In cold conditions in the Wilderness you can lose heat through a number of different processes - radiation, convection, conduction, evaporation and respiration (breathing).

Radiation:
When you sit by a good campfire and feel its warmth, you are benefitting from heat radiated by the fire. The other way to look at this is that while you are getting warmer, the fire is losing heat to its environment. It's the same for you when everything around you is cold: You will radiate warmth and lose heat to your environment.

Many people think that heat loss from radiation occurs only from exposed skin. But even if you are clothed, heat radiates from your body to your clothes, then from your clothes to your surrounding environment.

Convection:
Convective heat loss occurs between a surface and a moving fluid or gas in contact with it. The air nearest your skin is warmed by the body. If this warm air is allowed to move away from your body, colder air will take its place and you will lose more heat in warming this cold air. Putting on insulating layers of clothing helps to trap warm air near to your body. It's the air trapped in your clothes that keeps you warm, not the clothes.

Particular attention should be paid to insulating your core body area; the head, neck, trunk and groin. There is always a good blood supply to your brain and there is always a lot of warm blood passing close to the surface of your neck. Therefore, it is always good to have a warm hat with you, even in summer. It doesn't need to be bulky. A merino wool beanie is excellent. In colder conditions, you should also include a head-over or scarf to prevent heat loss from your neck. These items, although small, can make a massive difference to how warm you are on a cold day, if the weather turns bad, or if you have to spend an unplanned night outdoors.

Strong winds and low temperatures can create the potential for massive convective heat loss. Unless your outer layer is windproof, cold air passing over your body will disturb the warm air and take heat away from you.

When you are looking for a site to build a shelter from natural materials in a wilderness survival situation you should first evaluate the natural wind protection of the site itself. Even if it's not windy today, think about the direction of the prevailing wind and make sure your camp has some protection from it. If you are building a shelter you will want to make sure it is well draught-proofed. This is particularly the case if you are without any sleeping equipment. When you think you have put enough materials on your shelter put on at least 20% more!

Conduction:
Heat loss through conduction comes through direct contact with cold surfaces or objects.

Aluminium is an extremely good conductor. Water is a good conductor of heat. Air is a poor conductor of heat. That's why air is so good to have trapped in your clothes to insulate you. So its good to insulate your body from cold surfaces with a poor conductor. A common example of this is the use of a sleeping mat when camping out. If you sleep directly on the ground, you lose an awful lot of heat into the earth. If you are building a shelter, make sure you build a good bed, with adequate insulation.

Avoid handling cold objects, particularly metal, with your bare hands. Do not touch metal objects with your bare hands in temperatures much below zero degrees Celsius (32 degrees Fahrenheit) as they can cause near-instant freezing of skin. Even if you are using gloves, limit how much you handle metal objects in sub-zero (Celsius) temperatures.

Evaporation:
To convert a liquid to a vapour or a gas requires energy. What this means is when sweat on your skin or moisture in your clothing evaporates, this evaporation draws heat away from your body. This is called evaporative heat loss.

Evaporation is a very effective mechanism for getting rid of heat. It explains why sweating works so well at keeping you cool. The dark side of evaporationis that if you get wet in a cold environment, you can get very cold, very quickly. Hence, it is critical that you do everything you can to stay dry whilst in the wilderness.

Your first line of defence is effective waterproof clothing to stop rain, and other water, getting into your clothing. But it's not just about stopping water getting in; you should aim to minimise the amount of sweat in your clothing. When you exert yourself, getting wet from the inside can be a bigger problem than water coming in from the outside of your clothing. Breathable fabrics that allow perspiration to escape will help to an extent but if you are working hard, you can still get sweaty and damp.

If you take account of the environmental temperature and your level of exertion you can help to maintain an even body temperature and minimise sweat by adjusting layers, particularly taking layers off before you start exercising. Ventilation also helos - open up front zips, expose the neck, etc. Adjust your clothing as necessary during exercise. We can also regulate our activity level; if you are still getting hot and sweaty despite the above measures then you should probably slow down a bit and pace yourself.

Respiration (breathing):
As you breathe in cold air it is warmed before it gets to your lungs. You then breathe out warm air and you are effectively losing heat every time you breathe. There is nothing we can do about breathing. We have to do it to stay alive! In comparison with the other mechanisms of heat loss, however, heat loss through breathing is relatively insignificant.

Conclusion:
So these are the five ways you lose heat to the environment in the wilderness. Radiation, convection, conduction, evaporation and respiration. If you understand these processes well and combine this knowledge with your expereience of the outdoors, it should help you stay safe and less likely to suffer from hypothermia.