The wind was blowing briskly as I stepped out of the tent, but the sun was shining and it didn't feel too bad. When I had been out earlier, briefly, answering nature's call, the air had been still, and despite it being 40C it had seemed quite warm in the sunshine. I had decided to wear only a cotton windproof over underwear and fleece salopettes. It was amazing how little one needed to keep warm as long as you kept on working hard.

Ran and I took down the tent and packed up our sledges. The south pole was only 30 km away and, with luck, we would reach it within two days. It helped to have it so close. We had been going 12 hours a day for more than two months, and the effort had taken a terrible toll. It had been both mental and physical hell. It was not long after we set off that I realised my mistake. As well as only putting on a single jacket, I was wearing only thin contact gloves inside outer mitts and, after an hour, with the wind rising even more, my hands were suffering badly and not warming up despite moving. They became so bad that Ran had to help me put on the extra mittens from my sledge, my fingers were too useless to get them on. When we set off again, I was getting generally chilled. After the long stop fighting with the gloves, I found that I could barely pull the sledge with my cold muscles. I was in trouble, and I realised I would have to stop and put my fleece jacket on as well, but to do this meant removing my outer jacket completely and once again my fingers were useless and I was unable to do up the zips. Ran was there to help again, but I had entered a vicious circle. My thinking was beginning to fade, and although I kept walking for another half-hour or so, I was never with-it. It is only through Ran's description that I know what happened next.

I had apparently begun to move very slowly and to wander from side to side. When Ran asked if I was OK, I had been unintelligible, and he had realised immediately that I must be hypothermic. He then tried to get me to help with the tent, but I just stood around doing nothing. So he put it up alone and pushed me inside. Eventually he got me in my sleeping bag and forced me to take some hot drinks. After an hour or so I recovered, but it had been another close call. Obviously we were getting vulnerable and we discussed pulling out at the pole...

The above is an excerpt from my book, Shadows on the Wasteland, about my crossing of Antarctica with Sir Ranulph Fiennes. Under the circumstances, it was perhaps not surprising that I became hypothermic, for cold easily creates casualties and can even kill. Yet, with the correct preparation, man can operate successfully even in the harshest of climates. The secret is to match the body's heat production - chiefly dictated by activity - with its heat losses - chiefly governed by clothing and shelter. You should aim to neither overheat nor cool down. Both can have unwelcome consequences.

An inactive adult produces about a light bulb's-worth of heat (100 watts), which is not really much to keep the whole body warm in the face of the cold, wind and rain. It is, therefore, generally wise to keep moving for the most of the time in cold conditions, until you have either reached or created proper shelter. However, many reasons, such as getting lost or injured, may force you to halt or lie up under adverse circumstances, and you are then going to need to reduce your heat losses to less than the 100 watts that you will be producing. This may be an impossibility if ill equipped or conditions are really harsh. If you can't reduce heat losses enough, your body will cool and you will start to shiver. This can increase your resting heat production to as much as 500 watts, but even this may be inadequate and the shivering itself is uncomfortable and tiring for the muscles. If cooling still continues, you will become hypothermic and can be in great danger. It is definitely best to carry enough protection to deal with getting stuck out in the worst possible conditions you may meet.

When you are active, things are quite different. Working hard leads the body to produce as much as a good room heater - 2,000 watts or even more. It is therefore more common to get too hot rather than too cold, even in the worst conditions. Initially, getting too hot may not be important, but it does lead to sweating, which can ruin the insulation of your clothing by wetting it from the inside and later, when you have decreased your activity or the conditions have worsened, this wet clothing will have lost its ability to protect you properly. Sweating may also lead to dehydration, which in turn will make you vulnerable to fatigue, and it is with the onset of tiredness and the ensuing slow-down in activity that heat production will start to fall and you will cool rapidly to become at risk from the 'exhaustion/hypothermia' syndrome. Even the most experienced of people have become victims under such circumstances.

In order to match heat losses to heat production, clothing must have the flexibility to be both cool and warm. It must also be able to provide windproofing and waterproofing. Such flexibility can only be achieved by the use of layers, which must be easy to put on and take off and comfortable to wear together. In all but the very coldest regions - where rain or melting snow won't occur - I would favour the use of modern synthetics in the insulation layers as they tend not to degrade very much when wetted by sweat or the environment, and they also dry spectacularly quickly. If affordable, waterproofs/windproofs should be moisture vapour permeable (MVP) since these will limit the accumulation of sweat and condensation in inner garments and will allow the evaporation of some sweat, which will help to keep you cool if overheating. However, it needs to be remembered that even MVP garments are only partially vapour permeable (especially in the cold, when water vapour will condense or even freeze on the inner surface of the garment and will then be trapped by its waterproof qualities), and so it is always better to remove the waterproof if it is not actually raining and activity is making you too hot.

Additional flexibility when trying to maintain a comfortable body temperature can be granted by changing your head covering. In the cold, when wearing good clothing, as much as 90 per cent of your heat losses can come from your head, so by putting on or taking off a warm hat or balaclava and by adjusting a windproof hood, you can make enormous changes to your heat losses much more easily than by adjusting other garments. It is often said that if you get cold hands you should put on a hat.

Eating is also an important factor in keeping warm. Even at rest a meal will rev up your metabolism and make that 100-watt bulb glow brighter, while during exercise it will considerably increase your heat output for any given level of activity. More importantly, food also helps to sustain the supply of fuels to the muscles, and this will allow you to continue working, or for that matter shivering, for longer. In addition, it will make it much less likely that you will develop a low blood sugar - a factor now thought to be important in the onset of some cases of exposure/exhaustion. Almost any food will help, but it is probably best for it to contain a fair amount of carbohydrate. Grain-based snack bars are as good as anything, but snacks based on chocolate are also excellent, even if there is a greater fat content.

When hypothermia does begin to occur in an individual, a number of changes are seen that make the diagnosis pretty easy as long as the possibility is carefully considered. Unfortunately, the person suffering from the cold is often unable to consider things properly, since he or she may not realise what is happening and often, after feeling cold, shivery and miserable initially, they may feel quite happy and even warm. It therefore goes without saying that a problem may only become evident when things have already become quite bad, and that if a victim is alone or everybody in a party becomes hypothermic simultaneously, things are very serious. The signs to watch out for are quite similar to those seen when a person becomes increasingly drunk. At first the victim may slur speech and begin to be unnaturally happy with the situation. This normally corresponds to a core temperature of around 35C, compared to the normal 37C, although the actual temperature varies from individual to individual and some people feel quite unwell at 36C. Then, as cooling continues, the victim may begin to stumble or stagger and may go on to become aggressive or confused. This often correlates with a core temperature of around 3334C. Eventually, at a core temperature of around 32C, they will collapse and become unconscious, and they can go on cooling to stop breathing at around 27C. However, their heart may not stop until core temperature is as low as 22C, and so it is vital to remember that, however bad things seem, attempting rewarming and resuscitation may still work.

When someone first starts getting cold, act quickly by increasing clothing insulation, increasing activity or by seeking shelter. However, if choosing to shelter, remember that it may entail lying up in bad conditions and the loss of activity will cut heat production right down. This may have devastating results, and so the decision to go on or to seek emergency protection requires great judgement. Generally, I would recommend that if the victim is only just beginning to cool, push on if proper warm conditions are likely to be reached reasonably quickly. Hot drinks and food are also of great value, but will only be helpful while the victim is conscious and cooperative. Once again, however, remember that sitting around preparing them may have adverse effects.

If the victim is worse and is actually showing signs of staggering or confusion, the situation is becoming dangerous. Obviously additional clothing, hot drinks or seeking a course out of the wind remain of paramount importance, but the question of carrying on becomes more difficult since now it is probably better to stop if reasonable shelter is available. When going out in cold environments, you should always plan to carry some sort of windproof and waterproof bivouac protection - noting that, although tempting weight-wise, lightweight silvered survival blankets have been shown to be no more effective than a plastic sheet and definitely worse than a plastic or more rugged waterproof bag. You may, of course, be planning on camping anyway, in which case you need only ensure that your tent is adequate and that you have practised pitching it when the wind is up. It is no good finding out that it cannot be done with your model when you need it in emergency. Ideally, you should also be carrying a sleeping bag, even if you had no plans to get trapped outside, for there is no doubt that putting a victim in a good bag, and if necessary getting in it with them, is the best course of action if you are forced to stop.

Obviously, shelter can be sought as well as carried. In an emergency, it is a nice warm building that is best, but this is not normally an option. The priority then becomes getting out of the wind and wet, and any natural feature that you can get under or into the lee of is of great value. Also remember that effective shelter may often be found close in on the windward side of an object, particularly if it has a vertical side that will generate back pressure and a 'dead spot' immediately in front of it. Much to many people's surprise, the shelter there may even be better than to leeward since swirling vortices of snow do not come curling round and drifting over you. In conditions with decent snow cover, compacted snow or ice can be used to create a whole range of possible shelters, ranging from simple snowholes to multiple-roomed camps, but really you need to have been taught how to make them and be carrying a suitable snow shovel. Reading about building such shelters cannot replace experience, and before going out in really severe conditions one should have practised in safe conditions. Ideally, you should have attended a proper course on winter survival such as those run by the British Mountaineering Council in Scotland or North Wales.

If a victim has cooled so much that they are unconscious, they need medical attention urgently. However, while this is sought or awaited, every measure mentioned above should be made to protect them from further cooling. As a general rule, never give up trying to protect and warm them, even if they appear to be dead. People have been successfully resuscitated many hours after they have apparently stopped breathing, and you cannot rely upon being able to feel a pulse or hear their heart. It is said that hypothermia victims are 'not dead until they are warm and dead' and so, generally speaking, it is impossible to be sure while you are still out in the field.

I would reiterate that, with the correct preparation, you can operate safely and relatively comfortably in terrible conditions, but doing so is an art. That art needs to be learned and it is a mixture of education, preparation and forethought. Remember that hypothermia could happen to you or one or your party even in a temperate climate and indeed, it is more likely to happen in milder, wetter conditions than in the truly cold regions of the Earth.

I will finish with another extract that illustrates just how easy it is to be caught out, and how simple it is to remedy the situation.

His voice was slurred and I looked at him more closely. Although he smiled, there was a strange, wild expression on his face and his eyes were slightly glazed. He wasn't shivering any more but his skin was as white as marble and I noticed that he had taken his gloves off and they were nowhere to be seen. He was also swaying as he began to almost rip at his clothing, frustrated by his fruitless attempts to pull down the zip with cold fingers.

The effect was quite spectacular. As we entered the lee of the Cwm, the noise and buffeting that we had endured all day ceased and the world became an almost silent place. It seemed so much warmer that as I hurried downward, I began to sweat, but for my companion, who I almost dragged along beside me, the move into shelter brought a different experience. Although he, too, began to warm, it only brought him back towards the normal and, with it, he began to shiver and feel miserably cold.

I could scarcely believe what I had just witnessed. It was only September on Snowdon, yet my father had been to the edge of disaster...

Remember, always treat the cold with respect and never underestimate what even the UK weather can produce.