This recent cold spell has once again shown that people (especially the media!) are often confusing windchill with an actual temperature. This appeared in an article today in the Toronto Sun - "The Edmonton International Airport recorded a low of -46.1C, with the mercury dropping to -58.4C with the windchill, outfreezing even the Arctic." Well, that's not exactly true. The mercury did indeed drop to -46.1C at Edmonton airport, but it never dropped to -58.4C. That was the windchill factor which isn't a temperature at all, but rather a value that estimates the cooling rate of the air given a certain temperature and wind speed. With no wind, your body will lose heat at a certain rate depending on the air temperature. If there's a wind, your body will lose heat at a faster rate because the wind erodes the warm insulating layer near your skin that keeps in heat. The faster the wind, the faster the heat loss, and the "colder" it feels since your body feels like it's losing heat at the same rate as a colder temperature.
For example, let's say it takes 20 minutes for you to feel cold when it's -25C outside and no wind. And let's say it takes you 10 minutes to feel cold when it's -40C with no wind. Now, let's say it's -25C with a 30 km/h wind. Because of the wind, your body starts losing heat more quickly, and you start feeling cold after only 10 minutes. Your body will tell you, hey it must be -40C out here because it took 10 minutes for you to start feeling cold. But is it -40C outside? No, it's -25C. To you, it just feels like -40C because of the wind that accelerated your body's heat loss. People will say it "feels like" -40 with the windchill.. which is a way of saying that the wind makes it feel colder than it actually is.
Because of this, windchill should never be stated as a temperature value (like degrees C).. it really should be expressed as a cooling rate (e.g. -58.4CU "cooling units") but in Canada, it's officially displayed without any units. Media has become overly fascinated with the windchill (I guess because it sounds so much worse than the actual temperature!) to the point that often they're just giving windchill values instead of the actual air temperature! The problem with this is that people start quoting windchill values as actual temperatures (as in the Sun article), which they are not.
Temperature is a measure of how much heat energy a parcel of air holds. The more heat energy, the higher the temperature. Windchill on the other hand is an index that estimates the RATE of energy loss of an air parcel given its original heat energy AND a prevailing wind that helps to accelerate the amount of energy leaving the air parcel (i.e. cooling rate). So, temperature is the amount of heat energy an air parcel has.. windchill is the rate of cooling of that air parcel.
Look at it this way... You have a bucket of water at +5C. You put it outside, where the air temperature is a constant +2C, but a wind of 50 km/h is producing a windchill of -5. Will that bucket of water freeze? Of course not. The water will only cool down to the ambient air temperature.. +2C. It can't get any colder than its environment unless it's cooled by some other method (e.g. refrigeration) The windchill of -5 merely implies that the water will cool down to +2C at a rate equivalent to that if the water was outside at -5C with no wind. So instead of taking one hour to cool down to +2C, it will only take 10 minutes thanks to that 50 km/h wind. But the water temperature will never go below +2c, no matter how extreme the wind or windchill is. That's the effect of windchill. It describes an equivalent RATE OF COOLING, not an actual temperature. So if it's -20C with a -40 windchill, your car will likely still start if it's not plugged in because the battery will never get below -20C. On the other hand, if it's -40C with no windchill, your car will not start if it's not plugged in because the battery will cool off to -40C (given it's outside long enough). See the difference?
Before 2001, the Prairies used a windchill index that was given in watts/sq metre (e.g 1800 W/m2) This was a more scientifically valid measure of what windchill actually was.. a rate of cooling, expressed in the amount of energy loss in watts per square metre of area. However in 2001, the windchill was re-calculated and standardized for all of North America to the windchill index we use today, which was mainly based on the more common "equivalent temperature" index being used in Southern Ontario and the United States. A survey conducted at that time showed that most people found the watts/sq metre unit "too technical" a term to fully understand.