Guide: Car windshield fogging is caused not only by temperature difference between inside and outside, but also by influence of air humidity, saturated air pressure and windshield hydrophilicity. glass surface. Spot anti-fog, otherwise not only will not be able to remove fog, but will aggravate fog.
Recommended reading time: 3.5 minutes
Fogging windshield while driving is a headache for car owners. It takes time to stop to wipe it off, and continuing to move affects safety. Some cars obviously have anti-fogging function, but driving effect is slow and bad.
Where does fog come from and how to remove it with maximum efficiency? There are also different opinions on Internet, today I will take this hackneyed topic and talk about it from a professional point of view.
#1
Fog is not just hot and cold
Regarding causes of foggy windows, most older drivers can tell a little: because temperature outside car is lower than inside car, glass is cold, so water vapor in air in car meets cold. The glass condenses into small water droplets on it, which is source of mist.
I want to say that this statement is correct, but not complete, it only explains one of reasons for fogging of glasses. In fact, cause of fogging car windshield is much more complicated.
If all fogging is due to glass being "cold" then simply turn on warm air so that hot mist blows on glass and fogging should disappear immediately, but experienced car owners should find that sometimes glass is covered with warm air The fog in sky did not decrease, but became more serious. Why is this?
#2
Ignoring "saturation pressure" and hydrophilicity of glass surface
Air saturation pressure is too complicated to popularize science. For ease of understanding, I'm only talking about knowledge points related to fogging up car windows. Simply put, air contains moisture, but this moisture cannot be infinite, and it will be saturated when it reaches a certain proportion.
Sometimes we hear that "humidity" in a weather forecast refers to saturation of air with water.
Saturation pressure refers to atmospheric pressure when air humidity reaches 100%. It rises with an increase in temperature. The colder weather, lower saturation pressure, and vice versa. In other words, air in winter can "hold" much less moisture than in summer.
Once humidity in air reaches saturation or barometric pressure drops suddenly, moisture in air will do its best to leave air and condense into small droplets of water. Then water vapor that wants to leave air will go to hydrophilic substance, and then, being adsorbed on its surface, will condense into water droplets.
Pure glass is actually not very hydrophilic, but there will be a layer of dust on surface of glass, but dust has good hydrophilicity. In addition, solar film pasted on inside of glass also has good hydrophilicity, so that it becomes a water vapor paradise .
#3
Different fogs require different strategies
It can be seen that amount of fogging of windshield depends on at least four factors: glass temperature, air humidity, saturated air pressure and hydrophilicity of glass surface. The causes of fogging windows are different in different situations, and different strategies should be used to eliminate fogging as quickly as possible. Here are some common situations:
1. Fogging windows in winter
The main reason for this mist is that temperature of glass is too low, so heating glass is most efficient way. Just turn on warm air to heat glass.
2. Fogging glass on rainy days
There are two main causes of fog on rainy days: high air humidity and low saturated air pressure. The easiest and fastest way is to turn off external circulation and at same time turn on air conditioning compressor (A / C button), so that, on one hand, air with high humidity from outside car cannot get inside. car, and on other hand, humidity inside car can be quickly reduced to achieve anti-fogging effect. Some car owners turn on hot air to blow on rainy days and then find that fog suddenly becomes especially severe because all moisture outside car and in heater tank is blown onto glass.
3. Misting on outside of glass
Misting on outside of windows is generally a relatively rare occurrence. Typically, car is parked overnight in an underground parking lot, and after lowering to ground, fogging begins to appear on outside of window pane. This is due to high humidity outside car, low temperatures inside car, and low glass temperatures. The quick solution is very simple, just use glass cleaner to wipe it off, but it will reappear a few seconds after it was wiped off because glass is still cold, so turn on warm air to heat glass as soon as possible. At this time, it is better not to open window, because once high humidity air from outside car enters car, it will form fogging from inside of glass, causing fogging to appear on both sides of glass at same time. time.
#4
Does anti-fog spray work?
The answer is that it works, but not for long. The principle of anti-fogging spray is to change hydrophilicity of glass surface so that water vapor cannot easily condense on it.
However, as I said, hydrophilicity of glass surface is mainly determined by amount of dust attached. Everyone knows about air quality in China. It will be covered with a thin layer of dust, so anti-fog will only last for a maximum of half a day, and spraying once every half a month, as in advertising of some products, is completely impossible.
Of course, if you are in a city with beautiful scenery, clean air and a month without car washing, then effect of spray should be good.
Overview
Summarizing, we can say that fogging of windshield of a car is caused not only by temperature difference between inside and outside, but also by influence of air humidity, saturated air pressure and hydrophilicity of glass surface. only fog will not remove, but will make fog more serious and affect safety.
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