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If you live somewhere that gets cold in the winter, you’ve probably seen cars parked outside covered in a thin layer of snow On a cold morning. but what is the reason for this frostAnd how can you get rid of it quickly?
I am a mechanical engineering professor who studies how water vapor interacts with air particles under different atmospheric conditions. Frosty windshields are similar to some of the thermodynamic questions I study in the lab, and they’re also an annoying problem I deal with on my way to work every winter.
windshield condensation
The air in Earth’s atmosphere always contains a certain amount of water vapor, but there is only so much water vapor that the air can hold. Scientists call that limit 100% relative humidity. Dew point refers to the temperature at which relative humidity reaches 100%.
Wet air has a higher dew point temperature, while dry air has a lower dew point temperature. With each degree drop in temperature, the air gets closer to its dew point temperature – or its water vapor carrying capacity. Any cooling after the dew point temperature is reached causes water to condense on surfaces, or turn into fog.
all night long, car Windshields facing the cool dark sky are cooled by radiation, meaning they give off heat to their surroundings in the form of visible and invisible light. As soon as the air comes in contact with the cold windshield, it can reach its dew point temperature. Then, the water vapor condenses on the windshield.
When this radiative cooling drops the temperature at the windshield’s surface below the freezing point, 32 degrees Fahrenheit (zero degrees Celsius), the layer of condensed water on the windshield turns into frost.
defrosting your car
To defrost an icy windshield, you can follow a few different methods, some of which take longer and require more effort than others.
One option is to spray a small amount of hot liquid directly onto the layer of frost to help melt it. For this approach to work, the spray liquid must be hot enough to raise the overall temperature of the frost layer above the melting point. But the temperature cannot be hotter than the glass temperature otherwise your windshield will crack.
A better way to melt ice without damaging your car is to spray your windows with a warm liquid that has a lower freezing point than water, such as a mixture of rubbing alcohol and water. This hot mixture will melt the ice layer without heating the glass, and the freezing point of the resulting liquid layer on the windshield will be lower than that of water. It will remain liquid and you can wipe it off with your windshield wiper.
Similar alcohol and water mixtures – for example glycol – are commonly used to maintain the icy surface of skating rinks.
About the author
Suresh Dhanyala, Bayard D. Clarkson Distinguished Professor of Mechanical and Aeronautical Engineering, Clarkson University.
This article is republished from Conversation Under Creative Commons license. read the original article,
This approach can melt ice fairly quickly and easily, without much effort. You don’t even need to start your car.
If you have a little more time, you can start the car and run the air defrost system to blow warm air — aiming above 80 degrees Fahrenheit — inside the windshield. This will heat the windshield and eventually melt the ice layer. Once you see some melting, you can use the windshield wipers to wipe away the remaining ice.
This option consumes more energy, because your car must heat the windshield, but it doesn’t require you to do much.
Using the defrost system to blow warm air toward the windshield will also help clean the inside of the windshield when it fogs up due to condensation. Otherwise, if it’s dry outside, you can also clear the windshield fog by opening the car window and letting in outside air.
If you’re in a hurry or need some exercise, you can use a snow scraper to remove snow on your windshield, which can cause small islands of ice to form. The windshield wipers can then mechanically remove the pieces by moving them around and melting them. It requires more energy on your part, but it doesn’t require more energy from your car.
If your day starts off easy, you can let the sun warm the windshield and slowly melt the layer of frost for you. This technology saves energy in every possible way.