Why do windshields always start fogging on the bottom?

[Bugman]

Just wondering. Old windshields always fog from the bottom up. Even on cars without visors to shield the top part of the window. Why is that? Thanks

-Jeff

[C9]

Probably because the bottom of the windshield is closest to a heat source.

[Bugman]

And while I'm at it, why do they fog anyway. I've heard that it's because of the glass delaminating. Is that true?

-Jeff

[Fraz]

Yes. The little plastic film between the glass starts to discolor and dirt, grit, and water get in there, make it fog.

[Rocky]

Just to piss us off, Jeff. They crack for the same reason.

[stealthcruiser]

cuz' water leaks into the channel in the windshield that the glass is nested in,and stays there till it evaporates.
every time it rains.
the fog usually(from what i've seen)starts at the bottom and migrates up the corners.

[Junkyard Dog 32]

Because it feels good.
I usually start fogging on the bottom, too.
Then I start fogging on the top.
Then I turn her around and fog her from behind.

JOEhttp://www.jalopyjournal.com/ubbthreads/images/graemlins/cool.gif

[old beet]

Water runs downhill!!.............OLDBEET

[tunglegubbin]

I think it's called inversion.
This happens when air meets a vertical glass surface and the glass is colder than the air, typically in the morning when the air warms up faster than a solid object e.g glass or when warm sweaty humans for some reasons are inside a cold car.

What will happen is that the air closest to the glass will become cooler thereby heavier and sink. The farther is sinks the cooler it gets until it gets to the point where the water vapor in the air condensates on the glass = fog.

[Bugman]

I mean the white hazyness that starts appearing at the bottom of the window, not the kind that goes away with windshield wipers. And not that kind of fogging either Joe. That's back seat fogging, not windshield fogging http://www.jalopyjournal.com/ubbthreads/images/graemlins/tongue.gif

[old beet]

Simply moisture getting into the plastic lamination. The plastic turns white, because it sits in water........OLDBEET

[Bugman]

Ah, I see. Just not through that part of the window cause it's all fogged up http://www.jalopyjournal.com/ubbthreads/images/graemlins/grin.gif