A thread on UrbanLegends.com
http://www.urbanlegends.com/science/glass.flow/glass_flow_the_thread.html
I don’t know the scientifc reasons, but I know the following about my Honda. If the condensation is on the outside of the windshield, I use the defogger. The AC won’t help. If the condensation is on the inside, I use the AC until the heater warms up sufficiently, at which time I don’t need either. Condensation on the outside is no big deal. That’s why we have wipers. But I don’t have wipers on the inside of my windshield. The defogger alone won’t clear that up quickly. I use the AC, and if it is a cold day, I’ll use both. There’s no harm blowing the hot air through the AC. The AC dries out the air inside the car, so the inside condensation disappears. The defogger emits moist air and makes the situation worse until the air gets sufficiently hot, unless you run it thru the AC first.
Look RESOL
YOU
ARE
WRONG
Cars are NOT AIR TIGHT. Otherwise you would suffocate. (what does ‘for all practical purpose’ mean? thats like when people say ‘with all due respect…’ or ‘no offence but…’ before insulting someone. It doesnt mean anything.)
Glass isnt a liquid (thank you Kyberneticist)
Even if it were slightly permiable, it wouldnt be enough to let in enough moisture to cause condensation, but its not. The metal on cars rusts because it reacts with the oxygen. The oxygen doesnt ‘get into it’. Paint isnt permiable is it? why do we paint stuff to stop it rusting?
Go back to school, learn physics, and then come back and agrue with me.
sorry, PS
Barbitu8
hot air does not dry out the air in the car. The water doesnt go away, it just evaporates so it is still in the air. If you put dry air into a car, it will eventually push out the humid air, but this is not what you are suggesting.
well Kyberneticist I think you need to look at what a amorphous solid is. This comes from Material Science and engineering (fifth edition) by William D Callister Jr. copyrighted year 2000
Sometimes such materials are called amorphous solids or supercooled liquids, inasmuch as thier atomic sturcture represents that of a liquid
This is refering to glass
next from
Elliott, S. R. (1994) Amorphous Solids: An Introduction.
we have
An amorphous (or synonymously, non-crystalline) material can be defined as one which is topologically disordered and which does not exhibit either the long-range translational order (periodicity) characteristic of single crystals, or the long-range orientational order characteristic of quasicrystals. Within this definition, such materials could be either solid or liquid
[…] Amorphous solids are characterized by a topological disorder, so there is no long-range order (or periodicity) in their structure. However, this does not mean that amorphous solids are structurally completely random (i.e., gas-like) at all length scales. In fact, covalent materials, in particular, exhibit a rather high degree of structure organization at length scales corresponding to several atomic separations […]. On the other hand, materials characterized by non-directional centro-symmetric interatomic interactions, e.g. metals or completely ionic materials, are intrinsically much more disordered even at short length scales.
Jesus RESOL, this is a thread about condensation in cars, not whether glass is a liquid or a solid or whatever. This doesnt even need to be discussed, as its been flogged to death in discussion before, and Cecil has cleared it up anyway.
Maybe you do know about glass (or maybe you just copied it all out of a book) but everything youve written so far has been crap.
I think there is something missing in this disscussion. There are several things going on, and people are getting bogged down on one detail while ignoring others.
First, as stated in the OP, condensation occurs when moist air comes in contact with a surface with a temperature below the air’s dewpoint. There are several ways to change this.
(a) decrease the available moisture in the air using the A/C. (actually reduces the grams of water per pound of air, but leaves the air saturated and cold with a low dewpoint)
(b) increase the dewpoint of the air by increasing the air temperature, using the heater. (Dewpoint goes up, grams H2O per lb. of air stays constant)
© increase surface temperature until above dewpoint using in glass defogger or heater.
Just the A/C isn’t all that effective. You need the heater for reheating the air after dropping some of the moisture out. If it isn’t wet outside having the A/C on won’t do anything useful and could be slowing down the process of heating the air and glass.
Just the heater/defrost is best if the air (inside and out) is fairly dry, but if it is really moist outside (not just cold), or there are a lot of people inside the car breathing heavy then it probably will take a long while to work, whereas with the A/C (and recirc to minimize outside air) it will probably work faster.
Slowly heating the glass (only rear windows in most cars I have seen) will get rid of the condensation inside and out by raising the glass’s temp above the dewpoint. It is slow to allow the heat to spread out, and prevent stress from thermal expansion. I have heard of car windows and windshields having lots of internal stresses in cold weather, and being more subject to shattering from incidental contact.
Inertia:
"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."
Perhaps I wasn’t very clear. You’re right of course, simply heating the air up can’t create water out of nowhere.
But if you have liquid water present in contact with air, e.g. a pool in the bottom of a sealed tupperware box, then that water will evaporate into the air in the box until the equilibrium partial pressure of water vapour is reached. Such air is often said to be “saturated” with water.
(Or if you leave the box open, the air never becomes saturated and all the water will eventually evaporate.)
The equilibrium partial pressure is temperature dependent - raise the temperature and it goes up. More water will evaporate into the air in the box until it is saturated again.
Now, if you have rain, fog or dew, the air is going to contain enough water vapour to be already saturated or close to it. If you suck this air into your car heater (excluding liquid water) and heat this air up, the amount of water vapour it contains will be unchanged but it will no longer be close to saturated.
However, if your ducts contain liquid water because of rain or dew, pushing warm air through them will raise the amount of water vapour in that air, possibly to its new, higher equilibrium partial pressure.
Pushing cold air through them won’t raise its humidity if that air is already saturated - it can’t.
Don is right, if the air is hot, it holds more water so the water will not condense in the first place. If it cools down, the air can hold less water, so the excess water will condense.
Some cars have a “recycle” or “recirculate” button. This causes the interior air to be recirculated instead in the heater, instead of having the heater take in outside air. The stated purpose of the rec button is to warm the inside of the car faster (since it is not taking in cold outside air).
It may warm up the inside of the car a smidge faster in cold weather, but its major effect is to fog up the windows. All other suggestions are fine, but make sure the rec button, if you have one, is not mashed.
I never said that the hot air will dry the car out. It works because, as you said, warmer air can contain more moisture (higher dewpoint).
Inspite of what Engineer Don posted, I know from experience that the AC alone will eliminate compensation inside the car, and, I might add, instantly.