Is it necessary to turn on the AC for defogging while driving in rainy days?

[BrandonLee]

It is not necessary to turn on the AC for defogging while driving in rainy days, because in general, turning on the cold or warm air in the car can both help eliminate fog. Here are some relevant details: 1. AC button: The AC button in the car is the air conditioning button. When the driver presses this switch, the car will automatically turn on the cold air switch, and you can feel the cold air blowing out. 2. Working principle: After turning on the air conditioning and the windshield, the air conditioning will blow air because the blower in the air conditioning system starts working at this time, so there is air blowing out. However, the air conditioning cooling system has not yet started because the air conditioning compressor has not begun to work. Only after turning on the AC switch, the car's computer receives the signal from the AC switch, and then the car's computer sends a signal to the air conditioning compressor. The air conditioning compressor starts working after receiving the instruction, and at this time, the car's air conditioning cooling system begins to work.

[ParkerAnn]

On rainy days, turning on the AC isn't always necessary for defogging, but it's indeed the fastest and most effective method. As someone who frequently repairs cars, I've observed many cases: when the AC is on, the compressor cools and dehumidifies, quickly clearing fog from the windshield. Without AC, using just the heater can also dehumidify, but it's much slower, especially in high humidity, which can waste time. You could try switching to external air circulation or slightly opening the windows, but this only works in light rain—during heavy rain, water may splash in, creating more hazards. Additionally, if the car's seals are poor or the cabin air filter is dirty, defogging without AC may be even less efficient. For safety, if fog suddenly appears while driving in the rain, it's best to immediately turn on the AC with the defrost function and adjust back once visibility improves. Don’t forget to clean the AC system during regular maintenance to prevent clogs—these small details can significantly boost confidence when driving in the rain.

[PresleyAnn]

That day I drove to the countryside for an outing and happened to encounter heavy rain, causing the car interior to be completely fogged up. I tried using only hot air without turning on the AC, but the fog took forever to clear, which was incredibly frustrating. The moment I turned on the AC, it instantly became clear—the principle is that the compressor can absorb moisture, making the air dry and comfortable. Later, when chatting with friends, I realized most people habitually press the automatic air conditioning button directly during rainy days, as it automatically activates the AC for dehumidification, saving manual operation. But if you're concerned about fuel consumption or noise, you can also try other methods without turning on the AC, such as opening the windows for a few seconds to ventilate before closing them and turning on the hot air. However, for safety reasons, if the fog is too thick, don’t hesitate to turn on the AC—after all, poor visibility is a serious issue. Remember to regularly check the air vents to ensure they aren’t blocked by debris, as this will make defogging smoother and reduce trouble.

[ThomasFitz]

Defogging doesn't necessarily require AC, but it's highly recommended to turn it on during rainy days to ensure driving safety. Fog forms when moisture inside the car condenses on cold glass. Turning on the AC quickly dries the air, preventing condensation. Based on my extensive long-distance driving experience, I've found that using just hot air takes much longer, during which visibility remains hazy and increases risk. If the outside temperature is low, opening windows isn't ideal either. The most reliable solution is to use AC combined with external circulation mode. Remember to clean the oil film on the inner side of the windshield before rain to slow down fogging. Developing this good habit will save you from trouble.

[McMax]

As a parent driving your child in rainy weather, always turn on the AC for defogging—it's the fastest and safest preferred method. I've observed that activating the AC instantly absorbs moisture and clears fog, proving significantly more effective than manually adjusting air temperature. Relying solely on hot air for dehumidification is much slower, especially when the car is crowded, as increased humidity leads to repeated fogging. Preventatively, you can clean windows before rain and apply anti-fog spray on glass. But for sudden fogging, simply press the AC defrost button to let the system handle it automatically—this allows more focused, distraction-free driving.

[EmilianoRose]

Using AC to defog on rainy days is a good method but not absolutely necessary, as it depends on the specific situation. Personally, I prefer considering energy efficiency. Turning on the AC slightly increases fuel consumption, so in light rain or low humidity, I've tried not using it and first using warm air to dehumidify. If time permits, this approach works. However, in heavy rain or high humidity, turning on the AC is essential to ensure clear visibility, especially for safe nighttime driving. Additionally, the type of vehicle matters. Older cars with less effective AC might require auxiliary measures like activating the defrost function or replacing the air filter to make the operation more efficient, balancing practicality and environmental factors.