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The New Windchill Index
Thermal comfort and cold hazards are prime topics for discussion throughout the North American winter, particularly for weathercasters on the local news. But beginning in 2001, cold, blustery days might psychologically seem a little warmer than in past winters, for a new windchill formulation has been introduced in North America. Its values do not drop as quickly with increasing wind speed as the old index, and it is derived for conditions more relevant to modern life.
There are several sections to this piece. Readers may wish to jump to specific sections rather than read all this piece at once. Click on any of the titles below to jump to the appropriate subsection or related article.
Before looking at the details of the new windchill index, let me take a moment to set some background to the topic.
In summer, a commonly held cliche states: "It's not the heat, it's the humidity." The winter has its counterpart: "It's not just the cold temperature, it's the wind." For the last four decades, the media, particularly the broadcast media, have bombarded us with reports of current or forecast windchill conditions, often at the expense of the true air temperature reports. Critics have charged that competing stations vie to report or forecast the coldest possible, local windchill temperature in order to drawn an audience at the expense of accuracy. Not having cold local conditions to report, those media in the warm tourist states such as Florida, Texas, Arizona and southern California, likely have done their best to remind visitors how lucky they were not to be back in their frigid hometowns.
The windchill index or windchill equivalent temperature issued over the past four decades enters the winter 2001-2002 with its first major make-over since Paul Siple derived the original equations in 1939. The new windchill index emerges from several decades of research into our understanding of how we "feel" when exposed to hot or cold environmental conditions.
I'll briefly touch on some of the background in thermal comfort, but for additional technical details on cold comfort and the old windchill index, see the Weather Doctor article: "Cold Impacts on Human Health and Comfort."
As warm-blooded animals, or poikilotherms, human physiology employs various body processes to maintain the central core temperature near 37oC (98.6oF). With a skin temperature around 33.9oC (93oF), the core-skin temperature differential is "optimal," and we generally feel neither hot nor cold -- a state we term "comfortable." (Please note our discussion here is general in nature and assumes a healthy human body. Individuals may vary widely in their responses to their thermal environment due to a large number of body factors.)
When heat cannot be lost quickly enough to maintain the body's ideal temperature differential, internal heat builds up and we feel "hot." The body's thermal regulators soon respond to increase our heat loss through increased perspiration, urination and desire for water, and decreased metabolism.
On the other side of the thermal regulatory continuum, when internal heat is lost too rapidly, we begin to feel "cold," and the body again reacts. This time, shivering, "goose flesh," decreased perspiration and increased metabolism are some of the main responses.
How we feel under prevailing environmental thermal conditions, therefore, is more a response to the rate of body heat gain or loss rather than to the absolute temperature. Thus, we may feel cool in the summer under temperatures considered balmy during winter.
Unlike other animals, humans have the short-term option of aiding the body in heat regulation by adding or removing clothing to maintain an overall comfort level. Thus, we rarely venture out into the cold outdoors naked. If we did, we would lose heat rapidly, and under extreme conditions could face death in a few hours of exposure.
How fast we lose heat depends primarily on the air temperature and the wind speed. (Exposure to full sun can counter some of the heat loss to cold temperatures and windy conditions, and being wet can increase heat loss through evaporation.) Under calm or near calm conditions, we lose heat faster as environmental temperatures become colder. But when the wind blows faster than a threshold speed, we begin to lose heat at a rate, much more rapid than the loss due to temperature differential alone.
The Early Experiments
In the early years of cold hazard studies, frostbite and its onset were felt to be the limiting factors in cold exposure. These concerns arose from the experiences of polar explorers during the decades spanning 1900 to reach the poles.
Intuitively, we know that we feel colder when exposed to a strong wind, so the first windchill factor simply combined wind speed and temperature by multiplying the number of Celsius degrees below freezing by the wind speed in metres per second. Then in the late 1930s, Paul Siple undertook the task of determining how quickly extreme conditions could produce frostbite on exposed skin.
Siple conducted his research in the Antarctic as part of the third Byrd expedition. While on the frozen continent, Siple measured the heat loss rates and freezing times of water-filled, plastic cylinders under a variety of air temperature and wind speed combinations. He believed that this experimental set-up best mimicked the heat loss from exposed human flesh.
From his data, he produced an empirical formula for the rate of heat loss as a complex function of temperature and wind speed. The experiments were only run at subfreezing temperatures, so strictly speaking, Siple's windchill formulation should not have been applied to conditions where temperatures are above freezing. As wind plays an important role in cooling the body (this is why we use fans) under all but the hottest conditions, Siple's equation has also been applied for temperatures above freezing -- a logical extension, even if inaccurate.
Since Siple's original research of 1940 (whose findings were finally published in the open literature in 1945), the form of the equation has not changed although its coefficients have been recalculated to provide better fits to the data and to accommodate other units of measure. Siple's original research focused on heat loss rates and time required to cool the water to freezing. However, it was soon realized that the equation had other uses.
By equating the heat loss rate for a given set of wind and temperature conditions, it is simple mathematics to calculate the equivalent temperature for lighter, near-calm, winds. Thus, a windchill temperature, which has been touted to be the temperature that a set of wind and temperatures "feels" like, became the prime use for the equation. In the 1960s, Siple's equation for windchill temperature became a regular part of winter weather reports and has remained so until this year (2001).
Note that while in the past, the windchill effect has been reported as a temperature, doing so has led to much confusion about the nature of cooling and wind. Many believed that reporting windchill as a heat loss index in terms of energy lost per unit area of the body was a better means of reporting windchill than the "feels-like" temperature equivalency. In part, this stemmed from the fact that many high latitude residents did not feel as cold as the windchill temperature suggested they should.
Thus, Canadian meteorological services provided a windchill index as a heat loss rate in watts per square metre for residents of the Prairie Provinces and Arctic regions rather than an equivalent temperature for many years. Warnings attached to various values indicated the hazards of windchill to the population. For example, above 2400 watts per square metre, the windchill, which might feel like minus 60 C, made outdoor conditions dangerous even for short periods of time, and the ceasing of all non-emergency outdoor work was advised.
For many years, researchers have felt that, based on scientific arguments, the windchill concept needed to be altered. They argued that a beaker of water is hardly a good model for heat loss from the heat-generating human body. Thus, intensive research over the past decade or two led to a new model and windchill formula based on modern heat-transfer theory instead of Siple's experiments. The current model is based on heat loss from an exposed human face, chosen because it is the part of the body most often exposed directly to severe winter weather. The model assumes the rest of the body to be clothed appropriately for the weather conditions.
The task of developing a new windchill index was lead by researchers from Indiana's Purdue University and Canada's Defence Civil Institute of Environmental Medicine in Toronto. The new formula was tested on human subjects in a chilled wind tunnel at Canada's Defence Civil Institute of Environmental Medicine in Toronto during the Summer of 2000. In the wind tunnel, the faces of six men and six women were exposed to various combinations of temperatures and wind speed, and the rate of temperature drop of the exposed skin was measured and their assessment of the "feel" of the cold recorded.
The resulting equation for an equivalent-temperature windchill index was accepted by Environment Canada and the US National Weather Service in 2001. It is also important to note that the windchill is not a temperature in the strict sense, but a temperature-like number that quantifies the sensation of cold. Thus, in Canada, it will be reported without the degree designation. (I will use it here, however, to distinguish between the metric and imperial windchill numbers.) The former practice of issuing a windchill factor expressed in watts per square metre used in parts of Canada will be dropped with the introduction of the new index.
Specifically, the new Windchill Temperature Index:
The metric formula for windchill is:
Twc = 13.112 + 0.6215 Ta -11.37 V0.16 + 0.3965 Ta V0.16
where Twc is the windchill, V is in the wind speed in kilometres per hour, and Ta is the ambient air temperature in degrees Celsius.
The equivalent formula for Fahrenheit temperatures and wind speed in mph is:
Twc = 35.74 + 0.6215 Ta -35.75 V0.16 + 0.4275 Ta V0.16
The new windchill values won't sound as scary as the old ones since the drop in windchill temperature at higher wind speeds is not as great under the new equations. For example, at minus 15oC (5oF) with a 50-km/h (30-mph) wind, the old formula would produce a wind chill of minus 40 (C or F), but the new formula reports the chill at minus 28 C (minus 19 F).
Under the new windchill formulation, windchills of minus 28 C (minus 19 F) and colder can cause frostbite on exposed skin in 15 minutes or less.
On October 1, 2001, Canada officially began reporting windchill using the new formulation. The Americans followed suit on November 1, using the Imperial unit version of the same formula.
For official Windchill information from the Canadian and American weather services, go to their sites listed below:
United States: New Wind Chill Temperature Index
Research on windchill is continuing to expand the concept for use under other potential frostbite conditions, such as including the effect of solar radiation into the windchill index and windchill under wet conditions. A "wet windchill index" could be useful, for example to mariners exposed to freezing spray conditions.
Before I finish, I want to address some misconceptions about windchill and some of the drawbacks, particularly to the application of the old Siple equation.
First some problems with the old Siple formulation.
The old windchill temperatures were usually calculated from wind speeds reported at official observing stations such as the local airport. The wind at these locations is properly measured at a height of 10m (33 feet). But most of us inhabit the lower 2m (6-plus feet) of the atmosphere where winds are usually much lower in speed than those at 10m. (Typically winds measured at a typical exposure height of 1.5m (5 feet) are about one third lower than the wind speed at 10m.) Thus, windchills derived from 10-m winds are highly overestimated. In the new formulation, the wind speed appropriate for 1.5m above the ground is used in the calculation rather than the 10m speed. But since wind will still be measured at 10m, a simple correction factor is included in the windchill formulation to convert it for the lower height.
The Siple calculations also assumed a stationary body. Movement at a given speed would cause a higher relative "wind speed" for calculating the cooling rate. For example, walking at 4.8 km/h (3 mph) into a 10 km/h (6 mph) wind at exposure level would be the equivalent to a measured wind speed of about 15 km/h (9 mph). This becomes particularly important for skiers, snowmobilers and others moving rapidly through cold air. The new index assumes a moving body at a walking pace of 4.8 km/h (3 mph).
For those involved in faster moving activities such as snowmobiling or skiing, consideration should be given to adjusting the exposure windchill to the higher speed of motion into the wind. Remember too that other factors may alter the local exposure wind speed, such as topography, building effects causing wind channelling, or wind breaks, and thus can influence the effective windchill exposure of an individual.
There is always a degree of misstatement when the announcer reports: "The temperature is zero but it feels like Minus 20!" What it feels like is quite a matter of personal responses and even that can depend on our circumstances at a particular moment. For example, we may feel colder under a given condition when anxious than when calm. The new windchill determination formula is more consistent in providing general physical sensations than the old Siple equation.
While the new windchill index corrected many of these problems, several major misconceptions about the nature of reported windchill values will likely remain for some time.
One longstanding misconception is that the windchill temperature applies to "calm" conditions. In the old Siple equations, "calm" was a light wind condition of 8 km/h (5 mph). The new formula applies to a minimum wind speed of 4.8 km/h (3 mph).
Another misconception is that the windchill temperature value applies to all life forms and inanimate objects. Strictly speaking, it applies only to humans. Most mammals and birds, for example, have more protective natural coats than humans and thus lose heat much slower than our relative nakedness.
The heat loss concept underlying the windchill can, of course, be applied to any warm object, living or not, exposed to colder environmental temperatures, at least until it reaches temperature equilibrium with its surroundings. Thus, while the specific windchill temperatures may not apply directly, heat will be lost at a faster rate by animals and objects such as buildings when the wind is higher. Thus, the impact of windchill on objects or liquid surfaces can reduce the time required for their temperatures to reach equilibrium with the ambient temperature.
For example, high winds can cool a house or building faster than a light wind by forcing cold air through cracks and crevices and by altering the effectiveness of the building insulation layer. Increasing heat loss rates thus require higher energy usage to keep interior temperatures at a stable level.
Saving the best for last, in my mind, the most important misconception commonly held about windchill is that when an air temperature above freezing and a strong wind combine into a subfreezing equivalent windchill temperature, water or other liquids will freeze.
This is utterly false. The wind speed, however strong, does not change the actual air temperature. If we placed two identical thermometers side-by-side, one exposed to the wind and the other sheltered, they will read the same unless other factors such as radiation or moisture on the sensor are affecting one of them.
Thus, no matter what the equivalent windchill temperature may be, the temperature of a living body (human or animal) or an inanimate object (e.g., a car radiator) exposed to the air will never drop below the actual air temperature unless wet (when evaporative cooling may lower the surface temperature). Therefore, exposed water or other liquids will not freeze unless the air temperature is below their freezing point.
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