Why do you sometimes scratch your car window when it hasn’t frozen? And how do you prevent this? | Science & Planet

Ice forms on our car windows when it starts to cool down at night. During the day, the sun heats the Earth’s surface, everything on it and the air layers above it. But once the sun disappears behind the horizon and night falls, everything loses its warmth again. How quickly this happens depends on the weather. For example, a thick cloud cover and a lot of wind will slow down the process.

During this cooling process, your car window transforms from completely dry to covered with a layer of ice. The colder the air, the less water the air can contain. This causes the air to become saturated with water vapor. One then speaks of an air humidity that is 100 percent. That water has to go somewhere and gradually drops appear on cold surfaces like your car. When the temperature then drops even further and reaches freezing point, the layer of water transforms into a lot of scratching fun. But if there is insufficient moisture in the air, your car windows will remain spotless, even at a few degrees below zero. This is a ‘dry frost’.

However, sometimes you find yourself scratching ice in the morning while the thermometer shows a positive temperature. How is this possible? Your car can cool down faster than the surrounding air, causing the window to reach a temperature below freezing more quickly. However, you can prevent this by, for example, placing the car closer to a building or under a canopy. Your car will then lose heat less quickly, so you won’t be scratching your head in the morning. You can always use a protective cover to prevent ice on your window.

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