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Picture this: You wake up in the morning to go to the gym or go to work and the moment you turn your car on, the windows fog up. Now, you have to figure out how to defog your car’s windows before you can get moving. Odds are, you’ve been in this exact situation at least a couple of times. Whatever your past experience with foggy windows, we’re here to tell you the best ways to defog them.

Why Do Windows Fog Up?

Windows can fog up for many different reasons. It all boils down to some finicky science that basically just proves how frustrating Mother Nature can be sometimes. But, at the same time, it’s not all that difficult to overcome the problem once you understand the cause. Here are the four most common reasons why a car window might fog up.

Temperature

If there’s a significant difference in temperature between the inside and outside of your vehicle, there’s a good chance that your windows will fog up. In other words, a hotter interior can make for fog. The higher that difference is, the higher the chance is that you’ll have to deal with foggy glass. But why? That answer is actually simpler than you might think.

When hot air and cold air meet, the two temperatures affect each other and form condensation. When there’s a layer of glass in the way, that condensation forms on the glass instead. This is particularly common in more moist or humid climates. So, if you live in Florida and wonder why your windshield fogs up every single Summer morning, it’s probably because of the 90% humidity.

Air Quality

Cabin air filters tend to filter out all the little particles (like moisture and smoke) that could stick to and fog up windows. Unfortunately, cabin air filters weren’t really commonplace until the turn of the century. So, cars manufactured before 2000 are likely to have more problems when it comes to foggy windows.

In smoggy areas, you’re likely to see your windows fog over a little more than others. As some of the smoke and fog gets into the cabin of your vehicle and sticks to the interior sides of the windows, visibility can be reduced. This is much less of a problem for newer vehicles with more modern air filters.

Passenger Capacity

The more passengers in your vehicle, the more likely the windows are to fog over. Many people find this claim hard to believe, some might even call it a myth, but on the contrary! It’s tried and true. This goes back to what we said earlier about the temperature difference between the inside and outside of the vehicle.

The more people you have in the vehicle, the more people in the vehicle are producing body heat. The more those people breathe, sweat, or move around, the more condensation is likely to form on the vehicle’s windows. Have you ever exhaled on a window and seen the fog form? That’s basically what’s happening here, except on a larger and slower scale.

Dirty Windshield

Dirty windshields are dangerous for a variety of reasons, but we’re here to focus on one: If your windshield is dirty, that dirt can soak up moisture in the air and lead to foggy windows. Of course, it really depends on what you mean when you say your windshield is “dirty.”

If your windshield has literal dirt on it, it’s likely to turn muddy before it fogs over (which can be an even bigger problem as it gums up your windshield wipers). On the other hand, if you have leaves and twigs on your windshield, those naturally moisture-carrying debris can increase the likelihood of your windshield fogging over.

How to Defog a Windshield: Hot Temperatures

Temperature differences are the number one cause of foggy windshields. So, let’s take the time to talk about the process for defogging your windshield when the outside temperature is hotter than the inside temperature. Examples of when this might be the case include summertime travel and when parking in direct sunlight.

Step 1: Use Your Windshield Wipers

First and foremost, activate your windshield wipers. As we mentioned earlier, the biggest contributor to foggy windows is moisture. Well, windshield wipers are quite literally designed for the specific purpose of removing moisture from your windshield.

Turning on your windshield wipers causes them to scrub the moisture (and any potential debris) off of your windshield. This provides immediate relief for foggy glass—but that relief is only temporary. The windshield wipers remove the condensation that has already accumulated, but they don’t stop more from accumulating.

Step 2: Turn Down the AC

Now that you have some temporary relief, thanks to the windshield wipers, the goal is to stop more moisture from condensing. When it’s hot inside the vehicle, turning on your AC can help to quickly reduce the amount of moisture building up on the windshield.

Turning your AC to the coldest temperature possible will help speed the process up. Once the fog starts going away, you can dial back the AC and bring it to a more comfortable temperature. Depending on the weather, that should be enough to significantly reduce the amount of condensation and fog on the vehicle’s windows.

Step 3: Switch Off AC Recirculation

In some cases, you’ll also need to turn off recirculation in your car. This is a common practice for drivers in particularly humid and moist environments where the weather tends to throw a little extra condensation your way.

The recirculation button looks similar in most vehicles, but “similar” is not “the same.” What might be a circular arrow icon in one car might be a back arrow under a front arrow in another. And the list of possible designs is pretty much infinite. Your best bet is to know your car’s features inside and out and, if necessary, read the manual (or just Google it).

Switching off recirculation helps to balance the temperature and moisture levels on the inside and outside of the vehicle. That will, in turn, help to reduce the amount of condensation that builds up on the windshield.

Complete all three of these steps correctly and odds are you’ll not have to worry about any fog for the rest of your drive. Unfortunately, this can take some time to take full effect. Give it about 5 to 15 minutes before you decide the process didn’t work for you.

How to Defog a Windshield: Cold Temperatures

Summertime isn’t the only time windows fog up. Winter can be even worse. In winter, not only can the hot air inside your vehicle mix with the cold air outside to cause condensation and fog to build up; the frigid air outside the vehicle can actually freeze that condensation. If you thought fog was bad, just wait until your windshield is as white and opaque as snow.

Step 1: Turn Up the Heat

Unlike with fog caused by warm temperatures, your windshield wipers aren’t going to be much help in this situation—but your AC sure will. As soon as your engine warms up enough to start producing hot air for your AC to vent into the cabin, pump the heat up as high as it can go. We’re talking the absolute highest, hottest temperature your vehicle can safely produce.

The goal is to pull as much moisture from the outside air as possible to equalize the interior and exterior moisture levels. The temperature change should happen quicker and facilitate that change faster if you set your heat to the highest possible level.

Of course, you probably won’t be comfortable driving for long periods of time if you leave the heat at that level. So, once you’ve seen the fog start to recede, turn your heater down to a more sustainable level.

Step 2: Switch Off AC Recirculation

Once your heater has started the process and set the temperature to the ideal range to fight back the fog, it’s time to fully correct the moisture levels. Odds are, the moisture inside the vehicle will be higher than the outside of the vehicle, especially after step one. To correct that, turn off the recirculation feature of your vehicle’s AC.

This will cause your vehicle to pull air in from outside of the vehicle, bringing with it cooler, drier air. The key is to bring in enough dry air to balance out the moisture levels but not too much so as to cause the temperature to drop and fog the windows up even worse.

Step 3: Open Your Windows

As you drive, your body heat, breath, etc. will increase the temperature and moisture within the vehicle. Eventually, your windows might start fogging up again despite having done steps one and two perfectly right. When that happens, open your windows.

This tends to help rapidly defog windows. It draws in the outside air (cold and dry) and pushes out the inside air (hot and humid), thereby balancing things out. To do this right, however, you’ll need to find the middleground. Let too much air out and you risk having to start over from step one. Don’t let enough air out and you risk not fully defogging your windshield.

Step 4: Hit the Defrost Button

Of course, if your vehicle has a defrost button, you’ll want to press it. This feature is common in newer vehicles, especially when manufacturers intend to sell those vehicles in colder climates (like the northern United States). 

What it does is simple. Most of these features are enabled by little black lines that crisscross the back window. Each line contains a small electronic wire, which heats up to quickly equalize the temperature and reduce humidity. Windshield features tend to take the form of a special vent that blows air on the inside of the windshield to achieve a similar effect.

Tips To Prevent a Foggy Windshield

Now that you know what causes a foggy windshield and how to defog one when the situation arises, it’s time to learn how to prevent your windshield from fogging up in the first place. Here’s a table containing a few tips and tricks to do just that. After all, preventative maintenance does tend to be the best maintenance.

Tips and Tricks	Explanation
Use an ammonia-based window cleaner.	The ammonia in the windshield cleaner fluid helps to reduce buildup on your windshield, which in turn reduces fogging levels.
Rub shaving cream on your windshield.	When applied to the inside of your windshield, shaving cream creates a protective film that prevents condensation from forming. Of course, be sure to wipe off the shaving cream before attempting to drive.
Activate the defroster.	Do this while warming up your car and you should see much faster improvements in the amount of fog on your defrosted windows.
Don’t use the recirculation AC setting.	Vehicles may reuse humid or moist air if the recirculation setting is on. Disable it to prevent this issue.
Wipe off your shoes.	Removing wet dew or snow from your shoes can prevent the accumulation of moisture in a vehicle. Remember: high moisture levels are one of the leading causes of window fog.
Get your HVAC system inspected.	A malfunctioning HVAC (a.k.a., AC) system can hamper efforts to defog car windshields. The better your AC system works, the better it will be at defogging foggy windows.