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keeping warm when asleep

Discussion in 'Bushcraft Chat' started by sapper, Apr 11, 2012.

  1. sapper

    sapper Quite Talkative

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    O.K, some of you will know this but with the weather as it is I thought now would be a good time for a refresher.

    The reason we get cold when sleeping is because we are generating very little heat.This is because we are not active.What heat we do generate must be kept around us,to do this we need to insulate ourselves from anything that will draw heat away from us.Things that draw heat away from us are anything that is at a lower temperature than us.
    Nature strives for balance and heat transfer is one of the things it really works hard at.If you place an ice cube into boiling water it will melt as nature tries to equalise the overall temperature of both ice and boiling water,the end result is warm water.The boiling water has overcome the ice and brought it as close to boiling as it can,due to the energy expended in doing this the boiling water has lost some of its heat .So the ice has warmed up and the boiling water has cooled and both are now at the same temperature.The same is true in reverse.
    Your body heat is essentially placed in a huge area of cold air,the cold air will try to lower your body temperature to the temperature of the air.If you are in contact with the air then ,because of the vast quantity of air ,you will soon reach the same temperature as the air.When the temperature is less than 37 celsius then your body will start to chill,as the temperature decreases so does your body temperature and before long hyperthermia sets in and you will,if untreated,die from hyperthermia.This is fact HYPOTHERMIA WILL KILL IF NOT TREATED.
    HOW DO WE AVOID THIS IF WE ARE ASLEEP?

    We insulate ourselves using a sleeping bag.The loft of a sleeping bag is what provides the insulation.All sleeping bags work on the pricipal of still air will stay warm and so use either synthetic insulation or down.I'll go into the differences later.Down and synthetic materials work in basically the same way they trap air and therefore preserve the heat they surround.The more still air you have around you the more heat is retained.Buy the best sleeping bag you can afford it will save your life.

    Another way of losing heat is through conduction to the ground.Open your freezer and touch the ice,what happens ?Your fingertip gets cold.Leave it there for as long as you can and see how much of you hand starts to get cold.This is the heat from your body being drawn away to try and equalise the temperature between you and the freezer,stay at it long enough and you will be frozen.This is exactly what's happening when you lie on the ground.The ground is trying to equalise the temperature between itself and you.Eventually your body will be the same temperature as the ground.

    HOW DO WE PREVENT THIS?

    We insulate ourselves from the ground using a layer of still air,the best way to do this is with a sleeping mat.Either a closed cell mat or an open cell mat.A closed cell mat is a solid foam mat often issued to the armed forces.They do insulate form the cold but don't take the lumps out of the ground and eventually will allow the cold to get through due to conduction.
    Open cell mats ,also known as self inflating mats are made from a foam that is similar to sponge and covered with a nylon cover to keep it air tight.These work by trapping the air in the foam and stopping conduction.
    Another type of sleeping mat is the inflateable li-lo type these are best avoided as they create convection currents through the air inside them and take the heat away from you and then you get cold.
    There are 2 exceptions to this ,thermarests neo air and expeds downmat and synmat.These are all inflatable li-lo types with one major difference .The exped ones are filled with insulation and the thermarest has a secial lining.These all prevent air moving inside and therefore stay warm and keep you warm.

    Why do I get cold when sleeping under a tarp?
    The main reason is the air is moving through the tarp and cooling the outside of the sleeping bag which in turn draws heat from you to rewarm the bag.
    So we need to stop the air around us moving,this is done by lowering the tarp so that the edges are touching the ground and preventing the wind blowing through.The tarp needs to be pitched so that it keeps the wind off you and still presents a windshedding profile.You can also use a bivi bag under the tarp as this has the same effect as a tarp in as much as the wind passes over without taking much heat with it.It will also keep you dry if the damp air gets in the tarp.
    Another way to achieve this is to use a tent,again it must be pitched to present a windshedding profile.

    We've covered how to keep warm and dry but we need to be warm before we get into our sleeping bag.The sleeping bag keeps you warm,it doesn't get you warm.
     
  2. Gazo

    Gazo Administrator Staff Member Site Donor

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    Good write up.
    Another good tip on getting cold hands warm again.

    I know Bear Grylls is not the best but he has some good tips and one thing I have tried is when you have cold hands is spinning your arms to get the blood rushing to your fingers. I have tried this and it does work.
    So a good tip to try if you have cold hands before getting in your bag.
     
  3. Rob

    Rob Very Addicted

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    I normally get with the bird :)
     
  4. elliott92

    elliott92 Very Addicted

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    good write up mate
     
  5. Ian

    Ian Site Manager Staff Member

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    Some good information there. I work outdoors all year round and one thing I have found that helps to keep warm is mind control techniques. Its a bit difficult to explain in type but it does work, I am now experimenting with the same techniques to keep me cool in warm weather.
    Obviously this isn't much use when you are sleeping:eek:
     
  6. Rob

    Rob Very Addicted

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    thats a good one M8 PMSL please explain
     
  7. HillBill

    HillBill Very Addicted Site Donor

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    Sounds like he takes control of someones mind and gets them to stoke the fire, fill hot water bottles, make brews and cuddle him in his sleeping bag, come summer he will have them wafting palm fronds :p
     
  8. Ian

    Ian Site Manager Staff Member

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    Yeah, that's about the size of it:rolleyes:

    There are Siberian, Arctic fishermen who can still tie knots in their nets and lines in freezing sea water, they can do this because their blood vessels at their extremities do not close up to keep the core warm like most people. I know it sounds weird but I believe this is all done in your mind. Your mind controls your body, not the other way round;)
    One of my winter jobs is to clear the various small ponds at one of my gardens and after some practice I can now keep my hands working under freezing water for a surprising amount of time with no ill effects. Obviously it doesn't make you super human but it does help me get the job done;)
     
  9. HillBill

    HillBill Very Addicted Site Donor

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    Wear a hat and the heat doesn't escape through your head, therefore hands and feet stay warm as the blood is not needed else where. ;)

    But i totally agree that the mind controls the body and people really are capable of doing amazing things when they really put their mind to it.

    A classic example is soldiers who get hit fatally, but dont realise they have been hit. So they carry on fighting, then when they realise they have been hit, they drop dead. Its happened loads of times and i have seen it personally once.
     
  10. ReubenBushcraft

    ReubenBushcraft Quite Addicted

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    Actually, for the ice cube and boiling water one, does in not depend on the heat potential of each of the objects? Amoun of heat per cm^3