Alwyn, all depends (Before you ask why am I commenting, as we never get ice here... I've lived in some pretty cold places)
Take a glass, place in freezer, leave for a few hours, take out, and fill with warm water.
Wear gloves.
The point is, the screen may expand and crack.
This is especially poignant if the wind has been blowing - the screen may be 5-6 degrees BELOW the air temperature, due to accumulated windchill.
Another problem is the run-off, which can get into gaps and refreeze when you start moving. Wiper arms, door keyholes, under window seals - all very susceptible, and easily damaged.
Hope this helps.
(Phew! what a scorcher here, by the way...)
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Ian
Oh dear, the old wind chill fallacy again.
Windchill was devised as an approximate means of assessing the additional heat removal effect of wind on the human body. The human body tries to maintain a steady temperature. Air isn't a particularly good conductor of heat, but if the wind blows then the air immediately adjacent to the body is constantly removed, and the body loses heat faster. Wind chill factor tries to calculate an equivalent static air temperature which equates to the actual air temperature and the heat removal effect of the windspeed.
However it cannot and does not have any relevance to inanimate objects like cars and windsreens. If the air temperature is,say, 0C no matter how fast the wind blows your windscreen will not get below 0C! Yes, the car will cool down faster, but to drop below the temperature of the surrounding air would contravene the laws of physics! Heat can only flow from a hot body to a cold body, and once the car and the air are in equilibrium that's it.
This one usually occurs with the instructions for 'add-on' external temperature gauges which are suggested installed behing the bumper 'to avoid windchill'. No surprise that Vauxhall and BMW (to name but two who who understand the science) ignore this advice!
regards
JS
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John,
My head is both bloodied and bowed!
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Don't worry about it - there are plenty out there who need it explained - go spread the word!
Regards
John
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Aaah! Dug out the details, and I was wrong ... but right!
It transpires that, under ceratin conditions, the temperature drops to silly levels overnight. Over a period of time (4-5 hours) , the INSIDE of the vehicle chills down, and as the outside air heats up, the inside remains cooler! SO! When the outside expands, so the inside (up to 5 degrees colder) won't expand at the same rate, hence the water cracking the glass.
Again, my apologies for my earlier ignorance, but the point is - DON'T pour water on the windscreen!
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Wonder why glass tumblers don't craaack when we put our hot LemSip in them.
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On a frosty night, a cooling system filled with plain water might not freeze. But driving the car before the thermostat has opened can result in the water in the radiator freezing, due to the wind chill. I realize that the radiator is being cooled by the airflow to the ambient temperature, but in practice if not theory, windchill is going to cause the subsequent problems.
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Steve
The effect here is that when the car is standing overnight it takes much longer for the engine and coolant to cool. It may not get down to freezing point. Once you drive it, the air flow increases the rate of cooling, but it is not possible to get the water below the temperature of the air.
regards
john
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Alwyn
It depends how thick they are. Thick glasses will be more prone to cracking under those circumstances. The heat from the hot liquid will take a finite time to travel through the whole thickness. In a thick glass, this will result in stresses being induced as the hot inner surface attempts to expand, but is restrained by the cooler outer surface. These stresses can be sufficient to cause the glass to shatter. Small imperfections will act as stress raisers, and cracking will start at these points. I don't recommend you have your Lemsip from a cut glass tumbler for that reason!
Pyrex doesn't suffer from this problem as it is borosilcate glass with a very low coeficient of expansion.
You'll remember from Chemistry lessons at school that laboratory glassware is very thin - and thus heats through quickly and doesn't crack.
regards
John
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ian (cape town) wrote:
>
> This is especially poignant if the wind has been blowing -
> the screen may be 5-6 degrees BELOW the air temperature, due
> to accumulated windchill.
Bzzt. Wrong.
Wind chill refers to a *rate* of cooling. No matter how much wind there is it can't cool something to a temp colder than the actual temp.
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Alwyn wrote:
>
> Why not use warm water to de-ice a recently discussed? Works
> fine and no waiting.
I do sometimes - I find the scraping quite soporific and by the time I've gone back to the house it's doesn't take that much longer.
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