h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

If a bucket of water is left outside on a day where the air temprature is +1 degree C but the wind is blowing, making the wind chill factor -2 degrees C, will the water in the bucket freeze?

No. If the air temperature is +1 degree, the lowest temperature the water can reach is +1 degree.

Wind chill only affects things that are hotter than ambient temperature. It causes them to cool quicker than if there were no wind. But they can't cool to lower than the ambient temperature.

I thought that the wind chill factor was a bit of a fudge factor with no real scientific basis, that is to say, no real way of computing it. A bit like asking the neighbours how cold they think it is, and taking it as the "actually felt temperature".

Wind chill factor is a very scientific factor. I've a formula for it somewhere. It is completely precise.

It says that when the wind temperature is A and speed is B, you, at temperature C, will cool down at the same rate as if there was no wind and the air temperature was D.

Of course, that's if you're exposed in your bare skin to the wind. If you are well insulated, you won't cool down at all, either in wind or in still air.

You are warmer than the surrounding air. So the air around you warms up. That is to say, you lose heat, and the air gains heat. If that air stays around, that's fine. But if that air then goes away and is replaced with different air, due to wind, you then have to heat that air up. And then the second batch of air goes away and you get a third batch to heat up. So you're losing heat all the time. (In the real world, air doesn't come in batches.)

So yes, the wind-chill factor is real, but it can apply only to things which are warmer than the surrounding air.

(I worked this out in my mid-teens, musing on how car-thermometers can work.)

TRiG.

Thanks for the explanations Gnomon and TRiG

So for example, if you left a bucket of boiling water out in sub-freezing temperatures, it would freeze. But it would do so quicker if it was windy than if it was calm.

To answer that yourself - what do you do with a hot drink if you want to cool it enough to put it to your lips...

I didn't ask anything. I thought an explanation might help.

Good point, sorry for misreading.

#1 "If a bucket of water is left outside on a day where the air temprature is +1 degree C but the wind is blowing, making the wind chill factor -2 degrees C, will the water in the bucket freeze?"

Just to add to what others have already said - it's all to do with heat transfer and heat content, and since "heat cannot pass from a cooler to a hotter body" - the water can only fall to a temperature of +1 degree C in this scenario.