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Be careful if the screen is to cold it will crack ,It was -22c in Dresden today and as warm water freezes faster than cold water you would end up with ice an inch thick.Cold water is much better and safer or use a car top.
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> and as warm water freezes faster than cold water
Oh, Andy !! You of all people.
Warm water does not freeze faster than cold water.
If you think about it, water which was 10 degrees could not freeze faster than water which was 5 degrees, since at some point the 10 degree water would become 5 degree water and adopt its characteristics. To all intents and purposes cooling does not have inertia.
What it does do is lose more heat per time period, since it has got more heat to lose (greater temperature differential).
M.
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You do not know your physics Mark, the cooling is not linear and hot water freezes much faster than cold water .Water at 20c taken into a room at -20c will freeze twice as fast as water at 5c.The hot water will take seconds and the cold water minutes .Its all that sun you get
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> You do not know your physics Mark, the cooling is not linear
> and hot water freezes much faster than cold water .Water at
> 20c taken into a room at -20c will freeze twice as fast as
> water at 5c.The hot water will take seconds and the cold
> water minutes .Its all that sun you get
Cooling is not linear, I agree. Heat loss from warm water will be faster I agree - for as long as the water *is* warmer. When it is the same temperature as the other water heat loss and cooling will match.
However, hot water will not *freeze* faster than cold water although it will *cool* faster.
Andy - I bet you a virtual beer that I am right. In fact, I am so confident make that *2* virtual beers.
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> Andy - I bet you a virtual beer that I am right. In fact, I am so confident make that *2* virtual beers.
Aww, goddamn it.
www.weburbia.com/physics/hot_water.html
Mark. (off to look for conflicting arguments and pointing out that it says under *some* circumstances not *all* and not even *most*)
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Your wrong Mark,Two vitual beers behind the bar in Omalleys Sau paulo.Difference of opinion apart pouring hot water onto a cold screen and sealing rubber can be disasterous.Having said that the thought of a beer sounds good of to bar next door.If you get down to Coratiba try get up to the German town in the mountains behind it its brilliant there is even a small train that will take you.
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Favourite bar in Sao Paulo is Charles Edward on Juscelino Kubitscheck. If you don't know it, its opposite Dado Beer which you *must* know.
Don't like Curitiba, it rains all the time.
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Gents, Stop fighting. I did not say hot water, I said warm water.
I have been doing this for 40 years - not continually - and never had a screen crack yet. Use plenty of warm (just) water and it will defrost both inside and out.
I have often wondered why hot water might freeze faster than cold. (Yes I have heard it before) As has been pointed out, as the hot water cools, it will fall to the same temperature level as the cold water and will forget it was once hot water and behave the same as the cold water.
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The old metal ice-cube makers, filled with warm water, would partially melt the ice in the fridge compartment, thus giving a metal-to-metal contact, which would freeze the cubes quicker than if the (cold) ice maker had been resting on an uneven icy surface.
This is probably how the myth originated.Now, where's my gin and tonic?
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instead of a car top, use sheets of newspaper on windscreen the night before, works 100% on frosty mornings, however if it rains it can make a mess!
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There was a similar case in Africa, where a class was set to make ice cream. One of the class declared it was made quicker from mixture at ambient temperature rather than pre-cooled. The teacher, bless him, decided to divest the pupil of his notion by demonstration rather than didactically, and found that what was claimed was so.
It may be in the files of Nature.
Anyway, I'd be dubious about pouring hot water over a very cold windscreen - I imagine the freezing temperature of water is irrelevant, the temperature differential and thermal shock is what counts - but our neighbours seem to get away with it. Probably a laminated windscreen is less at risk than the old toughened variety.
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