Why Does a Bridge Freeze Before the Road?

[misskitty]

This is something I have been thinking about since last night and I cannot figure it out!

The question is:
Explain how conduction causes the surface of a bridge to freeze sooner than the road surface on either side of the bridge.

The only thing that I can think of is that the air passing over and under the bridge creates a thermal energy transfer between the pavement and the air, cooling the bridge first rather than the road that doesn't have a space for the air to pass underneath it...but for some reason I don't think that's right.

Your thoughts?

 

[misskitty]

I hate to double post...but could someone please help me with this question? I really would appreciate your thoughts on it. I really am not sure if that's a vaild valid answer to the question.

Please someone post!

MissKitty

 

[Ba]

It would seem logical, I actually think it more likely to be radiation not convection. The only time I notice this though is on wood bridges, this is because the wood does not hold as much heat. The air would carry away more heat, that is true because of the increased surface area as long as the air is colder than the bridge.

 

[DaveC426913]

Convection wasn't one of the choices. Conduction is heat transfer between surfaces in contact with one another.

Keep in mind: Conduction works both ways. Not only can it cool a target down, but it can also prevent a target from cooling, if in contact with something relatively warm - or something that won't be rapidly cooled by wind exposure - like, say, the Earth.


Does that set you on a better track?

 

[misskitty]

I think it might help...but I'm just guessing at why the bridge's surfaces freeze faster. I didn't remember that air can also be used as an isulator. Thanks for reminding me.

 

[kevinalm]

Don't think about why the bridge freezes faster, think about why the road freezes _slower_.

 

[misskitty]

Ohhh...ok that makes more sense. Thanks, I think that clears stuff up for me.