A normal, naked human body functions best when the air temperature is 27°C. One way of defining the word “cold”, then, is any temperature below 27°C. Below this temperature we need to be proactive, whether by adding clothes for protection, going inside or moving around to raise our blood circulation. Wind and rain dilute our perception of the cold.
There is a saying in Greenland that “if you sweat, you die”. This might be a little drastic, but it illustrates a point - when moisture enters the picture, frostbite often becomes an issue, even at temperatures above 0°C. Water conducts heat 25-30 times better than air and rapidly transports energy away from the body. (Air, on the other hand, is a poor heat conductor and provides insulation – a characteristic that sweaters, down jackets, sleeping bags, etc., make the most of.) This is why sweat, snow and water are actually a threat to your well-being, particularly if you are on a long trek and are unable to dry your clothes and equipment at the end of the day.
Wind and wind chill
When the wind is not blowing and you are standing relatively still, the air around your body warms up and acts as a layer of insulation. If the wind starts to blow, this warm air is pushed away by new, cold air. Your body reacts by once again warming up the air around you. If this continues for a long period of time, your body temperature will drop.
The more the wind blows, the cooler you become. At a temperature of –15°C and winds of 8 metres/second, the wind chill factor on your bare skin is the same as wind-still conditions at –34°C. This relationship between the wind, temperature and effective temperature on your bare skin is illustrated by the wind chill index below.
The wind chill index shows the effective temperature on bare skin at different wind speeds.