A surprising number of people don’t understand the causes of condensation. If you ask a stranger on the sidewalk, “Does condensation happen when cold air encounters a warm surface, or when warm air encounters a cold surface?,” many people will shrug their shoulders.
Here’s an example of this type of confusion: When drivers see condensation on their windshield during the summer, they are often unsure of the best remedy. Should they turn on the heater or the air conditioner?
Let’s look at four different scenarios.
This is the most common type of cold-weather condensation on a windshield.
Where did the moisture come from? The moisture came from the outdoor air.
What’s the cause? Due to night-sky radiation, the temperature of the windshield dropped below the dew point of the outdoor air.
What’s the solution? If all you have is dew, you can jump in the car and use the windshield wipers. If you have frost on your windshield, you either have to scrape the outside of the windshield, or let your engine idle for a while so that the engine coolant gets warm enough to provide heat through your defroster vents.
This phenomenon only happens when all of the vehicle’s windows are closed. It occurs more rarely than exterior condensation.
Where did the moisture come from? The moisture came from the air inside the vehicle.
What’s the cause? This phenomenon happens when the interior of the vehicle is very damp (as might occur if the carpeting has been soaked by melted snow from the driver’s boots). At night, the outdoor temperature drops below the temperature of the air inside the car, and the windshield is chilled by the outdoor air (even on a night without significant night sky radiation). The temperature of the windshield…
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