Ever got to your car in the morning and found the inside of the windshield covered in a layer of ice? What’s worse, few things are scarier than having the windshield suddenly freeze up while you’re driving, completely blocking your view of the road ahead. The problem is compounded if you don’t have air conditioning and the engine is cold, so you can’t blast dry, warm air over the windshield to help clear it up.
Fortunately, you can use a few tricks to help clear your windshield or, even better, prevent it from frosting up in the first place. In this article, we’ll explore the physics behind why windshields fog and freeze up and what you can do to help in this situation.
How to Quickly Clear a Frosted-up Windshield
Even if you take all the preventative measures in this article, sometimes the windshield will still fog up and freeze suddenly. If it happens, don’t panic. Follow these steps to restore your visibility and continue driving safely and comfortably quickly.
Blast it with some warm, dry air
The first step to clearing the frost on your windshield is to melt it. The best way to do it is to throw in some warm air, so wait until the engine is hot enough and turn on the defroster. If you have air-conditioning, turn that on as well, and turn off the air recirculation. Do not attempt to drive with a frosted windshield, no matter how late you are running. It’s just not safe.
Wipe it with a clean rag
Once the ice in your windshield melts, the quickest way to get the resulting water off the windshield is to wipe it away simply. It’s a bit archaic, but it works. A good practice is always to keep a clean rag handy, as you never know when you might need it. But remember to renew it occasionally; a dirty rag might help take some excess water off the windshield in a pinch, but it will introduce oils and dust in the glass, making it easier to attract more condensation.
You don’t want to be wiping the windshield your entire journey, do you? Luckily, there are a few tricks to prevent your windshield from freezing over from the inside, but in order to get to them, you must first understand why frost forms on the inside of the windshield in the first place.
Why Frost Forms on the Inside of the Windshield
Frost forms inside the windshield when the humidity in the air inside the cabin condenses over the glass and then freezes. This usually happens in the winter because the colder it is, the less moisture the air can hold, making condensation easier. This condensation then freezes over more easily due to the low temperature.
Have you ever seen small water droplets build up on the outside of a cold bottle in the summer? Or condensation forming on the bathroom mirrors and tiles when you take a warm shower? Of course, you have. The same phenomenon is behind condensation forming on the inside of cold car windows; understanding the physics behind why condensation forms inside of the windshield is the first step towards becoming proactive at stopping it from happening.
1. Relative humidity, saturation, and the dew point
You must first know the key concepts of relative humidity, saturation, and the dew point. Relative humidity is a percentage that indicates how much moisture content is in the air compared to how much moisture the air can hold at that temperature.
Relative humidity of 100 percent means the air is saturated, and can no longer hold water. When that happens, that water has to go somewhere: it becomes condensation. The dew point is the temperature below which the air becomes saturated, assuming a constant amount of moisture and air pressure.
2. People in the cabin release moisture
Then there’s also the human factor. People, by their very existence, release moisture into the air, be it through breathing or sweating. Ever seen your breath after exhaling on a cold day? That’s because your warm breath is highly saturated with moisture, which immediately condenses as it meets the cold air outside.
Well, you and your passengers also breathe inside the car, which adds even more moisture to the already saturated cabin air. While we don’t recommend you stop breathing, it’s important to know how that affects your driving conditions, especially when it comes to fog on your windshield.
3. The temperature gradient across the glass
Still with us so far? Okay, so now imagine a typical winter day. You have the heat on, and the car’s occupants release moisture, so the air inside your car is warm, and the relative humidity is high. On the other hand, the outside is very cold, and therefore, so are your car’s windows. The warm, saturated air inside the cabin then meets the cold surface of the glass.
If the temperature of the glass is below the dew point of the air around it, condensation will form. So, basically, condensation will form on the inside of the windows if the temperature outside is below the dew point of the air inside.
Sounds complicated, but the takeaway is simple: water vapor on warm, humid air that encounters a cold surface will condense on that surface. Now that you know this, you can work the physics in your favor to stop your windshield from condensing and frosting over.
How to Prevent Your Windshield from Frosting Up Overnight
Arriving at your car in the morning and finding out that the inside of your windshield is frozen up can be quite annoying. Murphy’s Law essentially mandates that it will happen whenever you are in a hurry. Here are some handy tricks to make sure it won’t happen again so that you can be on your merry way.
1. Keep the inside of your windshield clean
Moisture likes to attach itself to dirt particles. Keeping the inside of your windows clean creates less of an ideal environment for water to adhere to. When you wash your car, make sure to clean the inside of the windshield as well. You can use a dedicated windshield cleaning product, but normal glass cleaning fluid and a lint-free rag will work just as well.
2. Apply a water or fog repellent
There are many products in the market that help repel water, keeping your windshield clear. The most popular of which is Rain-X. Using it is rather easy and basically involves cleaning the surface thoroughly, applying the product to a dry cloth, and spreading it evenly across the windshield with a circular motion.
Be careful to follow all the instructions in the bottle for the best effect – it’s well worth the effort. There are specific anti-fog products you can use as well. Check the labels to see if they can be used together for maximum effect.
You can apply it to the inside of the other windows as well, to ensure maximum visibility even in terrible weather, or to the outside of the windshield to help the wipers with their job of clearing standing water (or, you may not even need to use them at all!).
As an alternative, you can put some shaving cream on a clean rag, smear it on the windshield, and then buff it out so it leaves no streaks. It will also act as a water repellent of sorts and is cheaper, though it might not be as effective as a dedicated product.
3. Remove and dry your floor mats
During the rainy and snowy winter months, extra doses of moisture are usually introduced into the passenger compartment in the form of wet or snowy clothes and shoes. Once the cabin heats up, they evaporate, becoming the dreaded airborne moisture that then deposits itself on the windows.
Whenever you can, clean off all snow from your boots and jacket before getting into the car, make sure to shake off excess snow from floor mats and leave wet mats outside to dry in the sun.
4. Use a moisture absorber
Another way to get rid of the water inside your car is to use a moisture absorber. You know those little silica gel packets you have in your closet, and which often come in shoeboxes? That’s what they are for. Please don’t throw them away; they help dry your car’s air. Be mindful that silica gel harms your health if ingested, so be careful to keep it away from children and pets.
If you have a cat, you can also use an old sock filled with silica kitty litter and tied into a knot. Just make sure both the sock and the sand are clean, obviously! You don’t want to introduce bad odors in your car.
5. Check for – and stop – any water leaks
As you have probably concluded by now, any source of moisture is your enemy when it comes to keeping frost away from your windshield. Other possible sources of humidity inside the car are worn or cracked window and door seals, as they will let rainwater inside the cabin. It is a good idea to apply some rubber reconditioning product on the seals occasionally, as that will keep them fresh and malleable, helping them seal better.
Coolant leaks from the heater core or its tubing can also introduce water into the vehicle, so keep an eye out for wet carpeting or a strong, sweet smell, which may signal a coolant leak inside the cabin. What’s worse, the warm, defroster air may carry a fine mist of coolant along with it, which is quite greasy and will stick to the glass like glue. If you find that the inside of your windshield is covered in a sort of thin, foggy film that keeps coming back no matter how much you clean it, you probably have a leaking heater core.
Finally, make sure the air conditioner drain hose is unclogged, as that can also cause water to pool up inside your car. A handy trick is to turn off the air conditioning a few minutes before you park your car, but leave the ventilation fan running. That will eliminate any condensation that may be left on the evaporator and prevent water from being left behind. Another positive side-effect of this is that you are less likely to develop mold and/or bacteria that cause your ventilation system to smell bad over time.
6. Replace your cabin air filter
Most modern cars have a filter for the heating and ventilation of air. If this filter is old and clogged, it may cause dirt particles to fly into your windshield when you turn on the defroster. We’ve already seen how a dirty windshield is more susceptible to getting foggy. Try occasionally cleaning off the filter housing with compressed air, and replace the filter every few years, according to your manufacturer’s recommendation.
How to Stop Your Windows from Frosting Up While You Are Driving
So the weather is bad and you have to drive on the highway. Have you had problems before with the windshield suddenly fogging up and robbing you of all visibility of the road ahead? Are you scared of it happening again? Then, this section is for you.
This can be summed up in most modern cars in one sentence: simply turn on the defroster. The car will automatically juggle temperature, humidity, and airflow, targeting the windshield with warm, dry air. However, driving an older car is not an option, and you might have to follow these steps manually. Here’s what you need to do to keep your windshield dry in the winter:
1. Turn off the air recirculation
Recirculation is perhaps the most misunderstood setting in a car. Still, its function is crucial for managing the moisture levels in the cabin and preventing condensation from forming on the windshield. When the recirculation is on, the car reuses the same interior air for ventilation. When it’s off, air from the outside is used, completely renewing the cabin’s air every few minutes.
Recirculation is more efficient than heating or cooling the outside air and is a good idea if you’re driving through an area where the outside air is contaminated, but otherwise, it has a few downsides. The first is that the interior air can become stale from being recycled too much.
The second, and the one that affects windows fogging up, is that it reuses the same air that you keep adding water vapor to (by breathing), increasing the relative humidity of the cabin air. By now, you know why that is a problem. You want to turn recirculation off to eliminate the humid, saturated air inside and replace it with drier outside air. Yes, even if it’s raining or snowing outside, this is better than simply recycling the interior air.
Have you ever looked at the button with a little symbol of a car with an arrow turning a circle inside of it and wondered what exactly it does? That is the air recirculation button. You want to press the button so that the light is off, indicating that recirculation is deactivated.
To further add to the confusion, some cars instead have a button with a little arrow coming into the car from the outside, which is basically the opposite thing. In this case, you want the light on the button to be on, indicating that the car is pulling air in from outside. Most modern cars will monitor the quality and humidity of the air inside and outside the cabin and automatically adjust the in-flow of ambient air to keep the cabin fresh.
2. Turn on the heating
Once you have ensured that the car is pulling in fresh, drier air from outside, you want to crank the heat up to the maximum and aim it at the windshield. This serves several purposes.
For starters, it will warm up the cabin. The best way to stop condensation from forming on the interior surfaces is to prevent the air inside the cabin from becoming saturated in the first place. Warm air can hold more moisture. Therefore, keeping the interior warm will help the water vapor to stay airborne, mixed in with the air, rather than depositing itself on your windshield in the liquid phase.
Secondly, the warm air heats up the windshield, reducing the temperature difference between the air inside the cabin and the surface of the glass. Remember, the cold glass causes the air temperature near the windshield to drop below the dew point, creating condensation; heating up the windshield makes that harder to happen. Finally, the warm airflow will actively drag moisture away from the windshield.
3. Turn on the air conditioning
As the moisture in the air passes over the cold evaporator piping in your air conditioning, it condenses (again, the same principles are at play here!). This process pulls humidity out of the air, which means the A/C on your car acts as an air drier of sorts. This is obviously beneficial if your main goal is to dry the air inside the car as quickly as possible.
Mind that turning the air conditioning on doesn’t necessarily mean you have to be freezing cold in the middle of winter. You still want the heat to be on max to blast warm air on the windows. In this situation, the A/C isn’t there to make the air colder but rather simply to remove moisture from the air, effectively acting as an air dehumidifier. Most modern cars will automatically activate the air conditioning when you turn on the defroster.
4. Focus the ventilation airflow on the windshield
If you want your windshield to defog extra fast, turn off the airflow from the front and side vents in the dashboard (the ones that face toward you). This will focus all the available, warm airflow directly on the windshield, helping to warm it up and reducing the temperature difference between the glass and the interior air. Again, most modern cars will do this automatically when you select the defroster, but if you drive an older car, make sure to do this manually.
Why am I getting frost on the inside of my windshield?
Frost forms on the inside of the windshield when the moisture in the cabin condenses over the glass and then freezes over. Condensation deposits on the inside of car windows when the glass temperature (roughly equal to the temperature outside on a very cold day) is below the dew point of the air inside the cabin, causing the water vapor in the air to condense over that surface.
Is it normal to have frost on the inside of windows?
Yes, having frost on the windshield inside when you arrive at your car on a cold morning is perfectly normal, especially if you park outside. However, if you keep getting a lot of condensation inside your car, it’s possible you have a water leak somewhere. This will generally come from either the door/window seals or from the heater core.
How do I stop frost on the inside of my windshield?
Stopping frost on the inside of the windshield prevents condensation from depositing over the glass in the first place. To pre-emptively stop condensation on the inside of your car windows, keep the windows clean from the inside and try to get rid of all the humidity in your car.
How do I get rid of condensation on the inside of my windshield?
Other than wiping it with a clean, lint-free rag, you can also turn on the defroster setting on your car to dry the cabin air and warm up the windshield. If you have an older car, also make sure to turn off the air recirculation manually, crank the heat up to the maximum, turn on the air conditioning, and turn off the front-facing vents on the dashboard. When you activate the defroster, modern cars will automatically do all these things for you.
Victor is a Mechanical Engineer BSc and certified Automotive Mechatronics technician from Portugal. He worked as an intern mechanic for Volvo and has been fixing his own vehicles for over 10 years. Writing and cars are his passion, so now he combines the two by creating content around the automotive industry. He specializes in automotive technology and maintenance.