Why AC Is On In A Car During A Heavy Rain To Remove The Fog In The Glass?

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Why AC Is On In A Car During a Heavy Rain to Remove the Fog on the Glass?


Have you ever wondered why the air conditioning (AC) is turned on in a car during a heavy rain to remove the fog on the glass? It may seem counterintuitive to turn on the AC when it is already chilly and wet outside, but there is a scientific reason behind this practice. In this article, we will explore the science behind why AC is used to defog the glass in a car during heavy rains.

Understanding Condensation and Fog

Before we delve into the role of AC in removing fog on the glass, let’s first understand the process of condensation and fog formation. Condensation occurs when warm air containing moisture comes into contact with a cold surface, causing the moisture to condense into droplets of water. This is a common phenomenon that happens when the temperature of the surface is lower than the dew point temperature of the surrounding air.

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Fog, on the other hand, is formed when the condensation occurs in the air itself, leading to the suspension of tiny water droplets or ice crystals. In the context of a car during heavy rain, fogging happens when the warm and humid air inside the car comes into contact with the cold glass surface.

The Role of AC in Removing Fog

Now that we understand the science behind condensation and fog formation, let’s explore why the AC is turned on in a car during heavy rain to remove the fog on the glass. When the AC is switched on, it does more than just cool down the interior of the car; it also lowers the dew point temperature inside the vehicle.

1. Cooling Effect

When the AC is turned on, it cools down the air inside the car by removing heat and moisture. This cooling effect helps to reduce the temperature difference between the cold glass surface and the air inside the car.

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2. Dehumidification

Another important function of the AC is dehumidification. As the AC cools down the air, it also removes excess moisture from the atmosphere inside the car. This process helps to lower the dew point temperature, reducing the likelihood of condensation and fog formation on the glass.

3. Air Circulation

In addition to cooling and dehumidifying the air, the AC also aids in air circulation inside the car. By blowing air across the glass surfaces, the AC helps to prevent the accumulation of moisture, further reducing the chances of fog formation.

Effective Use of the AC

While turning on the AC can be an effective method to remove fog on the glass during heavy rain, there are a few tips to ensure its optimal use:

1. Set the AC to the Fresh Air Mode

When using the AC to defog the glass, make sure to set it to the fresh air mode instead of the recirculation mode. Fresh air mode allows for the exchange of air between the inside and outside of the car, promoting better ventilation and preventing the buildup of moisture.

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2. Adjust the Temperature

While the AC is an effective tool to defog the glass, it is also essential to adjust the temperature appropriately. Setting the temperature too low can lead to excessive cooling and condensation on the inside of the car, counteracting the efforts to remove the fog from the glass.

3. Use the Rear Defogger

In some cars, there is a rear defogger feature that aids in removing fog from the rear window. This feature can be used in combination with the AC to ensure clear visibility from all glass surfaces of the car.


The AC in a car serves more than just the purpose of cooling down the interior during hot weather. It plays a crucial role in removing fog on the glass during heavy rain by cooling, dehumidifying, and circulating the air inside the vehicle. By understanding the science behind condensation and fog formation, and by following some tips for effective use, we can ensure clear visibility and safe driving even in challenging weather conditions.



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