Window Fog Evaporation: Why Does It Happen?

[Galap]

Everyone who lives in a region that gets cold knows that if you exhale onto a window, the water vapor in your breath condenses onto the surface. Because the air is dry, this condensation evaporates. What puzzles me about it is why it does so from the edge of the fogged region to the center, as opposed to all at once.

Of course the 'fog' is actually made up of many small droplets. Upon close inspection, it can be seen that the droplets shrink and disappear, but only those on the edge of the zone do this, as if they 'know' whether or not they are surrounded by other droplets. Why does this happen?

 

[Lavid2002]

I am not 100%, but by thinking this out this is what I have come up with.


Think about what caused the droplets to form on the glass. Your breath didn't only transfer moisture onto the glass, but heat too!

The glass directly in the middle of the condensed area will be marginally warmer than that of the outside area. It takes a while for the area to return to its original temperature, and since the glass just outside the condensation held its temperature (And conducts it to the condensed areas glass) this part will be the first to return to its original temperature, and shed the condensation to evaporation. This continues until all the moisture is gone.

Does that help?

 

[davenn]

I am not 100%, but by thinking this out this is what I have come up with.
Think about what caused the droplets to form on the glass. Your breath didn't only transfer moisture onto the glass, but heat too!
The glass directly in the middle of the condensed area will be marginally warmer than that of the outside area. It takes a while for the area to return to its original temperature, and since the glass just outside the condensation held its temperature (And conducts it to the condensed areas glass) this part will be the first to return to its original temperature, and shed the condensation to evaporation. This continues until all the moisture is gone.
Does that help?

yes but wouldn't that increased heat in the middle cause that to area to evaporate quicker ? ;)

I would more suspect that the density of vapor particles on the glass would less around the edges than in the middle of the "fogged " area meaning they would evaporate quicker than the denser region in the middle.

my 2cents worth :)

Dave

 

[davenn]

thinking on from my last reply ... a test
instead of breathing directly onto (perpendicular to) the glass, breath onto it as a low angle so that there would be one edge area that would be closer to the mouth than the middle or far side of the "fogged" area. Now watch to see where it starts to evaporate from the quickest and what area is the last to evaporate :)

does the far side now evap. soonest followed by the middle and finally the edge closest to where your mouth was ?

Dave

 

[Galap]

Davenn I tested both of your ideas, the first by moving my head in a cricle, creating a ring of 'center breath'. This did not seem to affect the evaporation, as if your first conjecture were true, the center of the foggy area would also evaporate quickly, which it did not.

Upon breathing at a low angle, the evaporation was still unaffected. It went from the outside inward.

 

[davenn]

hmmm interesting... I would have really expected the denser area of water particles to take longer and as a result a low angled breath would have shown that conslusively

temperatures are way too high here in Sydney, Australia at the moment hahhaha impossible to leave my breath on any glass surface... will have to wait till winter or some colder nights to do some tests for myself

Dave

 

[Lavid2002]

Correct me if I'm wrong, but isn't condensation only possible if the object that air is condensing onto is colder than the air that the moisture is in?

So it wouldn't be heat helping the glass wick away the moisture (Like a hot pan on the stove) Instead it would be cold temperatures, which the outside of the ring have more of...

 

[Vanadium 50]

At the boundary, the water vapor is just barely condensing. That's what it means to be "at the boundary". So when it starts evaporating, that region is the first to go.

 

[Antiphon]

Vanadium 50 is right. The fog has a density to it which is highest near where you breathed.

Temperature can in fact play a role but the temperature difference must be substantial. In my car, the window defogger will not clear the fog at a spot where there is ice on the other side.