Hypothermia, or exposure as it used to be known, occurs when the body loses heat faster than it can produce it.
The prime cause is becoming wet and cold but hunger, tiredness and low morale all play a part.
Symptoms to watch out for are shivering, lethargy, tiredness, irritability and unusual or irrational behavior.
Do not ignore these signs either in yourself or others in the party.
People may say they are fine but if you know them well enough to know they are acting out of character, suspect hypothermia.
If someone complains of the cold stop at once and do something about it and if it is you who is feeling cold do not be afraid to say so.
You cannot use will-power or toughness to deal with hypothermia.
Victims are often people who have been trying to keep up with others who are too fast for them and who do not want to hold the group up by admitting they are tired or cold.
They arrive last at rest stops and do not have time to put on extra clothing or eat properly, again because of fear of slowing the party down.
In bad weather this can easily lead to hypothermia.
It should not happen though.
Hypothermia is a killer of the unprepared and the careless.
If you are properly clothed, well fed and not over-tired you should be in no danger.
You must use your equipment when it is needed, of course.
People have died with warm clothing and bivvy bags in their rucksacks.
You should never leave essential gear behind either, even in the middle of a heat wave.
During the long, hot summer of 1995 a walker died in the English Lake District after he became lost because he did not have adequate clothing when a cool, damp evening mist caused a rapid drop in temperature.
Most hypothermia occurs in temperatures well above freezing, probably because people go out with minimal equipment due to warm weather when they set off.
Winter walkers expect cold weather and are usually well prepared.
If someone in your party shows signs of hypothermia take immediate action.
If you can, the first thing is to seek nearby shelter such as a large boulder.
Do not spend time searching for shelter, though, as the sufferer could be deteriorating rapidly.
The aim is to minimize further heat loss and start re-warming immediately.
The casualty should be put in a bivvy bag as soon as possible, with some form of insulation such as sitmats and rucksacks underneath them.
If they are conscious, they should be given hot drinks and food.
Ideally, wet clothing should be removed and replaced with dry warm items.
Unless you have a tent or group shelter this probably cannot be done without further heat loss, which must be avoided.
In that case just the wet outer layer should be taken off then dry clothing put on over the damp inner garments.
The only exception I would make to this is if the casualty is wearing cotton next to the skin, which they should not be doing.
Because lots of body heat is needed to dry out wet cotton I would remove this garment and replace it with wool, silk or synthetic top even at the risk of losing some heat in the process.
Putting the casualty in a sleeping bag, if you have one, will also help.
Because the casualty may not be able to produce enough heat themselves it is better if someone else gets in it and warms it up first.
If there is room someone could also get in the bivvy or sleeping bag with the casualty for extra warmth.
While doing all this keep reassuring the casualty that all will be well.
In a group everyone can huddle round the casualty for added warmth.
What you should never do is rub the casualty to warm them up as this speeds up the circulation, which slows down in hypothermia so that the core of the body containing crucial organs like the heart and lungs stays as warm as possible.
Once a person starts to become seriously hypothermic circulation becomes very slow and blood near the surface of the body becomes very cold.
If the circulation is speeded up this cold blood returns to the heart where it can cause serious problems.
This can also happen with re-warming by skin to skin contact so having someone naked get into the bivvy bag or sleeping bag with the hypothermia victim, a standard recommendation until very recently, is not a good idea.
Alcohol has the same effect too so forget the St Bernard and the brandy and save the contents of your hip flask for when you get back down and are inside in the warm.
If the casualty shows signs of recovery descend by the quickest safe route.
Further exercise will help the warming process once it has begun as long as the casualty stays warm and dry.
Do not make someone try to walk if they do not recover fully as this means the hypothermia is more severe and you have a serious situation.
Whether you move on or stay put send someone for help and make sure that the rest of the party keep warm.
If you do nothing for mild hypothermia it will rapidly become much more severe, leading to a lack of co-ordination soon followed by collapse, a coma and ultimately death.
You should never assume that an unconscious casualty is dead, even if there is no sign of breathing or heartbeat.
People have recovered from severe hypothermia so continue warming until help arrives.
The prime cause is becoming wet and cold but hunger, tiredness and low morale all play a part.
Symptoms to watch out for are shivering, lethargy, tiredness, irritability and unusual or irrational behavior.
Do not ignore these signs either in yourself or others in the party.
People may say they are fine but if you know them well enough to know they are acting out of character, suspect hypothermia.
If someone complains of the cold stop at once and do something about it and if it is you who is feeling cold do not be afraid to say so.
You cannot use will-power or toughness to deal with hypothermia.
Victims are often people who have been trying to keep up with others who are too fast for them and who do not want to hold the group up by admitting they are tired or cold.
They arrive last at rest stops and do not have time to put on extra clothing or eat properly, again because of fear of slowing the party down.
In bad weather this can easily lead to hypothermia.
It should not happen though.
Hypothermia is a killer of the unprepared and the careless.
If you are properly clothed, well fed and not over-tired you should be in no danger.
You must use your equipment when it is needed, of course.
People have died with warm clothing and bivvy bags in their rucksacks.
You should never leave essential gear behind either, even in the middle of a heat wave.
During the long, hot summer of 1995 a walker died in the English Lake District after he became lost because he did not have adequate clothing when a cool, damp evening mist caused a rapid drop in temperature.
Most hypothermia occurs in temperatures well above freezing, probably because people go out with minimal equipment due to warm weather when they set off.
Winter walkers expect cold weather and are usually well prepared.
If someone in your party shows signs of hypothermia take immediate action.
If you can, the first thing is to seek nearby shelter such as a large boulder.
Do not spend time searching for shelter, though, as the sufferer could be deteriorating rapidly.
The aim is to minimize further heat loss and start re-warming immediately.
The casualty should be put in a bivvy bag as soon as possible, with some form of insulation such as sitmats and rucksacks underneath them.
If they are conscious, they should be given hot drinks and food.
Ideally, wet clothing should be removed and replaced with dry warm items.
Unless you have a tent or group shelter this probably cannot be done without further heat loss, which must be avoided.
In that case just the wet outer layer should be taken off then dry clothing put on over the damp inner garments.
The only exception I would make to this is if the casualty is wearing cotton next to the skin, which they should not be doing.
Because lots of body heat is needed to dry out wet cotton I would remove this garment and replace it with wool, silk or synthetic top even at the risk of losing some heat in the process.
Putting the casualty in a sleeping bag, if you have one, will also help.
Because the casualty may not be able to produce enough heat themselves it is better if someone else gets in it and warms it up first.
If there is room someone could also get in the bivvy or sleeping bag with the casualty for extra warmth.
While doing all this keep reassuring the casualty that all will be well.
In a group everyone can huddle round the casualty for added warmth.
What you should never do is rub the casualty to warm them up as this speeds up the circulation, which slows down in hypothermia so that the core of the body containing crucial organs like the heart and lungs stays as warm as possible.
Once a person starts to become seriously hypothermic circulation becomes very slow and blood near the surface of the body becomes very cold.
If the circulation is speeded up this cold blood returns to the heart where it can cause serious problems.
This can also happen with re-warming by skin to skin contact so having someone naked get into the bivvy bag or sleeping bag with the hypothermia victim, a standard recommendation until very recently, is not a good idea.
Alcohol has the same effect too so forget the St Bernard and the brandy and save the contents of your hip flask for when you get back down and are inside in the warm.
If the casualty shows signs of recovery descend by the quickest safe route.
Further exercise will help the warming process once it has begun as long as the casualty stays warm and dry.
Do not make someone try to walk if they do not recover fully as this means the hypothermia is more severe and you have a serious situation.
Whether you move on or stay put send someone for help and make sure that the rest of the party keep warm.
If you do nothing for mild hypothermia it will rapidly become much more severe, leading to a lack of co-ordination soon followed by collapse, a coma and ultimately death.
You should never assume that an unconscious casualty is dead, even if there is no sign of breathing or heartbeat.
People have recovered from severe hypothermia so continue warming until help arrives.