Defogging the windshield of a car would seem like a simple enough task, since most modern automobiles come with a 'defroster/defogger'* installed. But the truth is, it's quite difficult for most people to achieve a clear windshield because they don't understand why or how condensation
forms on a windshield.
Your windows can get steamy for two different (but related) reasons:
- It's too hot and humid inside the automobile, and so water particles accumulate on the inside of the windows, upon the relatively cool surface of the glass. This is what happens when you park the car at night to, um, talk with your passenger(s). The windows 'steam' up because water vapor which has escaped from your lungs due to your, um, breathing, is trapped in the hot, stagnant confines of your car's interior.
- It's too hot and humid outside the automobile, and you have your air conditioning (or 'defroster')* running at a very cold temperature. In this instance, the water vapor is accumulating on the outside of the glass, because the interior of your car is very cool, relative to the outside temperature. Water vapor condenses on cold surfaces.
These are two distinct problems, but you can use your defroster, or any fan directed at the windshield, to accomplish windshield clarity, as I will describe. It is amazing (to me) that proper defogging techniques are not taught in driver's education classes (or at least, they were not taught to me), because not everybody understands the physics involved. I, admittedly, only understand a little bit about it, but I have developed a system for defrosting a windshield properly, efficiently, and effortlessly. And I share it below. But remember, if your windshield is compromising your view of the road, pull over to solve this problem -- do not attempt to solve the problem while also driving. That could be dangerous.
One common mistake in windshield-defogging is the belief that there is something wrong with the windshield defroster. There's not. The problem is you. I'm sorry, but I'm just being honest. You turn it on at full blast set for the coldest temperature (or the hottest temperature), and then get all pissed off when, after about thirty seconds of blissfully clear glass, you are dismayed at new formations of condensate. You give up, thinking your defroster is not working. But you just didn't give it the right instructions, that's all.
The first line of defense should always be fresh air. Very often, fresh air, either let in through the windows or the vents, can eliminate condensation without the need for a 'formal' defrosting process. But sometimes this won't work; if it's raining; if you're particularly sweaty; if it's particularly humid outside; or if it's too cold outside to roll down the windows.
In the event that fresh air does not work, (and for both scenarios above), you must attempt to minimize the difference in temperature between the two sides of the glass. Follow this process:
Turn on your car's defroster (and if that does not automatically engage the air conditioning, if you have one, turn on the air conditioning too). This applies in any weather. The 'air conditioning' involves a dehumidifying process which is very helpful at windshield defrosting. If you don't have air conditioning, but rather, just a fan vent pointed at the windsheild, turn that on instead.
Now, the most important step: Find the right temperature! If it's hot outside, start with the temperature adjusted to the hottest setting until all of the fog disappears from the glass. Then, slowly begin moving it to colder and colder settings until you see the first hint of frost beginning to form on the windshield again. Immediately turn the temperature back ever-so-slightly warmer, and you should have found the equilibrium point. If it's cold outside, you'll want to do the opposite: Start with the temperature as cold as possible until all of the frost disappears, and then immediately begin adjusting the temperature warmer and warmer, until the first signs of condensation reappear.
*There seems to be some confusion about the difference between a 'defroster' and a 'defogger'. There is certainly a difference between fog and frost, but many people use the terms interchangeably in the context of an automobile's windshield. Technically, what I refer to in this article is fog or condensate, but I use the term 'defroster' sometimes because that's the term by which I have always known the contraption.
Thanks to generic-man and unperson for helping me clarify this.