Hypothermia is a killer when you are out in the cold. Even if the temperature in your thermometer does not show it to be freezing, the prevailing wind conditions can drop the perceived temperature quite a bit ... and this can lead to hypothermia, and that in turn can lead to death.
Wind chill is the perceived air temperature on exposed skin due to wind and is usually lower than the air temperature, since the air temperature is usually lower than the human body temperature. The Wind Chill Factor is what the skin perceives the temperature to be due to the effect of the wind.
The rate of heat loss through the skin depends on the wind speed above and around the skin - the faster the wind speed, the more readily the skin will cool. The body’s response is to maintain surface temperature in an acceptable range in an environment of faster heat loss. This results in the perception by the body of a lower temperature and leads to an actual greater heat loss increasing the risk of adverse effects such as frostbite and death.
If the ambient temperature is -15°C and the wind is blowing at 40 kmph, the Wind Chill Factor is -27° and you can be frostbitten in less than thirty minutes of exposure.
Even on a relatively cold day, if it is bright and sunny, people tend to disregard the Wind Chill Factor because there is no wind blowing. Encouraged by what they deem to be great riding weather, they take off on their motorcycles riding at 100 kmph. With air temperature of -5°C and effective wind speed of 100 kmph, the wind chill factor is -18°C, a difference of more than 13°.
Whether it is windy or you moving into the wind, the chill can affect you. Be aware of it. Do not allow yourself to succumb to hypothermia because you disregarded the Wind Chill Factor.
Take a look at the Wind Chill Chart below to understand how the perceived temperature can change as a factor of the air temperature and the wind speed.