How to Defog Your Windshield in Rain or High Humidity

March 4, 2022

The last thing you want when you're running late is your windshield fogging up. It's something most people forget about until they start driving and realize they can't see! In these instances, you'll want to know how to defog your windshield fast. Luckily, we've got you covered. Read on to learn what causes windshield fogging and how to defog your windshield when it's raining or in high humidity.

What Causes a Foggy Windshield?

Ready for a quick physics lesson? Windshield fogging occurs due to the change in temperature and moisture levels between the inside and outside of your car. Fog on your window is really just condensation-like on a cold beverage or on your mirror after a hot shower. The fogginess occurs when warmer air-which holds a lot of moisture-mixes with colder air, causing the water vapor in the warmer air to condense. This is called the dew point-the point at which the air becomes so saturated with moisture that it forms water droplets.

So how does this work when it comes to your windshield fogging up? When you get into your car on a cold morning, your body heat and the moisture from your breath are usually enough to raise the temperature and humidity inside your vehicle to the point of fogging. When the warm air inside your car comes into contact with the cold air around your windshield, it deposits moisture on the inside of the glass in the form of condensation.

The colder it is outside your car than inside, the lower the relative humidity is, making it easier for water vapor in the air to condense into liquid water on your windshield. Things like wet clothes and umbrellas, snowy boots, or additional passengers will also raise the temperature and moisture level inside your vehicle and increase fogging.

But window fog is not just a cold-weather occurrence; your windshield can also fog up in warm weather. In this instance, fogging occurs for the same reasons, but in reverse. Warm, humid air from outside will condense against a windshield that's been cooled by air conditioning inside the cabin. When this happens, the fogging will occur on the exterior of your windshield rather than the inside.

How to Defog Your Windshield in Humidity or Rain

Humidity is the biggest obstacle when it comes to defogging your windshield. Rain makes the situation worse, not only by obscuring the view out of your windshield, but by raising the humidity in the air and making your windshield colder. The key to eliminating windshield fogging is to equalize the temperature and moisture levels on the inside and outside of your windows. However, the best technique to use will depend on whether the outside of your vehicle is colder or warmer than the inside.

Defogging in Cold Weather

If the air outside is colder and drier than inside your car, fogging will occur on the inside of your windshield. Try these steps to defog your windshield quickly:

• Activate defrosters & turn the heat to maximum. Since warm air holds more moisture, turning on the heat will help carry more moisture out of the vehicle and blow drier air against your windshield.
• Turn the A/C on. The air conditioner removes moisture from air passing over its cooling coils. Many vehicles automatically turn on the A/C when the defroster is active for this reason.
• Turn off air recirculation. Air recirculated through the cabin won't lose any moisture. Make sure this feature is turned off so you can exchange the warm, humid air inside for the cool, drier air outside.
• Crack open your windows. If it's not unbearably cold, try cracking open the windows to speed up the exchange of air and defog your windows more quickly.

Defogging in Warm, Humid Weather

If the air outside is warmer and more humid than inside your car, fogging will occur on your windshield's exterior. Try these steps to quickly clear the windshield:

• Use your windshield wipers. The wipers may help clear accumulating fog while you try to equalize temperatures.
• Turn down the A/C. Lower the air conditioning to its least-cool setting or even turn it off completely to match the humidity in your car with the outside.
• Turn off air recirculation. Bringing in air from the outside will help equalize temperature and moisture levels in the cabin.
• Crack open your windows. If the above steps aren't working, opening the windows for a short time is the fastest way to equalize conditions inside and outside your vehicle.

Failing Parts That May Prevent Your Windshield From Defogging

If you've tried the methods described above and haven't been able to defog your windshield, it may be because of a failing part. Any number of damaged parts in your HVAC system could be to blame. Here are some common causes for defogging trouble:

• Bad A/C receiver drier
• Fresh air intake at the base of your windshield is blocked by debris
• Clogged dashboard vents
• Low levels of anti-freeze
• Broken thermostat in your engine's cooling system
• Bad heater core
• Bad blower motor
• Damaged rear defroster grids

A foggy windshield makes it difficult to see the road in front of you. Defog your windshield with these tips, and get your Nissan cold-weather ready with genuine OEM replacement parts from Nissan Parts & Accessories.