The Wind Chill Factor
More Articles Related to WinterMany Survival Topics readers are no doubt familiar with what is known as “Wind chill” or the “Wind chill Factor”. But what you may not know is how wind chill is calculated and how it relates to the actual temperaturer as shown on a thermometer.
Related to wind chill is the time to frostbite of exposed skin in cold climates; very important information for wilderness survivors especially when temperatures dip below freezing.
The combined effects of temperature and wind speed can become dangerous very quickly as wind speed increases. Well rounded survivalists should understand wind chill and frostbite times in winter survival conditions.
There is a great deal of misinformation on the internet about the new wind chill charts and associated frostbite times of exposed flesh in very cold conditions. This is not surprising because the information as presented by NOAA (The National Oceanic and Atmospheric Administration National Weather Service) is somewhat confusing and layered in technical jargon.
What is Wind Chill?
The wind chill factor is a measure of how the combination of wind speed and temperature remove warmth from exposed skin, namely the face. As the speed of wind increases, air striking your body draws away heat at higher rates through convective heat transfer (see the Survival Topic How Body Heat is Lost) causing it to feel much colder than the temperature the thermometer displays.
The wind chill temperature is an equivalent air temperature equal to the temperature needed to produce the same cooling effect under calm conditions. Higher wind speeds cause you to cool down more quickly just as lower temperatures do.
For example when it is 20 F and there is no wind it will feel like 20 degrees F; no surprise there. However add a wind of just 10mph and the temperature will feel more like 9F as the moving air robs heat from your body at an increased rate.
Wind can be caused by natural movement of air as well as your speed of travel by vehicle or on foot. If you are walking against the wind, your speed of travel will be added to the natural speed of the wind thereby increasing the wind chill factor you are experiencing. You will feel warmer walking with the wind than when you are walking against it.
Higher wind chills will reduce your body temperature more quickly, potentially leading to hypothermia. This is why in cold environments the outer layer of your layered clothing system should be a wind resistant shell.
The New Wind Chill Chart
The following formula is now used to calculate wind chill:
Wind Chill = 35.74 + 0.6215T - 35.75V0.16 + .4275TV0.16
where T=temperature (°F) and V=wind velocity (mph).The following formula is now used to calculate wind chill:
or
Wind Chill = 13.12 + 0.6215T – 11.370.16 + .3965TV0.16
where T=temperature (°C) and V=wind velocity (kilometers per hour).
Factors that can affect wind chill at given temperatures include radiational heat such as that from sunlight, which can make it feel as much as 18 degrees warmer. You can often feel this effect immediately upon facing the sun on a clear day or being in areas of high sunlight reflection such as snowfields and glaciers.
The wind chill factor does not take into account sunlight and so assumes the temperature and wind you are experiencing are happening in the worst case scenario of being in total darkness.
Several cautions apply to the use of the wind chill model and tables. The exact effect of cold exposure due to wind chill on an individual will vary depending on the type and level of activity, length of exposure, moderating effects of clothing, partial shelter from the wind, solar radiation, and overall physical state of the individual. Obviously if you are well fed and healthy you will much better able to endure cold temperatures.
Time to Frostbite
Supplemental to the wind chill factor are the times to frostbite of exposed flesh. It has been found that 95% of people will experience frostbite when the temperature of their skin reaches 18F (-7.8C).
Again, frostbite times are related to a combination of temperature and wind speed. For example, with a temperature of 0F the exposed skin of the face will freeze in about 30 minutes when the wind is blowing 10mph. Increase the wind to 15mph and that time is reduced to just 10 minutes.
Wind Chill and Time to Frostbite
Many Survival Topics readers may observe that according to the new wind chill chart there seems to be an anomaly:
- At a temperature of -30F and wind speed of 5mph, the wind chill chart shows a wind chill value of -46F and exposed flesh freezes in 30-minutes.
- At a temperature of -15F and wind speed of 30mph, the wind chill chart also shows a wind chill of -46F but it also shows that exposed flesh freezes in 10-minutes (freezes much faster at the same wind chill temperature).
How can this be? Why does an equal wind chill temperature of -46F result in different times for frostbite to develop on exposed skin? I posed this question to Paul Stokols of the winter weather program of the NOAA National Weather Service. Paul’s excellent response included this tidbit concerning wind chill and frostbite times:
“stronger winds have a greater impact on frostbite times than temperature (or wind chill) alone due to increased evaporative cooling of and removal of heat from the skin surface and sub-layers”.
Paul also provided Survival Topics with this frostbite time formula:
Ft = ((-24.5 x ((0.667 x (V10 x 8/5)) + 4.8)) + 2111) x (-4.8 - ((Tair - 32) x 5/9)) -1.668
where Ft is the frostbite time in minutes, Tair and V are the air temperature in units of (°F) and wind speed in Miles per hour > 15 measured at a height of 33 feet
or
Ft = ((-24.5 x ((0.667 x V10) + 4.8)) + 2111) x (-4.8 - Tair)-1.668
where Ft is the frostbite time in minutes, Tair and V are the air temperature in units of degrees Celcius and wind speed in Kiliometers per hour > 25 measured at a height of 10 meters.
Wind Chill and Frostbite Survival
So there you have it. Wind chill has the effect of lowering the temperature you must plan for in order to survive in cold climates. When Time to Fostbite gets down into the range where exposed flesh freezes in minutes, you need to either make sure you have the proper winter clothing, make a shelter to protect yourself, build a fire, or avoid going outside entirely.
If you enjoy charts and graphs be sure to visit the Charts, Graphs, and Diagrams website for additional initeresting applications.
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Shelter is obviously the best method to get away from exposure. Also dressing up in layers according to your environment. I have an idea using saplings a little para cord and your folded tarp. Create like and tower sheild or umbrella that you hold in front of while you walk deflecting the cold air around you instead of on you. Make it quite tall and wide. This size idea is so you have a one minute leantoo when you want to sit and rest or dig your snow hut and you have your roof. Just being creative and prepared is it practical.