Every driver has noticed that car windows get covered with a condensate layer when it's cold, or put it simply, they fog up. Though, sometimes fogging takes more time and sometimes less. There is also a known fact: if a person has had a drink shortly before, a window fogs up faster. So is this the truth? What causes fogging? How do you think you could deal with it? We will try to give the most credible answers to these questions.
Once moisture caused by human breathing or damp clothes contacts a cold window, it turns to condensation by the rules of physics.
As you cannot get rid of the fog source, you want to either evaporate the extra moisture (airing a vehicle out) or increase the inside temperature (standard heating).
The thing is that alcohol dilates the blood vessels, thus stimulating better evaporation. Because of this, windows fog up faster.
As it is known from the school physics course, the hotter the air is, the more moisture it can hold. That's why human exhalation of over 30 °C is able to conserve a good deal of moisture. People quickly saturate a small and almost leakproof vehicle with moist air. This is not a big deal on a summer day, but in winter, windows get quite cold and always reach subzero temperatures—whatever the weather is. When the interior air contacts cold windows, it cools down and ceases to retain moisture bulk. Thus, the moisture straightway condenses across the nearest cooled surface, which is the windshield.
This question has been of interest to many people for decades. And you don't even need to drink too much—a couple of shot glasses with strong stuff are enough to cause a very intensive fog.
Various reasons have been proposed to explain this feature. For instance, some experts came up with the idea that alcohol vapors lead to dew point shifting, which means faster moisture condensation at a higher temperature. However, this requires massive alcohol vapors in the air, and even a dozen glasses are clearly not enough. So given the insufficiency of this version, we have to give it up.
Anyway, there is still another, a more realistic one. We know that even a small amount of alcohol makes blood vessels expand. And this applies to all body organs, including the lungs and skin pores. As a result, the body inevitably begins losing oodles of moisture, the interior air gets humid much faster, and windows fog up more vigorously. This version seems to be much more faithful.
Here you have nothing to do with theory but practice. But seriously, how to handle the fogged windows issue? There are a bunch of ways to do this, and one of the easiest is to use a heater.
If the surface of windows keeps warm, the fog will fail to form on them easily and will gather in colder areas, particularly on the side and rear windows, where it would do less harm. There is also an option to crack the window (if it isn't too cold outside) to provide sufficient ventilation, defogging the interior air.
You can use chemicals, after all! Mix a specialty glass cleaner taking glycerin and alcohol, one part to ten — both ethyl and methyl would fit. You only need to partly clean a window with the final liquid to spot the difference — this part will be dry and clean, whereas the surrounding will get fogged up quite quickly.
References: Links and sources