Wind Chill
With all the recent cold weather, the term “Wind Chill” has been extensively used to describe just how cold it FEELS outside. But what does that mean? How is it computed? What is the basis behind this seemingly subjective measure of “temperature”? I hope to answer some of this along with a brief history of the term “wind chill”.
History
Prior to 1945 everyone knew that it felt colder under windy conditions but there was no objective measure to report to the public. It was in 1945 that Paul Siple and Charles Passel published a paper quantifying the impact of wind on apparent temperature. They used a pretty simple experiment to test come up with their numbers. A vessel of near freezing water was set in various temperatures and wind conditions and was timed to observe how long it took to freeze. Of course, when you think about it, there are several problems with this original wind-chill methodology including, but not limited to, that they were measuring energy loss, not temperature. So you would not get wind chill temperature, you would instead receive a number representing the amount of energy lost in an hour over 1 square meter.
It was not until 1998 that Robert Quayle and Robert Steadman released a paper titled “The Steadman Wind Chill: An Improvement over Present Scales”. What Steadman did was expand on the work done by Siple and Passel but took into consideration the natural heating of the human body by its own metabolism. Steadman had experience with how heat is lost and gained in the human body and used this experience to develop his wind chill.
“The Steadman model of heat balance of the human body, originally conceived as a tool for textiles and clothing science, explicitly and implicitly includes all the major heat sources and sinks when estimating heat loss or gain to ascertain the comfort level of a person.” (Steadman 1998)
Some may be asking, “who cares about an accurate wind chill number” but Steadman addressed this in his paper by highlighting that a person who heard that the wind chill was -46 degrees could someday experience ACTUAL temperatures of -46 degrees and think that he will be fine. When in fact, the actual wind chill he felt was in the -25 range. It might seem minor but this work was done with the public’s interest in mind.
This is a very short, and likely incomplete, history of the development of something we all take for granted. A LOT of work has been put into creating an “accurate” wind chill number.
Description of “Wind Chill”
Before we get into the nitty gritty of describing wind chill, we need to all be on the same page as to what temperature is. Temperature is more than just a value you see in your thermometer, it can be thought of as energy. It takes high energy to keep temperatures high and lesser energy to keep temperatures cold. Cold temperatures can be thought of as having less energy while hotter temperatures, conversely, have more energy. Thermometers then, can be thought of as scales that measure energy.
If temperature can be thought of as energy, then we have to start thinking of how the universe works. The universe does not like isolated areas of high or low energy. It’s rather socialist in how energy should be distributed. With that even distribution in mind, a massive peak in energy must be flattened. When you are outside on a cold day, you are that peak in temperature and the atmosphere wants to spread your warmth to the surrounding environment. It does this by transferring the energy from you to the surroundings. How energy is dissipated is the same whether calm or windy, but the rate at which it happens is different.
What happens on a Calm day?
On a calm day, your energy is taken at a rate of speed proportional to only temperature. A thermal gradient forms between you and the ambient air. When you first step into the cold air, it takes a burst of energy to create this thermal gradient so you feel cold as you lose energy. The colder the air…the more energy needed to warm the area (thus the more energy you lose) and the more extreme the initial gradient. But the slope in this gradient is lessened naturally as you warm the air directly around you, and this air warms the air around it and so on and on. This naturally created a bubble is constantly warmed by your metabolism. Possibly contained only within a few centimeters, that is all you would really need to start feeling warm. If you were to stand perfectly still under perfectly calm conditions your body could keep an equilibrium for quite a while (this is the basis behind a wet suit). Of course, you don’t have nearly enough energy to warm the entire atmosphere so you’ll keep losing energy to the atmosphere and eventually….You’ll freeze.
What does Wind Do?
That bubble that forms on calm days…the wind takes care of that pretty quickly. In windy situations, there is no ability to form this bubble, the faster the wind, the less of a bubble. So with a strong wind you constantly get “new” cold air coming in contact with your skin and stealing your precious energy. As this energy leaves, you need to recreate the energy so your body is working much harder (using up more energy) to stay warm. If it continues to lose energy, blood flow to the extremities slows which causes the fingers, hands, feet, etc to lose their source of energy and they will tend towards the environmental temperature even faster.
What does it all mean?!?
Well really what it should mean to you is that wind chill is much less a temperature measurement and more a TIME “measurement”. The “colder” the wind chill the less time it takes for your body to lose energy and for you to freeze. It needs to be noted here that the wind can NOT bring any surface below ambient temperature. What this means is that if the ambient temperature is 33, a strong wind will not make water freeze, it will simply make the water get to equilibrium with the environment faster.
So next time someone says it is 15 degrees out but feels like -50, simply understand that it means you will die a heck of a lot faster on that day than on a day where there is no wind.
Notice the colored times in legend. This is what is important…not the temperature
Problems with Wind Chill
Wind chill does not include everything that can impact your core temperature. Your body mass, fat content, amount of hair, sunny day, cloudy day, time of year, time of day, walking vs. running…. the list can go on and on. What it does do well is warn people of the impact of the wind. There are many other formulas used by some companies (real feel, skin temp, etc) that may take more into account, but at the heart of it all is to warn people about a danger, and whether it is exactly the “feel” temperature is not important. A ball park figure (within about 5-10 degrees) is more than adequate.
Papers
(Siple, P. A., and C. F. Passel, 1945: Measurements of dry atmospheric cooling in sub-freezing temperatures. Reports on scientific results of the United States Antarctic Service Expedition, 1939–1941. Proc. Amer. Philos. Soc.,89, 177–199)
The Steadman Wind Chill: An Improvement over Present Scales – http://journals.ametsoc.org/doi/full/10.1175/1520-0434%281998%29013%3C1187%3ATSWCAI%3E2.0.CO%3B2