a small stove to brew a hot drink, the sheet is opened wide first and one stays awake to watch it, because the rule on the silent gas is not bent for cold or convenience. Later, off duty, that member thinks of the welfare operation, and sees that the person they will look for in a doorway tomorrow night faces exactly what they have just defeated: the wind, the wet, the cold draining ground, with no boughs beneath them, and that the first thing to do is get something dry under them and lift them out of the building's own cold sump. The thought is the lesson landing twice. What they build is certified by their instructors on the ground; that they sited it right, insulated themselves from the earth, drained the cold air below them, and kept the vent open belongs to this lesson.

Check Your Understanding

Shelter does three things against the cold. Name them, and explain why insulation beneath a resting person matters as much as the cover over them, naming the way heat is lost into the ground and why the ground is a worse thief than the air.
Why should a member usually avoid the low valley floor or a deep hollow when siting a shelter, even though it may be out of the wind? What kind of ground makes the best site, and why? Inside the shelter, what are the cold sump and the raised sleeping level, and how do they keep the sleeper warmer?
State the principle common to all good cold-weather shelters, name the kinds of shelter in principle and when each suits, and state the absolute rule that applies whenever any heating is used inside one, with the reason it is sharpest in a snow shelter. How do these ideas apply to helping a cold person on the winter operation?

Reflection (write a short paragraph): This lesson insists that getting a person off the cold ground is as important as getting a roof over them, and that for a vulnerable person shelter is the first thing that matters. Think about a doorway, a bench, or a patch of frozen ground where you might find someone on a winter night. What could you put between that person and the cold ground in the first minute, before anything else arrives, and how might you raise them out of the cold air pooling around them and into a smaller, more sheltered space, and why might these simple acts matter more than they look?

Summary

Shelter replaces the warmth a resting body no longer generates by moving; it gets the body out of the wind, out of the wet, and above all off the cold ground, into which heat is lost rapidly by conduction.
Because of that downward loss, insulation beneath a person, boughs laid thick and dry, a mat, a pack, a raised bed, anything dry that runs the full length of the body, matters as much as cover overhead; a roof alone over a body on cold earth will not keep it warm.
Choose ground out of the wind on the lee side, above the cold hollows and frost pockets where heavy cold air settles, off wet ground and on ground that drains, under natural cover and near fuel and water where it exists, and clear of falling dead trees, avalanche run-out, and flood.
Every good cold-weather shelter is the same idea: a small, enclosed, insulated space, well insulated from the ground. Shelters run from natural cover and a simple windbreak or shelter sheet, through improvised builds and issued tents, up to snow shelters (the snow trench, and the principles of the quinzhee mound and the snow cave) that use snow as an insulator; build small, take only fallen material, and leave no lasting scar.
Lay the inside out so the cold air drains away from the body: a cold sump below the sleeping level into which the heavy cold air pools, a raised sleeping level above it, and a small, low entrance on the lee side with a draught-block.
Whenever any heating is used inside an enclosed shelter, ventilation is mandatory because of carbon monoxide, a silent killer, and the danger is sharpest in a nearly airtight snow shelter (treated fully in Lesson 04, Fire, Heat, and Light in the Cold); and for a cold-stricken person, getting them out of the wind and wet, off the ground, and into a smaller sheltered space is the first and most important shelter, begun the moment they are reached and carried on through the routine of Lesson 07, Winter Routine and Looking After the Team. Staying dry, the other defence this shelter protects, is taught in Lesson 02, Keeping Warm: Clothing, Insulation, and Staying Dry.

Shelter from the Cold