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Wind Chill

J

John B.

Well-known Member
I have worked as a mechanic for over 30 years and have heard so many people say that their vehicle would hardly start because of the wind chill. I can"t believe how many people don"t understand windchill. Wind chill is not a temperature folks. It is just a calculated formula for how fast items cool down to the ambient air temperature. The wind makes the air feel colder to us because we are use to having body heat around us. The definiton of wind chill is; Ambient air temp combined with wind felt against exposed skin. If wind chill were a temperature thermometers would be going up and down continiously. A temperature of 30 degrees with a windchill of 15 means that items will cool down at the same rate as if it were 15 degrees without wind. Some swear their tractor starts better inside the shed than the one outside the shed. Well there is some heat from the ground that the shed will capture and keep inside. One thing to remember is that heat travels from hot to cold. Once the heat is gone it can"t get any lower than the ambient air temp, if an item is 20 degrees and the ambient air is 25 degrees the item will warm up to 25 degrees. So for those of you who think wind chill is actually making your vehicle colder you"re wrong, it just made it cool down faster to the ambient temperature. Wind chill is present all year long just not in the winter. That is why we use fans in the summer. There"s not two temperatures out there!!
 
John, you are completely correct. Wind chill is about rate of change, not temputure. It will not tell you how cold it can be before your tractor will start. All it does is tell you how quick you can die of hypotherma, a tempurture far higher that most understand. Without help it is easy to freeze to death in the mid 40 degrees, espically if you are wet.
 
You are right that wind chill shouldn't affect machinery, but wind chill has to do with evaporation of water from the skin, causing the skin (or a pan of water) to be colder than the dry bulb temperature. Since a good radiator is dry on the outside, wind chill as defined by the weather service has no effect. Sure wind will cool it faster, but that's not what the NWS wind chill number is. Dry air cools the skin more than damp air blowing by.

Actually the formula for wind chill was created by a couple Florida natives assigned to Duluth. MN and was basically how cold they felt for various winds and drybulb temperatures. A few years back the formula was revised by northern natives to not be so severe.

And the heat index in the summer was similarly devised by Duluth, MN natives working in Texas. At least one Duluth MN weatherman that I know starts to sweat at 65 degrees F. That heat index formula has also been calmed.

Gerald J.
 
No offence John, but I'd be awfully surprised if at some point in your life you weren't aware of that fact as well!

I'm fully aware that windchill has no effect on the actual temperature of a chunk of steel. In fact I'm even led to believe that the "cold" we commonly refer to is not the proper way to look at things. The "cold" we are referring to is actually the abscence of heat...............BUT despite all of that I still don't know $hit about the price of rice in China! ;)
 
BS IS BS BUT WIND CHILL HAS P;ENTY TO DO IF MY WATER TANKS FREEZE UP OR NOT BY THE SILO,,, THERE IS ALWAYS WIND BY THAT 75FT BOHEMOTH BUT IN THE WINTER SHE CAN FREEZE UP AT 35 DEGRESS IF THE HEATERS ARE NOT ON//////I AM NOT JOE FROM MINNISOTA;;;;;;; PS BUY THAT GM STOCK
 
Had the same type of argument with a terminal manager for Roadway relay station, back in 93, 94 winter. We had temps of -20, and the wind chill made it feel like -40. He would not idle engines for more that 15 min, was locked onto the fact that engines don't feel the cold from wind chill like we do, and I couldn't explain that ambient temp was -20 and it would cost him less to idle that call me out to thaw each truck's air stater system that froze open with each start. $20.00 a pop, with 25 to 35 dispatches per night, plus a teamster waiting on the clock for his truck. (always wondered how much fuel they did save!)
 
Yes Glenn the cold is the abscence of heat you are very much correct. Any one who's educated on air conditioning knows this. Everything on earth is naturally cold and heated by the sun or the earth's internal functions. As for Joe in Minnesota he needs to put a thermometer in his water tank and one on above the water surface to see which is colder.
 
Hi Joe, I got a couple questions for you on windchill. Have you ever blown on your hot coffee or soup so you could consume it? If so why it will cool down on it's own!
 
Buster, yes the terminal manager was right the trucks don't feel the wind chill they just cool down quicker to ambient temperature. A -20 degree temp would justify starting them every so often and let them run for 15 minutes that does the batteries good too in that temp because at 32 degrees a battery has lost half of its potential.
 
The only thing with Roadway is during the 90's, their trucks had only one 12v battery, and their air start systems froze open every time you hit the dump valve release. You always had to jump the battery for the series 60, because they were old and Big R would rather replace an alternator than a battery

"Stepping over dollars to pick up a penny"
That was my first experience dealing directly with a large company, and was amazed that they made money!
 
Awwwwwwwwww John, I was just trolling the trolls; figured any temperature expert could explain the wet bulb. :>)
 
A cousin finally proved that once and for all to a friend a few years ago. Got him in the passenger side of the car holding a thermometer out the window. Drove him 6 miles at 60 miles an hour. His fingers didn't work so good afterwords,but he finally got it.
 
This hits one of my pet peeves as well. Had a roommate in grad school (we were both MechEng. so lots of thermo and heat transfer type classes) always insisted on parking his car nose away from the wind. Smart guy, but could not get it across to him that the car was going to be at ambient regardless of the wind. Decided if he had trouble with it I would stop trying to explain it to anybody else.

Regarding the stock tank freezing... It is entirely possible to freeze the surface of water when ambient is still above 32 degF, but it has to do with the radiant heat loss to space on a good clear night not a wind chill. Black body of space is very close to absolute zero and can really suck the heat from the earth. Pay attention, on cloud covered nights it generally stays warmer overnight (hard to eliminate weather patterns in this observation, but you get the idea.)

Been a long time since I thought about heat transfer.

Kirk
 
Personally I think that keeping machinery away from the wind will help it.
Not because the wind will make it colder but because the wind will carry more heat away faster until the engine is almost stone cold.
Also when cranking the engine, the wind will carry the heat of compression away faster.
Those facts added to the reduced cranking speed is sometimes all the difference it takes for a no start or a start
 
I don"t care about facts; I always have and will continue to park with the back to the wind...
I know how cold it is to face into that wind, and I couldn"t sleep at night, knowing my poor old tired engine was out there...I want it to start in the morning, and I know it wants to start, too, to get warmed up, like any decent, responsible engine, so the least I can do for it is to get it out of the direct wind...maybe even drop a light bulb under the hood for it...(I don"t think it knows about engine heaters)...
 
OK answer this for me I was running my Case LA the other morning and it was about 40% outside. The outside of the intake was frozen over with ice. The only thing was the air passing though it.
Walt

PS I looked this up once on a website because of an argument on another tractor list. The same website contradicted itself on whether it gets colder or not by saying that you can get frostbite in 40% temps from windchill but won't cool a metal object down below ambient temp.
Funny HUH
 
Press the alt button on your keyboard down and while holding it hit 0176 and you will get the degree sign °. Isnt there some law about air passing through a restiction lowering the the temp.
 
Since I live in balmy western Washington, about the only experience I have with wind chill is when people ask how long the Mrs. and I have been married, and I have been known to reply, "38 years come February, but sometimes it seems like 50 with the wind chill."
 
You guys ought to come up here in Maine when its 45 below and the wind is blowing 30 mph.Youd understand wind chill then. Hoss
 
There is a vacum in the intake which causes the drop in temperature. Just like when you let the air out of a big tire, frost builds up on the outside of the metal stem. The rush of air coming out causes a void around the outside of the stem thus called a vacum. Air tools will do the same thing at the exhaust if used long enough. Frost always builds where the drop in pressure takes place like at the tip of the valve stem or just on the vacum side of the carburetor butterfly. They get really cold I know that for sure. Vacum cools and presure warms usually
 
Your air conditioner works on the same manner as your intake on your gas tractor. You have a low side and a hi side in the ac unit. Some where this has to take place and it is in the expansion valve or orfice. Look as your carb as being the orfice with atmospheric pressure on ones side and a vacum on the other side.
 
We understand what windchill is but some people don't. It's not a temperature but our minds make it believe it is. That's why the weather forecasters say the wind makes it FEEL colder. Next time you blow on your cup of coffed try to figure out why you're doing that. It will cool on it's own but you want to cool it faster. You won't cool it any cooler that the air that's coming out of your mouth. Heat travels from hot to cold not cold to hot.
 
A light bulb will help some but I would use a block heater. Remember windchill only blows the heat way once it's gone it can't get any colder. If the air was -20 and the enine was -30 the wind would actually warm the engine up to -20.
 
Good point, I agree with you to a point. but if the tractor has been sitting for days it won't help much but if it were run hours before being parked I agree.
 
Something to think about. Why do we blow on our hot sout? We know it will cool on its own but we want to cool it quicker. Also would an engine need a radiator fan if the radiator was huge enough? If not why not? Because the coolant would be in the radiator long enough to cool down on its own. All the fan does is speed up the cooling process. A fan is needed since we are limited on the size of the radiators. Engines can run hotter if you take out the thermostat why? Because the coolant isn't kept in the radiator long enough to draw the heat out. It's time not temp we're talking about with windchill !!
 
I never heard that. I don't see how when the air would actually warm it up. I have heard what feels colder wood or metal when they are actuall the same temp?
 
Ok, here is my take on it. I agree that 20 degrees is 20 degrees if the wind is blow'n 5 mph or 50. While the 50 mph feels colder to us, it also pulls heat faster from metal. I have found that if I plug the tractor up in the drive way, in the open over night and the wind is blowing real hard it will not start as easy as it will if it was parked in a barn. I figure with it pluged in the wind pulls more of the heat from it than it would in the barn. I may be way off base, it may be all in my head, but seems to me a pluged up tractor will start better in a barn than in the open on a windy morning.

Dave
 
Dave, You are correct! One thing to consider is you are trying to put heat into the tractor by plugging in the block heater. I was basically talking about one that is not plugged in. Your theory is correct but compare it to using a small torch to heat up a big hunk of iron it will take for ever and even longer if something is trying to cool it down by pulling the heat way. But a bigger torch would heat it up to make up the difference. The metal once cooled will never be any colder than the ambient temperature. Take two thermomters or tractors and let them set out side but cover one up and in the morning before the sun comes up see which one is colder. I've done it and they're the same temperature.
 
Dave, You are correct! One thing to consider is you are trying to put heat into the tractor by plugging in the block heater. I was basically talking about one that is not plugged in. Your theory is correct but compare it to using a small torch to heat up a big hunk of iron it will take for ever and even longer if something is trying to cool it down by pulling the heat way. But a bigger torch would heat it up to make up the difference. The metal once cooled will never be any colder than the ambient temperature. Take two thermomters or tractors and let them set out side but cover one up and in the morning before the sun comes up see which one is colder. I've done it and they're the same temperature.
 
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