Clothes don’t provide heat. The purpose of wearing clothing is to reduce heat loss and retain and maintain your warmth. Think about how you dress: small details make a big difference when the weather is harsh and your energy level is dropping. Clothes should insulate and support the body’s temperature equilibrium. At the same time that our clothing should help to retain heat that the body produces, it should also transport excess moisture and heat away from the skin.
Breathability and permeability (a.k.a. wind resistance) aren’t the same thing. Breathability represents how much perspiration vapor can escape through a fabric from the inside out, whereas permeability measures how easily wind passes from the outside in. These are different properties, it’s possible for a garment to be very air permeable but not very breathable—air could pass right through, while sweat could still bead up inside, leaving you feeling damp and cooled. The opposite is also possible—for example, a WINDSTOPPER® jacket blocks wind completely (zero air permeability) but will still let perspiration vapor pass through at a decent rate.
Retain heat, but let moisture out. The purpose of clothing is to retain heat that the body produces. The body puts off heat all the time, for better or for worse. This is a natural process, and we lose heat in five ways.
Have you heard the phrase “cotton kills”? Cotton is an extremely absorbent fabric. It holds water – like rain or melted snow or sweat – very well for a long period. And the problem is when you work up a sweat, cotton traps it close to your body. Add a little wind and evaporative cooling will happen. It can chill you very quickly to the point you are uncomfortable or potentially hypothermic. Cotton loses its insulating qualities when it gets wet, whether from rain or sweat and it takes a long time to dry. Wool or synthetic materials are much better suited to cold weather conditions.
- Protect against heat loss through your head by wearing a hat, balaclava, etc. One saying goes, “If your feet are cold, put on a hat.” A balaclava helps protect your face and neck from cold and wind.
- Be sure to carry plenty of dry socks, but do not wear too many pairs of socks at one time. If the blood flow to your feet becomes constricted, your feet will get cold regardless of how many socks you have on. Tightening your boot laces too tight will constrict the blood flow as well. Similarly, make sure your gloves, especially liners, are not too tight on your hands. If they are too tight, they can constrict the blood flow and keep your hands from warming up.
- Gaiters will keep snow, rain, etc out of your boots and therefore help keep your feet drier and warmer. Gaiters also add another layer of material around your lower legs to help keep them warm.
- Attach “dummy cords”, or security cords to your mittens to prevent losing them in windy or snowy conditions. Carry extra gloves or liners to change into if your first pair gets wet. Gloves can be dried out overnight in your sleeping bag.
In 2001, the U.S. government started using a more precise way to measure wind chill by testing how quickly people’s skin froze. Twelve volunteers were placed in a chilled wind tunnel. Equipment was stuck to their faces to measure the heat flow from their cheeks, forehead, nose and chin while they walked three miles per hour on a treadmill. One of the things they learned was how quickly frostbite develops on exposed skin. The information collected from the volunteers helped scientists come up with the complicated formula involving wind speed and air temperature to compute wind chill. For example, if the temperature is zero degrees Fahrenheit and the wind is blowing at 15 miles per hour, the wind chill is calculated at 19 degrees below zero. At that wind chill temperature, exposed skin can freeze in 30 minutes.
Windchill temperature is only defined for temperatures at or below 50 degrees F and wind speeds above 3 mph. Bright sunshine may increase the wind chill temperature by 10 to 18 degrees F. The new Wind Chill Table index:
- Calculates wind speed at an average height of five feet (typical height of an adult human face) based on readings from the national standard height of 33 feet (typical height of an anemometer)
- Is based on a human face model
- Incorporates modern heat transfer theory (heat loss from the body to its surroundings, during cold and breezy/windy days)
- Lowers the calm wind threshold to 3 mph
- Uses a consistent standard for skin tissue resistance
- Assumes no impact from the sun (i.e.,clear night sky)
For those wishing a more precise calculation than that provided by the table the NOAA website provides a Wind Chill Calculator where you can enter your exact temperature and wind speed to determine the precise wind chill factor.