The Big Chill
What’s the Deal with the Wind Chill Factor?
A term is often bandied about when the weather is frigid, such as we will experience this week. The Wind Chill Factor is frequently misunderstood and often associated with sub-zero temperatures only.
We’ll begin our examination with a short refresher course on the history of the Wind Chill Factor: In the years leading up to the Second World War, Two polar scientists, Paul Allman Siple, and Charles F. Passel, developed the mathematical equation we now call the Wind Chill Factor, or the Wind Chill Index.
The experimental method used to derive the equation was quaint by standard laboratory protocols. Siple and Passel hung a bottle of water above their hut, accompanied by an anemometer, an instrument used to record wind speed. Their resultant wind chill chart is based upon measurements made on the effect of wind speed on the rate at which heat is lost to the outside environment until it reaches the ambient temperature.
Wind chill does not mean the wind can bring temperatures below ambient temperatures. To illustrate, the thermometer on my back porch today reads 34 degrees F, and I noticed that my dog’s water bowl is still liquid, as expected.
Let’s say I put room-temperature water in the bowl; if the wind were to pick up, there would be a wind chill effect, bringing the water temperature down to 34 degrees much faster than on a still day, but it would not cause the water to drop below 34 degrees.
This is important to humans because we will lose body heat much more readily when exposed to moving air. This is called convective heat loss, and it makes us more susceptible to hypothermia and frostbite when we are exposed to wind or create air movement by activities such as cycling and running.
Take, for example, the Great Greenbrier River Race of 2015, in which several race participants were treat-ed for acute hypothermia on a day in late April that had a high of 41 degrees Fahrenheit.
The race has three legs: running, paddling and cycling, all of which require a lot of exertion. It started raining at some point in the race, adding to the moisture the racer’s clothing absorbed from perspiration and paddle splash.
Additionally, the cool air forced past the body while running, cycling, and even paddling will carry away body heat, increasing the odds of becoming hypothermic.
In a sense, the Wind Chill Factor is an effective temperature, meaning that the ambient temperature remains the same, but when you add the convective heat loss caused by air moving over your body, you lose heat faster as if it were much colder.
Consider the following fictional story about a couple who decide to go winter camping. We’ll call our cold-weather campers Susie and David.
Arriving at their campsite in the late afternoon, they erected their tent and got organized. As a gadget gal, Susie brought an anemometer to measure the wind speed and an outdoor thermometer to obtain the ambient temperature, which is the current air temperature.
Susie secured the anemometer atop the campsite post, ensuring nothing blocked the wind. She then hung the thermometer from a tree limb out of the sunlight, which could heat the liquid in the thermometer, affecting the displayed temperature. Then she pulled out the final piece of equipment from her pack, an electronic wind chill calculator.
After allowing the thermometer to obtain an accurate reading, she wrote it down—the current ambient temperature at their campsite was 35 degrees Fahrenheit.
Susie then took a reading on the anemometer: 15 miles per hour. According to the wind chill calculator, the wind chill factor, or “effectual” temperature, was 25 degrees Fahrenheit. She entered this in her camping journal, and she and David proceeded to make dinner and unroll their sleeping bags in preparation for the long winter night ahead.
Before falling asleep, David suddenly sat up and announced that he had left their water bottles on the picnic table. If Susie’s readings were accurate and the wind chill factor was 25 degrees Fahrenheit, the water would surely freeze. Susie assured David that her readings were accurate but that unless the ambient temperature dropped and the weather forecast did not call for this, their water would not freeze.
Susie knew that the wind chill factor is based upon the heat loss experienced by living organisms by air movement, sometimes referred to as convective heat loss. The wind chill factor does not affect inanimate things in the same way, such as water.
The ambient temperature, i.e., the thermometer reading, will not be affected by the wind; it will remain 35 degrees until the ambient temperature drops or rises. So, the wind chill factor will only affect their water in that it will bring the temperature of their water down to 35 degrees faster than if there was no wind at all, but it will NOT cause the water temperature to drop below 35 degrees.
On the following day, David and Susie decide to hike a nearby trail. The wind velocity was 20 mph, and the ambient temperature was 25 degrees, so they would lose body heat as if the temperature was 11 degrees.
In preparation for their hike, David and Susie dress as though the ambient temperature is 11 degrees rather than 25 degrees. They put on another layer of clothing, including a windbreaker, and wear their warmest mittens and head coverings.
The Wind Chill Factor is important because unusually low temperatures like we are currently experiencing, a person can suffer frostbite on exposed skin within minutes if there is wind. Most outdoor enthusiasts know how to dress for extremely low temperatures; it is in temperatures that are in the moderate range that even experienced people can get into trouble.
Wet clothing increases heat loss through conduction—heat is transferred from the skin surface to the outside via the wet clothing, resulting in a net heat loss. Even without considering wind speed, the forward movement of running, paddling, and cycling will cause heat loss at an even faster rate as cool air moves past your body. This is enough to cause a core temperature drop, which may lead to a hypothermic state. And when you throw in the fatigue factor, you greatly increase your odds of be- coming hypothermic.
In closing, you may be interested to know that the lowest wind chill index recorded in the United States was minus 108 degrees Fahrenheit on Mt. Washington, New Hampshire, in February 2023. The ambient temperature on the summit was 46 degrees F, and the wind speed was 97 mph. Now that’s cold!
Being aware and informed can prevent hypothermia and frostbite in cold weather. I wish you all a happy and safe 2025.
Ken Springer
ken1949bongo@gmail.com