Post by thread:[OT] (human?) wind chill (was: Commercial vs. Indu

At 20:35 03/18/99 -0500, Sean Breheny wrote: >I think RH is taken into effect especially because when they calculate the >reverse of wind chill:"Heat Index",it is based upon RH and temp. I know it >isn't exactly analogous since it doesn't take wind into account,but it >certainly seems to me that it is perfectly reasonable for them to include >RH as a factor and use a "typical" model for how much a person sweats. >Also,I think they assume exposed skin when calculating wind chill,they >don't take different types of clothing into effect. i gathered a few chips of "official" information about the "wind chill factor" (see below). it seems there is no "sweaty body model" involved, not even exposed skin :) if the plastic cylinders they used for the experiments were closed, evaporation is also not a substancial contributor (in any case it doesn't contribute to the calculation) -- to the official numbers. it of course contributes to the heat (or cold) perception... so it depends on what wind chill we're talking about: the "official" wind chill (the formula below) or the "real" wind chill (the change of our perception of temperature with varying wind speeds). it's a bit desillusioning, isn't it? :) ge ** about the official formula (from <http://usatoday.com/weather/wchilfor.htm>): The term "wind chill" goes back to the Antarctic explorer Paul A. Siple, who coined it a 1939 dissertation, "Adaptation of the Explorer to the Climate of Antarctica." During the 1940s Siple and Charles F. Passel conducted experiments on the time needed to freeze water in a plastic cylinder that was exposed to the elements. They found that the time depends on how warm the water is at the beginning, the outside temperature and the wind speed. The formula the National Weather Service uses to compute wind chill is: T(wc) = 0.0817(3.71V**0.5 + 5.81 -0.25V)(T - 91.4) + 91.4 T(wc) is the wind chill, V is in the wind speed in statute miles per hour and T is the temperature in degrees Fahrenheit. The formula to calculate a Celsius wind chill using V as the wind speed in kilometers per hour and T in degrees Celsius is: T(wc) = 0.045(5.27V**0.5 + 10.45 - 0.28V) (T - 33) + 33 [...] Note: When wind speeds are below 4 mph., the above formulas will give you a wind chill that is higher than the actual temperature. When wind velocities are near zero and you are standing still, your body heat warms the air near your body. This warm air near your body provides some insulation from the colder environment. As a result, it may actually feel warmer than the actual temperature. ** about the real meaning of the official wind chill factor (from <http://usatoday.com/weather/wchilpro.htm>): He [Kessler, from the University of Oklahoma] notes that the original 1940s experiments by P.A. Siple and C.F. Passel in Antarctica that led to the wind chill index measured how long it took a can of water to freeze at various temperatures and wind speeds. This obviously, does not take into account the much more complicated cases of humans exposed to cold and wind. Measurements of wind speeds also present problems. Kessler writes that the wind speed used in today's weather observations and forecasts is from a 10 meter (33-foot) high tower. Wind speeds at the ground are slower than the "official" speeds. As just about anyone who's been out in the cold can tell you, sunshine can make a big difference. Yet, the wind chill charts do not account for differences between sunny days and cloudy days or even nights. Kessler wrote: "... our use of the wind chill index entails all of attributions of substance to mere form, addiction to hyperbole, and passion for technological glitter. Scientists should not support this! An employee of the National Weather Service summed it up in a letter to me: 'Neither the heat index nor the wind chill serve any purpose other than to provide the news media with a tool with which to excite the general public.' "

Wow,you really did some research on this one,didn't you! I stand corrected. Sean At 07:51 PM 3/18/99 -0800, you wrote: > >i gathered a few chips of "official" information about the "wind chill >factor" (see below). it seems there is no "sweaty body model" involved, not >even exposed skin :) if the plastic cylinders they used for the >experiments were closed, evaporation is also not a substancial contributor >(in any case it doesn't contribute to the calculation) -- to the official >numbers. it of course contributes to the heat (or cold) perception... > [SNIP] | | Sean Breheny | Amateur Radio Callsign: KA3YXM | Electrical Engineering Student \--------------=---------------- Save lives, please look at http://www.all.org Personal page: http://www.people.cornell.edu/pages/shb7 spam_OUTshb7TakeThisOuTcornell.edu ICQ #: 3329174

Gerhard, I designed a PIC-based weather station that has been in operation over three years here in Portland, OR. During the design process I consulted the Federal Meteorological Handbook, National Weather Service, World Meteorological Organization, and local meteorologist. I also ran across that web page you quoted in your earlier message. Those equations are the accepted standard. Wind Chill is based on wind speed and temperature. Heat Index is much more subjective but it's based on RH and temperature. While my station displays indoor and outdoor RH, it also displays Dew Point which provides a better overall `comfort index'. For example, it can be raining `cats and dogs' with a humidity of 99% yet it does'nt feel `sticky' due to a low temperature. In general, when the Dew Point get's above 60, it feels `sticky'. Reminds me of when I was stationed down in Florida... - Tom At 12:08 PM 3/19/99 -0800, Gerhard Fiedler wrote: >At 14:30 03/19/99 -0500, Sean Breheny wrote: >>Wow,you really did some research on this one,didn't you! > >it was not much more than using a search engine ( "wind chill" :), >following three links or so and copying the stuff after some review. in >fact, i wanted to know what they're doing there. but it was not up to my >expectations :) > >ge

At 00:48 03/20/99 -0800, Tom Handley wrote: > Gerhard, I designed a PIC-based weather station that has been in >operation over three years here in Portland, OR. During the design process I >consulted the Federal Meteorological Handbook, National Weather Service, >World Meteorological Organization, and local meteorologist. I also ran >across that web page you quoted in your earlier message. Those equations are >the accepted standard. Wind Chill is based on wind speed and temperature. >Heat Index is much more subjective but it's based on RH and temperature. >While my station displays indoor and outdoor RH, it also displays Dew Point >which provides a better overall `comfort index'. For example, it can be >raining `cats and dogs' with a humidity of 99% yet it does'nt feel `sticky' >due to a low temperature. In general, when the Dew Point get's above 60, it >feels `sticky'. Reminds me of when I was stationed down in Florida... i don't know how they calculate the heat index, but dew point depends on the same variables. (actually, dew point is just another measure for the absolute content of water in the air; in that, it doesn't include temperature at all.) as i quoted also, it seems it is a standard, but not necessarily universally accepted :). it would be interesting to come up with a "real" win chill factor, which not only shows how fast water freezes... :) which takes into account some medium perspiration as well as the different thermal resistance. but maybe that's too individually different to make for a good measure. ge