-49c - Page 4 - PeachParts Mercedes-Benz Forum

vstech · #46 01-29-2008, 12:56 PM

Quote:

Originally Posted by Graplr

You don't like physics, do you?

Quote:

Originally Posted by Graplr

2)Does the rate of cooling by the wind vary greatly (assuming same wind speed) if the ambient air temperature is 10F or 100F?

rate of cooling is dependent on the ambient air temp AND the temp of the object being cooled.
so IF a 98°F object is in a 10°F room with a 10knot wind, heat will be removed faster than the same object in a 100°F temp room.
ALSO, heat absorption of moving air is determinate of the current temp of the air, AND the relative humidity or latency of the air... don't mess with an HVAC tech when it comes to heat transfer.

Graplr · #47 01-29-2008, 12:59 PM

Quote:

Originally Posted by Jordan G

Stop watching TV then. I have. This forum is a better form of entertainment.

I guess being a teacher I need to keep up on what is out there. I want everyone to understand the misconsceptions out there and be on the same playing field. I guess that is why I am a teacher!

(Make it three, no that was twice this time...four)

Jordan G · #48 01-29-2008, 01:00 PM

Yeah - I thought it was just semantics.....and no, you weren't overtly dangling your profession out there....it just didn't really change the crux of the discussion, tis all.

I remember you battling it out with Forced Induction - maybe that's why I recollect the science teacher background

I agree - it could be packaged as temperature and wind speed.....but when it comes right down to it, if my 8 year old daughter hears that it's going to feel like -15, she's more apt to remember her scarf than if it's 5F with 30kt wind.....so I suppose in that case, it has some value - albeit steeped in some media sensationalism, no doubt.

Of course, her mother would never let her out of the house without it - but that's another argument

Chad300tdt · #49 01-29-2008, 01:02 PM

Quote:

Originally Posted by Jordan G

Stop watching TV then. I have. This forum is a better form of entertainment.

I also have given up TV (mostly) to spend the time on my MB and the forum.

I always viewed "windchill" like blowing on hot soup. If you blow air over it, it cools faster.

Graplr · #50 01-29-2008, 01:06 PM

Quote:

Originally Posted by vstech

rate of cooling is dependent on the ambient air temp AND the temp of the object being cooled.
so IF a 98°F object is in a 10°F room with a 10knot wind, heat will be removed faster than the same object in a 100°F temp room.
ALSO, heat absorption of moving air is determinate of the current temp of the air, AND the relative humidity or latency of the air... don't mess with an HVAC tech when it comes to heat transfer.

Notice I said GREATLY? You are correct, it does vary.

But I was talking in terms of windchill. So if we are talking about cooling an object down the same amount or percent, it will vary according to temperature but not greatly enough to affect a windchill temperature that much!
My point in making that statement was that if windchill was such a great number then why don't they use it in the summer when wind can cool you down the same as it does in the winter?
Yes, yes...humidity plays a role at the RATE you cool. But that is not what I was talking about. I was talking about the wind chill number itself.

Man, my post number has really gone up!

vstech · #51 01-29-2008, 01:08 PM

Quote:

Originally Posted by Chad300tdt

I always viewed "windchill" like blowing on hot soup. If you blow air over it, it cools faster.

that is also due to evaporative cooling a liquid. removal of the evaporate aids additional evaporate causing more rapid cooling.

Graplr · #52 01-29-2008, 01:13 PM

Quote:

Originally Posted by Jordan G

I agree - it could be packaged as temperature and wind speed.....but when it comes right down to it, if my 8 year old daughter hears that it's going to feel like -15, she's more apt to remember her scarf than if it's 5F with 30kt wind.....so I suppose in that case, it has some value - albeit steeped in some media sensationalism, no doubt.

Of course, her mother would never let her out of the house without it - but that's another argument

Yes, that is true. But what if she had never learned the windchill number? If she heard that is was 5F with a 30kt wind and had only heard it that way would she be more apt to wear a scarf?

Maybe...guess we'll never know.

My 2 year old always tries to take off his stocking hat. I don't ever let him do it until we are inside the car. I'd side with your wife!

I really have cut back on my TV watching the past year. And now because of DVR I don't have to watch all the ads and can finish an hour show in about 35-40 minutes.

cjlipps · #53 01-29-2008, 01:42 PM

Convective heat transfer in a nutshell:
The fluid will not cool (or heat) the object in question to a temperature any colder (or hotter) than it itself is (excepting any phase changes). It will only get (or attempt to get) it to that temperature sooner than if there was no convection. In the case of our faces, a wind chill value represents that the rate of heat removal approximates the rate of heat removal at that temperature in still air. IOW, it "feels like" that temperature because heat is being removed from our skin at a rate that exceeds our ability to replace it from within. The temperature is not what makes us "feel cold", it's the rate of heat removal from our body. )Right now it's 27F here with a WCI of 11F so that means it's cold out and the wind makes it feel much colder.)
It seems the core of the argument here is that people either do or do not need to know this value. I think people want quantification in all things when possible and this wind chill idea is no exception. If you invite a friend to go to a restaurant with you and you tell him the prices are "cheap" or "expensive", he might want numbers assigned to those generalities so he can decide how much money to bring or whether to even go.

dataiv · #54 01-29-2008, 01:52 PM

Quote:

Originally Posted by Graplr

My point in making that statement was that if windchill was such a great number then why don't they use it in the summer when wind can cool you down the same as it does in the winter?

Explain how wind will cool you down the same in the summer as in the winter. That doesn't make any sense because windchill, as discussed, is based on how quickly a human's face will cool down. In the summer, with air temperature much nearer skin temperature, windchill is irrelevant. On the contrary, in winter, with air temperature much lower than skin temperature, windchill is relevant.

So if the air temperature is -32 C for example, your skin temperature of your face may be 60 or so degrees warmer (just a guess, but it won't be as high as your core body temperature). With a stiff wind, that heat will be taken away very quickly ... and if your face was allowed to cool off completely, the temperature would drop quickly at first, then slow down as it approaches the air temperature. At -32 C, with a wind of 20 km/h, windchill will be -46 C.

It's the difference between air temperature and skin temperature that makes a windchill value worthwhile to have. In the summer that difference is too small, and that's why there is a statement like this: "Windchill Temperature is only defined for temperatures at or below 50 degrees F and wind speeds above 3 mph. Bright sunshine may increase the windchill temperature by 10 to 18 degrees F." on the following website: http://www.weather.gov/os/windchill/index.shtml

vstech · #55 01-29-2008, 02:10 PM

Holy C$%^ !
who cares what wind chill is at 110MPH winds?

Graplr · #56 01-29-2008, 02:13 PM

Quote:

Originally Posted by dataiv

Explain how wind will cool you down the same in the summer as in the winter. That doesn't make any sense because windchill, as discussed, is based on how quickly a human's face will cool down.

Wind doesn't care what your body temperature is, only that it will take heat away from the source more quickly to bring both temperatures to an equilibrium. You are correct in that if the air temperature is higher than your body temperature that will be a problem.

Quote:

Originally Posted by dataiv

In the summer, with air temperature much nearer skin temperature, windchill is irrelevant. On the contrary, in winter, with air temperature much lower than skin temperature, windchill is relevant.

So if the air temperature is -32 C for example, your skin temperature of your face may be 60 or so degrees warmer (just a guess, but it won't be as high as your core body temperature). With a stiff wind, that heat will be taken away very quickly ... and if your face was allowed to cool off completely, the temperature would drop quickly at first, then slow down as it approaches the air temperature. At -32 C, with a wind of 20 km/h, windchill will be -46 C.

Yes, if your body can not cool faster than it heats up that will be a problem. So maybe I shouldn't use 100F. Say it is 70F and windy. That is below my body temperature, correct? I will be cooled off by a wind? Does the weather person tell me the wind chill when it is 70F? Nope.

Quote:

Originally Posted by dataiv

It's the difference between air temperature and skin temperature that makes a windchill value worthwhile to have. In the summer that difference is too small, and that's why there is a statement like this: "Windchill Temperature is only defined for temperatures at or below 50 degrees F and wind speeds above 3 mph. Bright sunshine may increase the windchill temperature by 10 to 18 degrees F." on the following website: http://www.weather.gov/os/windchill/index.shtml

Yes, you are correct. Temperature does make a difference for what humans feel. But feel is subjective. Windchill tries to put a temperature number associated with it. I don't think we need that number. The number is irrelavent because your body's temperature could never cool down to what the given windchill number is. If it is cold and windy I don't need the weather person telling me an arbitrary number that my body could never cool down to. All I (and everyone if we weren't fed these windchill numbers) would need to know is that it is cold AND windy. Bundle up.

vstech · #57 01-29-2008, 02:17 PM

this guy is NOT going to be convinced. everybody stop it.
HE (total assumption here...) does not need the number. He thinks it should go away. explaining to him what the number is or how it is needed is as useful as talking to a cement wall as it is falling on you...

dataiv · #58 01-29-2008, 02:18 PM

Agreed, windchill is not a real number, it is how it "feels" to a typical human face. I still think it is useful to provide this number to the public ... though certainly some people take the number to heart and say the temperature is the windchill value, which of course is not true.

It looks like everyone agrees here now

... just you feel a windchill value is completely useless and should not be published, while I do not.

Chad300tdt · #59 01-29-2008, 02:22 PM

Quote:

Originally Posted by vstech

OK, where's that dead horse emoticon?

Hold on, here's another one I've been looking to use.

Graplr · #60 01-29-2008, 02:54 PM

Quote:

Originally Posted by vstech

this guy is NOT going to be convinced. everybody stop it.
HE (total assumption here...) does not need the number. He thinks it should go away. explaining to him what the number is or how it is needed is as useful as talking to a cement wall as it is falling on you...

I like the horse. Actually looks more like a mule, but that is okay..

Maybe I dislike it because here (in Minnesota) we are bombarded with the number (compared to some warmer places). When these low windchill numbers get posted it is all everyone talks about even though they have no idea what these numbers mean. Which is why I stated earlier the media does it for shock value.

Yes, I would agree that everyone sees where everyone is coming from now. You guys see that I can be stubborn (maybe that is why it is a mule and not a horse

). And I see that some of you like having the number and see merit in it.

And yes, who really cares what the windchill is at 110mph? Actually, that brings up a new question...is there a windchill if the wind carries you away and thus you are travelling at the same speed as the wind? If so, what would that windchill be?