# If you leave your car windows up on a hot day....

…can the heat cause the window(s) to blow out? It’s just about to start raining here so there’s a mass exodus of people going outside to close the windows on their cars. Everyone seems to be in agreeance that the reason they do it is because if it gets hot enough the windows can be blown out.

On it’s face it sounds a bit silly, but heated air will expand, so what say we? I’ve never heard of such a thing before right now.

While air does expand when it heats, it is compressible. So, even if your car were air tight, the air inside couldn’t expand enough to blow out your windows.

And, since your car is not air tight, any expansion that does occur happily exits through the nearest crack.

While air does expand when it heats, it is compressible. So, even if your car were air tight, the air inside couldn’t expand enough to blow out your windows.

And, since your car is not air tight, and expansion that does occur happily exits through the nearest crack.

The cabin of a car is hardly air tight. If it was, people would suffocate on long car rides. Also, the amount of expansion from, say, 20 C to 40 C isn’t very much. In absolute temperatures, that’s 293 K to 313 K. That would cause less than a 1% increase in pressure given constant volume.

The real reason is to keep the car cooler by letting the hot air escape.

Awesome triple post. I didn’t think so, it sounds like a story someone made up to cover up the fact that they did something really stupid and didn’t want everyone to know. Thanks!

:smack:
It seems pretty clear that the Straight Dope’s mission is going to take much longer than was previously thought…

If your car was air tight and the air expanded you’d get a cool “Pshhhh!” sound when you opened your car door like opening a 2-liter of pop.

True, but we got a three day extension on our subscriptions, so maybe that will help.

And if this were only the dumbest thing I’ve ever heard from a coworker…

I’ve heard this when opening my gas cap.

Not only is it not airtight, there are vents in the back, usually under the rear bumper so you can run your AC/Heater without pressurizing the cabin.

Plus, the door would fly open forcefully and hit you in the junk.

I assume you’re saying this is the reason they leave the windows down in the first place (the obvious reason for their running out to close them being they don’t their cars wet inside).

Jillions of cars have been parked for jillions of hours with the windows up in hot temperatures without the windows being blown out. Furthermore, car windows are notorious for their strength. Special tools are made to break them to allow exiting a submerged car, because even strong people can’t kick them into breaking. That notion that air pressure could break them doesn’t have any evidence to support it.

What can happen sometimes is that car window glass will spontaneously shatter. It’s rare, but not unheard of. It happened on one of my cars, and it was parked with the windows shut on a hot day. There’s no way it blew out - it was an old convertible that leaked (air) like a sieve. And interestingly, most of the broken glass was on the inside of the car.

This comment FTW!

–FCOD

No.

Otherwise the car glass industry in the South and other parts where summer gets to triple degrees, easy, would have a field day every summer.

I’m cracking up thinking of this! Living in south Florida, I’m glad this isn’t true. I’d have to wear a cup every day in the summer!

I’ll tell you what is true, however:
It’s possible to crack your windshield by squirting washer fluid on it during a very hot day. I know, because I watched it happen to my Jeep.