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Why Condensation Forms on Your Car's Windshield

January 14, 2020

Why Condensation Forms on Your Car's Windshield

It is awfully annoying to enter your vehicle only to find the windshield is covered in condensation.  Windshield condensation delays your travels, forcing you to either scrape off the moisture or blast the defrost until you can see clearly.  The question is why condensation forms on car windshields in the first place?  Let's take a look at why this phenomenon occurs.

An Explanation of Condensation Formation

The average person understands moisture levels combined with air temperature determine whether condensation forms on car windshields and windows.  However, most people do not understand whether condensation occurs when warm air contacts a cold surface or when cold air contacts a warm surface or even for an entirely different reason.   Condensation resulting from cold weather is the most common scenario.  In this situation, the moisture stems from the outdoor air.  During the night, night sky radiation occurs.  Here's how it works: the windshield temperature falls below the outdoor air's dew point.  If there is strictly dew, it is possible to hop in the car and merely turn on the windshield wipers to clear away the condensation.  However, if there is frost on the windshield, it will be necessary to scrape the exterior of the windshield.  Alternatively, you can let the engine idle for a bit, allowing the engine coolant to warm and generate heat through the vehicle's defrost vents.

Condensation on the Interior of the Windshield

If all of the vehicle's windows are shut, there is a chance condensation will form along the interior of the windshield.  However, interior windshield condensation is rarer than condensation along the exterior of the windshield.  The moisture is the result of the air within the vehicle.  Here's how this phenomenon works: if the vehicle interior is excessively damp and the nighttime outdoor air temperature falls below the temperature of the air within the vehicle, the windshield ends up being chilled by the outdoor air.  This phenomenon can occur even if there is minimal night sky radiation.

Humidity Matters

Humidity leads to the buildup of water that transforms into water vapor, leaving condensation on the windshield.  If several people are in the vehicle, their natural breathing will heighten the chances of windshield fogging simply because that many more people will be breathing humid air.  If there is 100% relative humidity and the temperature decreases, the water will look for a way out and subsequently form condensation along the windshield.

Clearing the Condensation From Your Windshield

Thankfully, condensation and fog can be cleared from windshields in a variety of ways.  Though the same approach is not guaranteed to work every single time, if you try at least a couple methods, you will eventually clear the condensation off your windshield.  Here's how to do it: dry out the air.  Run the air conditioner in the vehicle yet do not turn it up to the point that it causes you to catch a chill or feel uncomfortable in your own vehicle.  If necessary, turn the air conditioner halfway up in temperature to ensure vehicle occupants remain comfortable.  As long as the outdoor temperature is above freezing and the air is not damp, the method should work. 

It will also help to evaporate as much water vapor as possible.  If you are in a rush and need a rapid fix, consider turning on the heat and running it through the vehicle's defrosters.  This approach forces the condensation to evaporate similar to moisture on clothing that evaporates when drying them in the dryer.  However, this approach will work best when the temperatures dip down low and you address the condensation soon after it forms rather than waiting until it has formed and spread across the entirety of the windshield.