With the arrival of Autumn, the wet and windy weather is starting to take hold of Scotland once again.
In this post Sean from the Walk Fife team explains why windy days seem so much colder than ones with only a slight breeze although the air temperature is the same.
The effect that wind has on our perception of cold is called the wind chill factor. Wind chill is one of those things that allow us to express how cold conditions really are around us.
It is important to have some understanding of wind chill because, as winds increase, heat is carried away from the body at a faster rate, driving down both the skin temperature and eventually the internal body temperature. Wind chill can make a fairly moderate winter day equivalent to a much colder one, sometimes dangerously so.
The wind chill factor is a calculated temperature that represents the ‘feel’ of a wind on exposed human skin in terms of an equivalent temperature in still air. While wind speed in the most important parameter in the calculation of wind chill, humidity and pressure also has some impact, but are generally ignored.
The chilling effect of wind comes from two sources:
- The disturbance of the insulating boundary layer of warm air over the skin
- The evaporative cooling effect of moisture loss from the skin.
In practice, wind can also cause drafts within clothing and therefore heat loss, but this effect is not considered in the chill factor calculation because of its obvious variability.
How do we measure wind chill?
The wind chill factor is commonly presented as a table of temperature versus wind speed.
The impact of wind chill
Wind chill is not an exact measurement and is not the only parameter that contributes to comfort in windy conditions. The warming effect of direct sunlight can provide some relief. Also there are characteristics that effect particular individuals. For example someone who is thin or someone wearing damp clothing will lose more heat and feel colder than someone who is not.
Wind chill should be taken very seriously. Wind chill below -50°C to an exposed forehead can be very painful and cause unconsciousness in minutes. If a significant percentage of skin is exposed, core temperature will drop rapidly and induce server hypothermia, coma and death within 10 to 15 minutes.
When the body’s core temperature is at -35°C, deep shivering will occur and recovery in a warm environment can take hours. At -34°C blood flow to the brain is impaired causing difficulty in walking, talking and sleepiness. Below 30°C the body no longer attempts to regulate temperature and coma is induced. At -25°C the heart stops. In iced water this occurs in less than 30 minutes.
Understanding the wind chill effect could save your life
Learning the facts about wind chill is one of the most important things you can do in relation to outdoors activities. It can be a potential killer and even the most experienced of climbers and outdoors professionals have fallen foul of its effect.
The next time you are planning a trip out into the wilds make sure you consider wind speed when looking at the weather reports for the area that you are heading to and take enough kit to keep you warm.