Modern humans spend their lives indoors in controlled environments and tolerate harsh cold only briefly.
To really spend time outside without freezing – whether for sight-seeing in a foreign city or hiking in a national park – appropriate clothing must be selected based on the expected conditions, and then used correctly.
The correct selection, and more importantly the correct use of cold-weather gear, follows these basic principles.
Normal body processes, like digestion and movement, constantly produce heat which is dissipated into the environment. We feel comfortable when the rate of heat production is balanced by an equal rate of heat loss. The cooler our surroundings, the faster we lose heat – so we need to either increase heat generation (through movement and exercise) or slow down the rate of heat loss (by insulating ourselves from the cold).
Still air is an excellent insulator. Warm clothes work by trapping air between the fibres, and it is the trapped air which which holds in body heat to keep you warm.
Thicker clothes feel warmer because they trap more air.
Wearing multiple layers of thinner clothes is warmer than wearing one thick garment because more air can be trapped between the layers. Layers should slide over each other easily so you can move freely.
Layering also makes your clothing more versatile – you can add or remove layers to change the amount of insulation.
Size & Fit
If your clothes are too tight, the insulating air gets squeezed out and blood flow is restricted. Loose fitting clothes have more air space and so provide more insulation.
Any openings in a garment – at the neck, wrists, waist, or ankles – are places where the trapped air can escape. Cuffs and hems should have fasteners or elastic to seal in the insulating air.
Whereas still air is a good insulator, water is a good conductor of heat. Replacing some insulating air with heat-conducting moisture lets body heat escape. To stay warm, stay dry.
Clothes are most often wetted from the inside, by sweat. All of the layers worn while active shouldn’t trap sweat, but should allow the sweat to move through them, evaporate, and be dissipated into the atmosphere.
A water-repellent or waterproof outer layer resists the penetration of water from outside.
Still air is an insulator, but moving air carries off heat. The outer layer should seal in the insulating air and prevent its displacement by the wind.
Sun rays carry heat, but also damaging ultraviolet (UV) light.
Sunburn from too much UV is painful and increases the risk for skin cancer. Protect all exposed skin with either sun-protective fabrics or frequent sunscreen application. Some clothes have a UPF rating that tells you how much sun protection they provide; in the absense of such a rating, choose tightly woven, dark-colored fabrics.
Surfaces such as snow, sand or water reflect sunlight and so increase exposure. The eyes are particularly sensitive to diffuse light, so eye protection is important on bright, cloudy days as well as on clear, sunny days.
What to Wear
Choosing garments requires first identifying the roles that the clothes must fulfil. While insulation is the most obvious role for cold-weather clothes, these principles illustrate the importance of supporting garments to limit the effects of perspiration, precipitation and wind.
Read about layering systems and what to buy.
For more detailed information about outdoor clothes, refer to the book Keeping Dry & Staying Warm (Part 1). Published in 2020, it contains up-to-date information about the principles of garment construction, insulation, and layering.
To learn about medical problems that can happen in the outdoors – and to see gruesome pictures of frostbite – the standard work is Paul Auerbach’s Wilderness Medicine. It also contains some discussion about selecting appropriate clothing.
A concise explanation of the body’s mechanisms to maintain heat balance can be found in the chapter
Body Temperature, Temperature Regulation, and Fever in Guyton & Hall’s Textbook of Medical Physiology.