Given that humans are warm blooded - and have evolved for a central African climate - we need to maintain a consistent core temperature of around 37°C in order to survive. To do that, we have to create a situation where heat production is approximately the same as heat loss.
In the outdoors, the main heat source (apart from a sneaky pocket hand-warmer) is your own body.
Heat production can be increased by moving around more; heat loss can be reduced by wearing more clothing.
On a typical day out in the British hills, when weather conditions and activity levels can fluctuate wildly, maintaining the balance can be a bit of a challenge.
It's also important - because even very minor changes in the body's core temperature can be uncomfortable, unpleasant or life threatening.
The easiest way to achieve this balance between heat production and heat loss is by adopting a flexible, adaptable clothing Layering System - made up of 5 Layers.
THE KEY TO KEEPING WARM
Wearing clothing that traps dry, still air is fundamental to reducing heat loss in the outdoors. Dry air has a tiny thermal conductivity - water, by contrast, is about 24 times more conductive than air.
So, basically, the more air your clothing traps - and the dryer you can keep it - the less heat you'll lose.
Keeps the surface of your skin dry and starts the process of the pushing moisture out of the clothing system.
Insulation. The thicker the base layer, the more air trapped inside.
Core Silver range or Ultra Silver range.
Wicks moisture effectively and dries rapidly. Low-friction fabric layers smoothly and comfortably with additional layers.
Traps air, provides insulation and helps reduce heat loss.
Wicks moisture out through the clothing system.
Microgrid or Microrib Stowaway Jacket.
Simple design, layers well with a fabric construction that traps air and increases breathability.
Improves the insulating effects of the layers underneath by eliminating wind chill.
Maximises breathability, and promotes the dispersal of moisture vapour.
Breathable, windproof, ultra-lightweight and phenomenally packable.
Traps dry air and reduces heat loss.
A layer you call on when your activity levels drop.
Thermally-efficient, lightweight, compressible and warm when wet.
Maximises the insulating effects of the under layers by keeping them dry.
Eliminates wind chill.
Vertex Overhead (Men's) or Vertex Jacket (Women's).
When used as part of a versatile 5-layer system, it offers complete wind and waterproof protection in an ultra-durable 3-layer construction.
The Rohan "5-layer" Layering System is a comprehensive clothing system for the top half of the body consisting of just five pieces.
Chosen carefully, these five items of clothing will protect you in all conditions, all year round, for walking and trekking in the hills and mountains of the United Kingdom.
All the layers are designed to be worn together if required. In cold winter conditions, such as a February backpacking trip in the Scottish Highlands, that's probably necessary. You'll certainly want all five with you - even if one or two of the layers spend most of the time in your rucksack.
In warm summer weather (with a reasonably dry forecast), you might opt for just a Base Layer and a Soft Shell. With a Mid Layer in your rucksack for when you stop for lunch.
Conditions are obviously crucial - but just as important is your planned activity level.
If you're walking to the top of a Lakeland fell to photograph the landscape at first light, you'll be inactive for quite some time while you make your pictures - and you might well need all the layers. If, by contrast, you're planning to run up and down the same fell - on the same day, in the same weather conditions - a Base Layer and a light Soft Shell may be all that's required.
As a general rule - whatever you set off wearing in the morning, it's worth having at least one additional layer with you - just for emergencies.