How to keep warm

Clothing is designed to help the human body regulate its temperature. The human body needs to be kept within a narrow range of temperatures to prevent us from overheating or even developing hypothermia – both of which can be fatal. Hypothermia starts to occur when the body’s temperature drops below 35C (95F). This is why it’s particularly important to know how to build an effective outdoor clothing layering system to keep your body at the right temperature levels when you’re out in cold temperatures or wet weather. Think sensible, start with your baselayers and work through the levels to jackets.

Our optimal body temperature

Man is a tropical animal that has adapted over the years to be comfortable without clothing at 27C (80F). At this temperature the body works at its most efficient and maintains a steady internal temperature of 37C (98.6F). To allow it to do so, we have to provide our bodies with energy in the form of water and food to enable our hearts to pump, our muscles to work and our brains to function. Approximately 70% of the energy supplied is needed just to maintain a constant core temperature.

Maintaining body heat

Body heat is created by burning the food we eat and is mainly produced by our muscles. The more work our muscles do, the more body heat we create. At rest our bodies’ normal heat production is roughly 80-100 Watts, but during intense physical work this can increase to 1000 Watts. When we are cold the body needs external help to maintain its temperature so we add energy by eating and drinking more, by keeping active and by dressing appropriately.

How our body reacts to cold

When the body becomes cold it tries to create heat by shivering. This can increase body heat production by 4-5 times although it uses up a lot of energy to do so. The body also decreases blood flow to the hands and feet in order to prioritise heat to the heart, brain and other vital organs. This is why you get cold hands and feet even when the rest of your body feels warm. Putting on a pair of insulating gloves and wearing the right socks can protect your hands and feet from becoming painfully cold – or in the worst case, developing frostbite in extreme conditions.

How our body reacts to heat

When the body becomes too hot, it tries to cool down by perspiring. Sweating cools the skin as the moisture evaporates. This works well in a hot climate, but in a cold one, heavy sweating when wearing lots of clothes can backfire because the moisture will actually cool you down. Breathable materials such as Gore-Tex and Seetex are used in country jackets, trousers and various other country clothes to ensure this does not happen! The Harkila Vector Jacket is an excellent example of a jacket designed for layering.

The body will lose 0.5 – 1 litre of fluid through evaporation from the skin every day. During hard physical exercise, this kind of evaporation can reach several litres per hour.

Women tend to feel cold more often than men. In general, men have a greater muscle mass which gives better blood circulation and creates more body heat.

The effects of wind chill on the body

From a practical perspective, we mainly dress to keep warm, but the cold can come in two ways – via the ambient temperature and from the wind. When the two combine, the wind literally blowing away the warm air layer closest to your body, you can experience wind chill. Its effects can be disastrous, dramatically increasing the chilling effect of cold temperatures. The body reacts by reheating the now cold air closest to it. If this process is repeated too many times, the body can rapidly become dangerously chilled.

Put into context, at a temperature of -10C with a wind speed of 8 m/s (18mph) the wind chill on bare skin will equal a temperature of -27C with no wind! This effect is not to be underestimated!

The Solution: Layering up for cold weather

Clothes don’t provide heat, but they do allow your body to retain the warmth it produces. This is where a layering system for warmth comes in.

For more information on the various layers of clothing and how to keep warm, view our Protective Layers Guide.

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