Monday, December 14, 2009

Windchill is not a temperature!

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.

23 comments:

  1. WOW!!!!

    That's quite the science lesson!!!!

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  2. Amen

    Interesting that the WindchillC Calculator at Physlink.com attributed to (Source: Weather University of Waterloo) produces different results than the EC Windchill calculator.

    Using -44.4C and 10Km winds of Feb 18 1966 produces a -56CU at Environment Canada and -51.8CU using Physlink.com

    EC Calculator at http://www.msc.ec.gc.ca/education/windchill/WindChill_Calculator_e.cfm

    Physlink calculator on my name

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  3. Doing my little part to educate the masses out there.. :) This is mainly a repost from last winter.. seems like I have to go through this windchill thing every winter!

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  4. In Brandon my 4 yr old plugless car/battery had no problem starting this am. The old dog backed into the wind though. Prairie educated winter habit.

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  5. I'm a dinosaur when it comes to windchill. I liked the old watts/square meter approach. In those days the rationale was that no one had experienced -50 °C and didn't know what it felt like, so they expressed heat loss in watts/square meter. Then they came out with the temperature method.

    It got my goat when the University of Manitoba web cam posted a temperature of -29 °C today. Those geniuses post ONLY the windchill temperature, not the air temperature and they don't even state that. Very good publicity for them. I've bugged them many times, but they are truly hopeless.

    MLBYWS recorded a low temperature of 28.6 °C overnight. Encouraging how the temperature has been rising though. Gives one hope that we'll get back to something near normal for a change before the Christmas event.

    Interesting sight from the downtown CWB web cam a few days ago. The buildings' exhalations, which I assume is mainly water vapor, went up and settled into a layer very close to the ground, just like it had hit a glass ceiling. I guess that phenomenon is a temperature inversion event.

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  6. If w/m2 is the Jurassic era for windchill, the Triassic era WC charts, which ironically are still in use today are @ http://www.main.nc.us/graham/hiking/windchill.html??

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  7. Rob -- It is the same there here in the states....the wind chill gets all the billing...with the media reading the absolute lowest reading possible as the actual temperature. As you said though minus 50 seems so much worse than minus 25.

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  8. It seems like we just can't get any snow around here!!!

    This is the driest pattern I have ever seen!!!!

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  9. I for one am in favor of WindChill having the prime billing in a meaningful to the public MEDIA format. IMO impacting John Q Public to make appropriate weather related decisions should determine the message format

    As a user for whom Media info is intended one's decisions are dependent on how well one understand the message. Watts didn't do it. IMO Feels like Equivalent Temps do .

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  10. Rob!
    The models are showing a couple weak impulses in southern Manitoba but yet the forecast calls for no snow!!!

    You feel we could get some snow this week????

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  11. Daniel P, the weather network has been calling for snow for a few days now. Though the enviroment canada forecast has been only saying chance of flurries. I think I go with the weather netork this time.

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  12. Bigger question for me is how far east the warm up will reach... arctic ridge finally dropping SE allowing for westerly pacific flow to cascade over Rockies. Temperatures at Pincher Crek soared almost 15 C at one point. models like NAM were depicting 0 C at 850 hPa reaching western RRV. Enivironment Canada guidance far too conservative with warm up for us it seems.. again I think SCRIBE or GEM output not takin into account the lack of snowcover. I think this will allow for a rapid recovery of tempreatures...

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  13. EDIT... that should read temperatures at Pincher Creek jumped 15 C in *one hour* last nite

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  14. Is there some kind of force field around Winnipeg?????

    Like tonight it seems like the snow was heading this way and just somehow can't work it's way into the area!!!

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  15. Fascinating! Thanks.

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  16. Kudos to TWN and their 2 week trend forecast one week ago. Bitterly cold with minimal snow until yesterday, then warming today and much warmer, -3 to -1, tomorrow until the 24th. That appears to have been accurate for 10 days at least.

    They have been generally consistent since, although slightly less generous on the strength of the warmup.

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  17. I'm not too concerned about the effect wind will have on a bucket of water at 2C.

    Exposed skin is what I am worried about. -46 and -46 with a breeze are dangerously different for your face.
    10 seconds of that on one's nose quite likely felt like a few minutes at -60 with no wind.

    watts per sq.m mean nothing to me.

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  18. Ok!
    Tonight there should be snow across all of Southern Manitoba!

    Hopefully ....cause our pathetic snowpack is looking kinda of dirty!!!

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  19. Pat..

    The water bucket analogy was simply a way of explaining the difference between temperature and windchill. I fully understand the importance of windchill to the public, but I see more and more misinformation and misunderstanding on the concept. I am merely trying to explain it a little more clearly. It's liking saying the BTU output of an oven is the same as the temperature. To the uninitiated, it may not make a difference, but clearly there's a difference between the two measurements.

    BTW.. nice website you have!

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  20. Hey Jordan K. How did that going with TWN forecast of snow work out for yeah. I was wondering as well why EC did not have much snow mentioned in the forecast for the past few days and TWN did. As it pans out EC seemed to be bang on again with just a chance of flurries. I have pretty much lost all faith in TWN how about you?

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  21. To Anonymous) Sometimes I find there straight on with there forecasts. But they have been wrong to many times for me to really count on them. I pretty much go with EC's forecast and I watch the weather rader on there website for even more proof if something is going to happen. By the way There 14 day trend really suck it changes every 4 hours!

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