Much of the United States is stuck in a deep freeze that includes plenty of snow and ice. If you have to clear the ice from your car in the morning, then you may think about dumping some hot water on your windshield rather than scraping it off the old fashioned way. Here’s why that’s a terrible idea.
There’s advice out there on the internet suggesting that hot water is the perfect way to clear your car. Hot water melts stuff so it seems like a good way to get rid of all that ice. This is where anyone with an understanding of thermodynamics (sit down engineers, we see you) jumps up and down and gets ready with science.
Things expand when they get warm, including the parts of your car like the windshield. This thermal expansion happens all the time when your car gets hot in the summer, but it happens slowly. The windshield doesn’t just suddenly go from -5 to 60 in the blink of an eye. Your windshield glass is fine with this process and survives unscathed.
The problem comes from heating up your windshield from freezing cold to boiling hot in just a few seconds. The glass still expands, but not all of it and not at a nice even rate. It expands very quickly only in the spots where you splashed that boiling hot pot of water.
The rest of your windshield is still freezing cold and it’s where the hot and cold meet that things go wrong. The hot bits expand, the cold bits don’t, and where the two meet you’ll end up with a nice crack in your windshield.
You’ll hear people tell you this only happens if you use boiling water. The idea here being that hot or warm water will still melt the ice while keeping your windshield in one piece.
While it’s true that less of a temperature differential isn’t as likely to cause problems, hitting the exact temperature that will guarantee your windshield doesn’t crack isn’t easy. What works for you one day, might fail miserably on another. You’re taking a heck of a risk every time you put hot or even warm water on your windshield.
It might take longer and it might result in some seriously cold fingers, but scraping the ice and snow from your windshield is, unfortunately, a better idea than hot water.