You know when its a cold morning (but not frosty) and you start the car up and you get condensation on the inside of the windscreen?
I had an argument with someone about whether you should blow cold air or hot air on the window. I thought it should be hot air because the hot air makes the condensation evaporate, but she disagreed, saying that the condensation was caused by the different temperatures in the car, and that you should use cold air to equalise them. So we kept changing it, and what we agree DOES happen is that when you switch between them, it makes it much worse. I dont see why we should sit and suffer in the cold - what do you think?
God, you don’t know how many fights I’ve had with my husband about this. I’m glad to hear we’re not the only ones. His car seems particularly prone to condensation on the windshield, and for some reason is particularly bad at getting rid of it.
Here are my thoughts on the matter. (He’ll tell you I’m crazy.)
Condensation is caused when the temperature of the windshield is below the dewpoint of the air. Dewpoint is determined by how humid it is: how much water vapor is in the air. (Side point that becomes important later: Warmer air can hold more water vapor.)
Therefore, to get rid of condensation, you should raise the temperature of the windshield by blowing hot air on it.
EXCEPT that if you have a cool windshield, and start blowing warm, moist air from the defogger/defroster (such as you’ll get if it’s a rainy day) onto the windshield, it will hit the windshield and moisture will condense, and you’re worse off than when you started.
So sometimes blowing cool air (which can hold less moisture, and may be even dried because it’s gone through the air conditioner) onto the windshield will help remove the condensation. However, if it’s a really humid day out, and you’re blowing cool air onto the windshield, it’ll cool the windshield enough to cause condensation OUTSIDE. No prob if you just turn on the wipers, of course, except under less than ideal conditions (driving in the dark, or sitting in a parking lot yelling at your spouse to leave the damn climate control alone) sometimes you don’t notice WHERE the fog is.
So which should you do? My feeling is that if you just blast the windshield with hot air long enough, eventually it’ll clear up.
I don’t think the equalization argument works, because if it’s cold out and you get condensation, cooling the windshield MORE would just cause more condensation–I think cool works, as I said above, because the cool air is drier.
Cold air has less moisture than warm air. If you blow warm air, the moisture in that air, if there is enough, can condense on the cold windows. Eventually you warm the window and the condensed moisture evaporates.
If you blow cold [dry] air on the window, the moisture will evaporate off the window.
Nothing dries like warm DRY air, and to accomplish getting warm dry air to blow onto the windshield, you need to what kind of car you have…and this is important, so I’ll explain:
:)1. American car: Set fan to Defrost, set Temp. control to warm. In an American car, the defroster will automatically prompt the A/C compressor to come on, thus removing moisture from the air, and clearing the windhshield asap. Even when blowing warm air in defrost mode, the A/C compressor comes on and you get the most effective warm dry air you need.
:)2. Non-american: Generally, the “defrost” setting just blows “fresh” air on the window, and whether warm or cool, it doesn’t do a whole helluva lot. Set to “Defrost”, set Temp. control to Warm AND turn on the A/C, it’s usually a button or seperate selector.
Any discussion about inside/outside temp is useless since the most effective way to dry something is to blow warm dry air onto it. You’ll be filling the rest of the cabin with warm dry air too, and hence, that’s less moisture that can get on the window via condensation. Cool dry air works, but warm air holds moisture better. That is, it doesn’t make it condense on the glass. 
Generally, my statements about American and non-American cars are true, but check your manuals. Most will explain that if defrost alone is insufficient, turn on the A/C.
Nothing like a good ol’ disclaimer…
Let’s clear this up…pun intended:
The most effective wat to dry something is to blow warm dry air onto it. The most effective warm dry air comes from the use of the warm setting and turning on the A/C, which happens automatically in most American cars.
Defrost problems? Chances are it is a non-American car and you need to turn on the A/C.
WARM DRY air dries surfaces the fastest. Period.
How does you back glass get defrosted? Most cars use heat- generating defroster strips. Heat dries. Dry heat dries faster. Ever notice how effective a hot blow dryer is? Or a hot curling iron?
Your friend is wrong in her reasoning, but may be right about which is faster - it depends on the car, the weather and the windows.
The condensation is caused by increased humidity inside the car, generally because of all the moist air you’re breathing out.
The heater can be slow because it can increase the humidity of the air it delivers to your windscreen. The heater should just take the outside air, warm it up and send it to you dry. But if it’s raining or foggy or there’s dew inside the ducts of your car, this might not be the case - the hot air can increase the humidity inside your car. Eventually the windscreen will warm up and clear the condensation anyway, but it takes a while.
Cold outside air removes the condensation by a different mechanism - it reduces the humidity by getting some airflow through your car, and prevents the moist air you’re breathing out from contacting the windscreen by providing a “shield” of drier air. (You can also increase the airflow by opening the windows, which you tend not to do with the heater on.)
I see on preview that our American friends have cars which will cool the air with the A/C to dry it, then heat it up again before it blows it onto the windscreen! Greenpeace must love you guys!
I was going to suggest that you test the theory in dry weather, but don’t bother - get an American car.
Ok, water condenses on the window for two reasons.
-
The temperature of the window is cold enough to cause a phase change from the gas pahase to water phase, duh we all knew that
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The amount of water outside the car is less then the amount of water inside your car. (I’m talkin winter here, I live in northern NY) The natural tendancy is for the water to diffuse to the air on the outside using the window as a membrane. Since the window is not as permiable as the water would like it doesn’t work to well and the water collects on the window. Your car has more water vapor in the air due to your breath.
Now, where does this put us? Simple, To REALLY clear out the window you need to replace the air in the car with the dry air from the outside. The air has very little moisture and well absorbe the water from the windshield. So make sure that your car is using outside air and not recycled inside air. Cold or hot it doesn’t really matter. Unless it’s really cold out and the inside of the window freezes over, then you need to scrape the inside…
Ah, a moment of revelation! I’ve finally connected the dots! Not long ago, I realized that the defroster on my truck kicks in the AC, even if you have it all the way to hot. I noticed it because the AC really puts a heavy load on the engine, and I was annoyed by it.
However, my husband’s car (a Toyota) has a separate button for the AC.
Now I can’t wait for a cool, moist day to try it out!
My eternal gratitude to all of you!
I’ve tried both hot air and cold air to defrost the inside of my windshield, and after much experimenting, I’d have to say that hot air defrosts the inside of the glass more quickly.
As others have pointed out, though, it must be hot, DRY air. Hot, moist air just gives rise to more fogging of the windows (e.g. the stereotypical fogging of windows caused by two people making out in the back seat).
By increasing the temperature of the air near the windshield, you increase the ability of the surrounding air to hold moisture. The condensation on the inside of the glass evaporates, leaving you with a clear windshield.
I’ve always assumed that this is how the rear window heating strips work to defrost the window.
Thanks guys, I tend to agree with Caldazar. I would much rather use the heat technique on a cold day.
A slightly related tip, but have you ever seen those people on monday mornings furiously scraping the frost off their windshield? Why not just use hot water? I do that all the time.
I’d be careful about pouring hot water on your windshield to clear the frost off. I’ve heard that this can crack your windshield! When the windshield is really cold and you pour hot water on it, it does not expand uniformly and if the stress is too much it will crack. If you continue doing this I suggest warm water instead of hot water and don’t do it in really cold weather.
This is the correct method. You don’t know how many fights I’ve had with my girlfriend over this.
Philster beat me to it explaining the A/C-defrost connection in american vehicles. Boy did that piss me off when I figured it out several years ago. When it’s -20°F I don’t want my defrost handicapped with A/C. I want the truck warm NOW!
And I don’t get frost or condensation on the inside of the windshield. Just the outside. And after the A/C broke 6 years ago, the frost melts much faster.
If I wasn’t so lazy, I’d actually scrape the frost off manually. But then I’d be late for work.
I think hot air gets rid of the condensation faster, but the problem is waiting for the hot air to get hot. My old car (Toyota) took at least 5 minutes of idling for the heater to blow air that was warmer than the outside air. The a/c gets cold straight away, so I think the fastest way to clear the windscreen after you jump in is to use the a/c then turn the heater on once the car is warm. It works for me, anyway.
Hot air works best period. Warm air can carry more moisture laden air to a cold window which will then fog or frost up even more.
Perhaps instead of rushing around and trying to find ways to make things defrost faster we should be starting our cars a few minutes earlier to allow the engine to warm and have the windows clear. When you start the car and get cool air from the defroster it will have some effect but as soon as the temperature rises to where the thermostat circulates hot coolant to the heater core you get the best results. My car has an automatic system with a thermostat inside the car. As soon as the engine temperature is sufficient the defroster turns itself on.
There are some products on the market that will prevent condensation from forming on your windows, they are similar to what I use on my helmet visor. We know that having your motorcycle helmet fog up is a bad thing.
It drives me nuts to see people here doing the “Canadian Crouch” on winter days… this is where one scrapes the ice off the windows which invariably then fog up. Said person then drives away peering through the small clear spot on the windshield where the defrost is beginning to have some effect.
Drive safe, you are sharing the road with the rest of us.
Okay Rufus, im not stupid, i dont use BOILING water or anything. warm water is what I mean
matt: “The heater can be slow because it can increase the humidity of the air it delivers to your windscreen.” Humidity is the measure of the amount of water vapor in the air. Simply adding kinetic energy (heating) is not going to just create water. As for the sources of water you’ve cited - dew in the ducts, fog, rainy day - these same sources would affect cold air intake also. Let’s make some sense please.
RESOL: “Ok, water condenses on the window for two reasons. 1) … 2) The amount of water outside the car is less then the amount of water inside your car. [snip] The natural tendancy is for the water to diffuse to the air on the outside using the window as a membrane” What the hell are you talking about? Are you suggesting that glass is permeable? Get a grip.
Nukeman: “Why not just use hot water? I do that all the time.” Because if it is a particularly cold day and your water is more than tepid your windshield is going to shatter. WARNING: This can also happen with just the defroster if it is a VERY cold day.
honkytonkwillie: “When it’s -20°F I don’t want my defrost handicapped with A/C. I want the truck warm NOW!” Perhaps you should get a foreign car that expects people to think about whether or not they want the A/C on.
inertia: matt is saying that the condensation will evaporate into the air making it more humid. (it wont be very much though) Also, the sources of water will not affect cold air as much as it holds less water than hot air. Go and look it up.
I agree that resol is talking bollocks. resol, the air in the car should have the same humidity as the air outside.
No, it really wont. I do it all the time and warm water is not going to have an effect. Plus, windscreen glass is stronger and different from normal glass. What do you mean by ‘really cold day’? a really cold day is only minus a few degrees, ie not much different than 5 degrees, so not as big a difference as between warm water and boiling water, so a really cold day wont have that affect. Have you ever heard of an instance when the defroster has broken the rear windshield? I havent.
Kinetic engergy is not heat. Why dont YOU make some sense.
OK, the air inside has more humidity in the winter. First off it’s warmer (or it should be) inside your car than outside. Also since the interior of your car can be considered air tight (it’s realy not of course but for all paractical purposes) and since every time you breath you are realising moisture into the air, the air stays in the car, hence your car air has more moisture then the outside.
Secondly, if the problem is taken from the engineering point of view on the molecular level (the whole diffusion thing) you could say glass is permiable but it’s diffusion constant would be very low (everything is permiable not just noticibley, how do you think your car rusts? O2 has to get into the iron somehow) Remeber glass is technically a liquid (which is why it sags in old houses)