Heat transfer question - ice on windshield

[freehat]

Ok, easy one for you.

When its a cold morning and you get in your car, start her up then spray water on the windshield, it freezes (obviously as the windshield is below freezing). You then scrape it off and drive. 5 mins later, while at speed, if you spray water on the windshield it wont freeze.

Why?

My school of thought is that the heat in the car (still very cold) wont be enough to convectivly heat the windscreen, and then conductivly heat through to bring the surface of the outside of the windcreen to non-freezing. I was thinking it would more be due to the fact that the car is a cold sink and so would be colder than the outside air (due to the fact that in the night the air temp was much less than in the morning) and so the convective "heating" of the outside air blowing on the windshield at speed, will result in the water not freezing.

thoughts?

 

[djack77494]

Wow, engineers talk about some strange stuff. Interesting though. I'd concur that evaporative cooling will overwhelm still air convective heat transfer. And let us not forget the very large thermal inertia of a large mass of glass relative to the small shot of cleaning fluid.

 

[Compositepro]

While evaporative cooling is a factor, I would say that a bigger factor is that methanol evaporatates much faster than water. Methanol is the most common antifreeze used in washer fluid. As the methanol evaporates there will certainly be some cooling but not enough to significantly change the temperature of the windshield. The most important factor is that as methanol evaporates the freezing point of the remaining fluid rises higher than the temperature of the windshield.