Know When Frostbite and Hypothermia Might Occur
What is Windchill? Basically, it combines the cooling effect of temperature and wind, driving down the “perceived” temperature. Interestingly, the wind does not change the temperature of the air; it’s changing the temperature of your body. See the Windchill Chart, how windchill is calculated, and how it all really works.
What is Windchill?
The Windchill Temperature Index is a “measure of the combined cooling effect of wind and temperature.”
Let’s explain: Ever noticed that you “feel” colder in the winter if the wind is blowing? You’re not imagining this! How cold it “feels” is not just about the temperature. It’s also about wind speed. As the wind speed increases, the body is cooled at a faster rate causing the skin temperature to drop.
Why? Our body keeps a “buffer” or think layer of air next to our skin to help us regulate our body temperature (maintaining 98.6°F). Think of this buffer like an insulating jacket! A strong wind can disrupt this buffer layer, making us feel colder.
When the wind picks up speed, it draws more heat away, so if your skin is exposed to the wind, your body will cool more quickly than it would have on a still day.
If you combine freezing temperatures with a frigid wind, the danger of frostbite and hypothermia increases. In northern climates, it’s not uncommon to hear wind chill warnings where exposed flesh can freeze in less than a minute.
Windchill Temperature Chart
The windchill chart isn’t technically measuring “how cold it feels” even though your body will certainly notice it’s colder. It’s really reflecting the rate of heat loss on exposed skin.
Wind chill IS a good indicator of how long it will take for hypothermia or frost bite to occur.
The Windchill Temperature (WCT) chart below was created in 2001/2002 by the Environment Canada (EC) and the U.S. National Weather Service (NWS). It replaced the prior 1945 Siple and Passel Index, created during a United States Antarctic Expedition in the 1939 to 1941.
The Windchill Temperature (WCT ) index gives the perceived temperature equivalent for the combination of cold air and wind. It shows air temperature in degrees Fahrenheit and wind speed in miles per hour. Here’s how to convert from Fahrenheit to Celsius.
The chart also includes a frostbite indicator, showing the points where temperature, wind speed and exposure time will produce frostbite on humans. Each of the three shaded area shows how long a person can be exposed before frostbite develops. For example, a temperature of 0°F and a wind speed of 15 mph will produce a wind chill temperature of -19°F. Under these conditions, exposed skin can freeze in 30 minutes.
|Frostbite occurs in:||30 minutes||10 minutes||5 minutes|
Example: when the temperature is 15°F and the wind speed is 30 miles per hour, the windchill, or how cold it feels, is -5°F.
How to Calculate Windchill
The windchill chart above was designed to accurately calculate how cold air feels on human skin. The index is based on heat loss from exposed skin was tested on human subjects’ faces.
For those interested, Windchill is a basic Algebraic formula:
The Fahrenheit version of the equation looks like this:
Wind Chill = 35.74 + 0.6215T – 35.75(V^0.16) + 0.4275T(V^0.16)
T is the air temperature in degrees Fahrenheit, and V is the wind speed in miles per hour.
The above formula makes some assumptions: that your exposed face is roughly five feet off the ground, it’s night, and you’re walking directly into the wind in an open field at 3 mph.
While that formula may not reflect your reality and you could date the way it’s calculated, the basic concept is important: Cold winds will cause exposed skin to cool more quickly and frostbite will happen more quickly. Pay attention when there are high winter winds!
Dangers of Windchill
- Frostbite: Windchill actually causes your body tisue to freeze! Frostbite is body tissue that has frozen and usually starts with the fingers, toes, tips of the nose, and ear lobes. If you lose feeling in these area or they are turning pale or white, immediately get inside and get medical attention.
- Hypothermia: When your body’s temperature drops too low, hypothermia sets in. Uncontrollable shivering, disorientation, and incoherence are signs of this issue and medical attention should be found immediately.
In both cases, take care to rewarm the body very slowly.
Windchill Safety Tips
Be smart about windchill! Here are some common-sense safety tips:
- Listening to the weather station. Windchill Warnings are issued when wind chill temperatures are life threatening. Windchill Advisories are issued when windchill temperatures are potentially hazardous.
- When there is low windchill, cover your exposed flesh, especially your face and hands! Consider a balaclava to cover your mouth and protect your lungs. Mittens are better than gloves.
- Wear layers of loose-fitting, warm clothing as the layers will trap air and provide insulation. Your outer shell should be water-repellent and hooded.
- Always wear a hat.
Check out the Beaufort Wind Scale to learn how to judge the speed of the wind!