It’s cold and raining! Do I still need A/C?

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The weather in Southern California went from very very very warm (over 100 degrees F) to cold and rainy in just one weekend.  You may think that you don’t need A/C anymore. But I’m going to tell you that you will need A/C now more than ever! Why, you ask?  Because your windshield defogger depends on it!

Ever wonder why your car windshield tends to fog up very easily when it’s cold and/or raining outside?  Without getting into a nerdy discussion about condensation, adsorption (yes, that’s with a “d”), triple point, saturation limit, or psychrometrics…whew (I majored in Electrical Engineering, but I reluctantly had to go through nearly enough Physics courses to get a minor in Physics…talk about spending too much time in school!)…anyway, getting back on topic…here’s the gist of what you ought to know:

1. When it’s cold outside and warm inside, condensation occurs, which causes tiny water droplets to form on the surface of the glass on the inside of the car.
2.  The higher the relative humidity level, the more condensation occurs. This is why your windows fog up more when it’s cold and raining more than when it’s cold and not raining.
3. When the surface of the glass on the inside the car is dirty from dust, streaks, smudges, etc., there is more surface area for the tiny water droplets to adhere to. This makes the fogging effect even worse. So, make sure you keep your windows clean on the inside too!
4. As long as A/C is ON when the defogger is ON, whether you set A/C temperature to cold or hot doesn’t really matter all that much. There are pluses and minuses either way, so feel free to set it to a temperature you’re comfortable with. I could explain more, but that could take a while.

The critical component to proper defogging function is A/C. In order to defog, the relative humidity must be reduced by removing water vapor from the in-cabin air. To do that, the A/C must be ON (hint: the A/C evaporator helps with this). Some vehicle defogger systems automatically set A/C to ON, while more primitive systems might require the user to manually set the A/C to ON. In any case, short of opening all your windows while driving in pouring rain, the only way you’re going to defog that glass is to have a working A/C system.

So, the final answer to the “Do I still need A/C” question is:  YES. All-year round. 365 days/year.