Winter has come, so I thought it would be nice to throw out some practical knowledge to keep you safe while shedding some light on something that happens every day in the colder parts of the world. We all like to play with foggy windows in the car, but have you ever wondered why that happens? And also, if you draw in the fog with your finger, why will it still be there when the fog returns?
Air consists of a number of different elements and molecules, including water vapor
Air is invisible (usually), but you can be certain it’s there from the feeling you get when you wave your arms around quickly, or hold a hand out of a moving car. On Earth the air is mostly nitrogen (78%) and oxygen (21%), but there is a lot of water vapor (1%) as well.
This water vapor has evaporated from lakes and oceans, so there tends to be more of it in the air near those large bodies of water. The measure of how much water vapor is floating in the air is called the humidity, of which there are a few different types. The one you are probably most familiar with is relative humidity, which describes the current water vapor content of air as a percent of the most water vapor air at that could be present at that temperature (this is the one that gets reported by meteorologists). But why is there a maximum amount of water content?
Water vapor is constantly condensing into a liquid and evaporating back to vapor, but the rates of these two processes depend on temperature
We’ve talked before about how the temperature of a material is describing how much the molecules that make up that material are wiggling. In a liquid the atoms and molecules are much less free to wiggle than they are in a gas because of how closely packed they are with their neighbors by comparison (solids are even more tightly held, and so typically wiggle even less). Even so, the molecules near the surface of a liquid can sometimes wiggle free of their neighbors and escape into the air above—this is the process of evaporation. The hotter the liquid, the more vigorously the liquid molecules will be wiggling, and the more likely some will be able to get free.
(But wait–a pot of boiling water on the stove has its bubbles rising from the bottom of the pot. That’s because the heat is greatest here, and some water molecules are pushing far enough apart to make a small region of vapor, that then rises towards the top of the liquid because it’s now less dense than the surrounding liquid. Sometimes these bubbles dissipate before they reach the surface—that’s because the surrounding water absorbed the extra heat energy of the bubble as it was moving, causing the water vapor to reform into liquid water.)
At the same time, some water vapor above the liquid surface will manage to wiggle into the liquid, give off some of its heat by colliding with its neighbors, and as a result become part of the liquid again. This is condensation, or the opposite of evaporation.
Both of these processes are always happening when there is liquid water and water vapor in the air above it. Which one wins is related to how much pressure they exert: a lot of water vapor in the air will exert pressure on the liquid water, which can keep the liquid water molecules from escaping. It turns out that the rate of evaporation and rate of condensation don’t scale with temperature in exactly the same way, so in warmer air the evaporation wins and you end up with more water vapor present in the air than if the air was cooler. Note that it isn’t really right to say that warmer air can hold more vapor, as you will often hear, since the air didn’t have much to do with it—it’s all about the relative rates of water evaporation and condensation.
If air is cooled quickly it will condense some vapor into liquid form—this occurs when cold air comes near a warm windshield
So far we’ve just been talking about water vapor in the air above liquid water, but condensation can happen on to a solid, too. In the case of your windshield, the air nearby on one side will most likely be warmer than the glass surface. If the surface is cold enough, water vapor that impacts it will lose enough heat energy to condense down into its liquid phase, forming a fog layer.
Say it’s a warm, humid day outside, so evaporation has beat condensation due to the temperature, meaning there is a lot of vapor in the air. You probably have your air conditioning set to cool inside the car, so the windshield is cooler than the outside air. The warm humid air outside the car will condense onto your cool windshield—this usually isn’t a problem, though, since windshield wipers will get rid of it just fine. If you wanted to mitigate the effect you could blow hot air onto your windshield to make outside air molecules lose less heat when they impact it, but most people will probably pick running the wipers over sweating inside the car.
To defog a windshield on a cold day, you want to evaporate the water with heat or remove water vapor by blowing dry air
Temperature isn’t the only effect, though: we have to think about the humidity of the air inside and outside of your car as well. On a cold day, the air outside has lower water vapor content due to its temperature. Inside the car you probably have the heater on, and what’s more you’ve tracked in snow on your boots and are releasing vapor into the air through your breath. You can see your breath in the winter because the high water vapor content in the warm air from you lungs is condensing in the cold outside air, forming fog.
Eventually there will be high relative humidity inside the car, and this water vapor will condense on the inside of your cold windshield. The first instinct is to blow hot air onto the glass, but sometimes this can be dangerous! If the air isn’t hot enough (which it won’t be if you just turned on your car) then you’re just blowing more humid air at the windshield, giving more water molecules the chance to stick to the windshield. Once the air heats up enough it will start to evaporate all that water, but it might take a while depending on your car, so be careful!
Actually, you can also get rid of this inner condensation on a cold day by blowing cold air. In this case you’re taking advantage of the constant evaporation and condensation going on at the window surface—cold air doesn’t contain as much water vapor, so you aren’t adding much that will condense, but you are blowing away the evaporating air, which will quickly get rid of the fog. Importantly, you can remove condensation with heat to speed up evaporation or with moving air to get the already evaporating water out of the way.
That’s not a comfortable way to drive on a cold day, though, so your best bet is to use the defrost setting on your car. Modern cars run the defrost by taking air from outside the car (which has less water vapor content, remember) and running it through the air conditioning unit, where the vapor in the air condenses its water vapor onto the evaporator coils. Then this dry air is run near the engine to be heated before being blown onto the windshield. In this case the air is hot, dry, and moving, which are all conducive to evaporating air and moving it away from the windshield. Actually, this is the same principle hair dryers work on—to evaporate something as quickly as possible, you want to blow hot, dry air across it.
Bonus physics—Dirty windows
It turns out that phase transitions, like that from gaseous water vapor to liquid water on a windshield, typically start around a little clump of dirt or dust, or a bit of roughness in the windshield. This is known as a nucleation site. It’s also related to why you can draw with your finger in the fog and it will re-appear the next time the window mists over—the oils on your finger prevent liquid water from forming just a little bit, so if they’re still on the glass you’ll see the picture or writing again! If you want to prevent your window from fogging up as much as possible, though, keep the inside clean!
Bonus physics—Evaporative cooling
The liquid phase of water wiggles less than the gaseous phase, which means changing water into vapor form is a process that requires heat—we call these endothermic processes. Since evaporation uses up heat, that heat often comes from whatever the liquid water was sitting on. Your body uses this to cool itself—evaporating sweat will remove heat from the skin underneath the sweat.