Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Wind chill -- How is it actually measured?

  1. Jan 30, 2019 #1

    zuz

    User Avatar

    How do they measure wind chill factor?
     
  2. jcsd
  3. Jan 30, 2019 #2

    jedishrfu

    Staff: Mentor

  4. Jan 31, 2019 #3

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2018 Award


    I hate that "Wind Chill" hype.
    It doesn't feel like the temperature is any lower,
    it feels like temperature is what temperature is and the wind is blowing..
     
  5. Jan 31, 2019 #4

    DrClaude

    User Avatar

    Staff: Mentor

    Hear hear!

    And even worse than the wind chill is the humidity factor.
     
  6. Jan 31, 2019 #5

    russ_watters

    User Avatar

    Staff: Mentor

    It's even worse when in the news they seamlessly mix or jump back and forth between the real temperature and wind chill.
     
  7. Jan 31, 2019 #6
    Since how something feels is very subjective, I would hope the formula is based on empirical data.
    For me a type of real measurement would be the time difference between
    a bowl of water at say -5 C without air movement to freeze solid vs the time with a 10 knot wind.
    The moving air would cause quicker heat loss and a faster freeze time.
    You would then need to then see what still air temperature correlated to the freeze time of the moving air test.
     
  8. Jan 31, 2019 #7

    zuz

    User Avatar

    Please correct me if I'm wrong, but I thought wind chill is a measurement of how cold it feels. A bowl of water does not feel anything
     
  9. Jan 31, 2019 #8

    jedishrfu

    Staff: Mentor

    I viewed the wind chill as something pretty cool (jk). It told me when I needed to really bundle up.

    I grew up in upstate New York and knew something of the coldness of winter while waiting for the bus each day. Such a cruel thing for children to endure especially with all that snow that needs to be spherical and launched in such a precise manner so as to hit its target. I remember definitely hitting the wrong target when my ball of icy coldness sailed through an open school bus window landing in the lap of he who shall not be named. (some embellished alliteration here for the PF community -- ie I can't remember who it hit as I ran home as fast as I could)

    However, I now know the power of the wind here in Texas especially when walking the dog at night. Its especially nice bundled up with my Notre Dame Football jacket on making me feel like an astronaut on the moon but my legs still shivering from the wind's relentless attempt to remove my thermodynamically generated warmth.
     
  10. Jan 31, 2019 #9

    Astronuc

    User Avatar
    Staff Emeritus
    Science Advisor

    'Wind chill' is about forced convective heat transfer, which is more effective as wind speed increases.
     
  11. Jan 31, 2019 #10

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2018 Award

    I completely agree.
    Having spent winters in in Florida, Idaho, Quebec , and central US,
    my testimony is +50F in ever-soggy Florida is more bone chilling than +15F in bone-dry Idaho.
    Specific heat of air is 0.24 BTU/lb, for water vapor it's twice that.
     
  12. Jan 31, 2019 #11
    I think of wind chill as how quickly heat is removed from an object, the object can be ether a person or a bowl of water.
    The persons perspective is subjective, the bowl of water freezing solid is not.
     
  13. Jan 31, 2019 #12

    gleem

    User Avatar
    Science Advisor

    The current wind chill index was determined by using human volunteers. They exercised on treadmills for 90 minutes while having their face temperature, their internal cheek temperature, and core temperature monitored. The heat transfer from and within their bodies allowed the experimenters to determine the cooling rate due to the changing wind speed. From "Understanding Weather and Climate" by Aguado and Burt.

    Temperature perception is highly variable both among individuals and within an individual and may not accurately represent the rate of heat loss. Keep in mind while wind chill and heat indices are related to comfort their usefulness is for predicting physical harm that could result.under extreme conditions.
     
  14. Jan 31, 2019 #13

    DaveC426913

    User Avatar
    Gold Member

    Frankly, I think y'all are missing the point.

    Wind chill factor and humidex are not comfort issues; they are health and safety indexes.

    If a thermometer reads 35 degrees Fahrenheit outside and the wind is blowing at 25 miles per hour (mph), the windchill factor causes it to feel like it is 8 degrees F. In other words, your body loses heat as though it is 8 degrees outside.

    That single number indicates how to dress, how long to be out, and how soon to seek shelter so as not to suffer from hypothermia, frostbite, etc.

    Likewise, with humidex. At 86F with a 60% humidity, you will overheat as quickly as if the temp is 100. That single number indicates how to dress, how long to stay out and how much effort to exert so as not to suffer from heat stroke.

    The weather peeps aren't trying to tell you whether you'll be comfortable, they're trying to keep you out of the hospital due to exposure.
     
    Last edited: Jan 31, 2019
  15. Jan 31, 2019 #14

    russ_watters

    User Avatar

    Staff: Mentor

    Your example was poorly selected for your point: there is no frostbite risk in an 8F windchill if it is 35F outside. Even when it is below freezing, wind chill is not well tied to frostbite risk. Worse, while for a given person the risk of both goes up with lower wind chill, for a person with more body fat the frostbite risk is increased while the hypothermia risk is lowered.

    More broadly, since windchill is calculated for exposed skin, walking into the wind, it can be defeated by ducking and is not valid for adjusting for heat loss through quality insulated clothing, for which wind is largely irrelevant.

    And none of these issues of the science have anything to do with the additional issue of news hype.
     
    Last edited: Jan 31, 2019
  16. Jan 31, 2019 #15
    When I lived in Florida, freezing friends (up north) would ask about the temperature.

    I would tell them "90 degrees... oh, that's with the windchill."
     
  17. Jan 31, 2019 #16

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2018 Award

    That's what aggravates me - the faux import and concern in the delivery ,
    They've already told me it's cold and the wind is blowing. I already know from motorcycle days that "cold plus wind equals damn cold."

    I know reporting the weather is usually mundane - and what's wrong with calm ?
    We need more calm-speak when a hurricane is approaching. Instead they feed hysteria.
    .
    Thanks Russ...
     
  18. Jan 31, 2019 #17
    Well, up until your body (or the bowl of water in another post) is chilled to 35 degrees. The 25 mph wind won't cool anything down below the actual temperature. Wind chill numbers are about rate of cooling, not final temperature*.

    *That might not be exactly correct for skin if you are perspiring. But I don't think that's the case in these "minus 55F windchill" conditions.
     
  19. Jan 31, 2019 #18

    zuz

    User Avatar

    As far as comfort is goes. I live in Buffalo NY and I would rather work outside when it's 20F and snowing as opposed to 40F and raining.
     
  20. Jan 31, 2019 #19

    DaveC426913

    User Avatar
    Gold Member

    I pulled those numbers from Google. Pretty sure I didn't suggest they were an example of frostbite risk in particular. I think you know that exposure and hypothermia can occur in a wide range of temps.

    It's easy to pick apart an imperfect system so long as you don't have to offer a better one.

    I don't see that as the core topic, more of an add-on.
     
    Last edited: Jan 31, 2019
  21. Jan 31, 2019 #20

    zuz

    User Avatar

    Again, correct me if I'm wrong, but I thought if you put a thermometer in front of a fan on a cold day it would read the actual temp.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook