I am sharing my story to shed light on a little-known phenomenum that we have dubbed “Thermal Snap” Why? Because it sounds a lot cooler than “Thermal Expansion of Toughened Glass.” And it could result in damage that may not meet your auto insurance deductible, and you’ll have to pay for it yourself.

The following interchange tries to make sense of what happened :

Last week, I left my car parked at the Metro station in the full sun. Upon my return 10 hours later, I found the back window completely shattered. Did I get hit by a meteor or a large bird? I looked around but I could not find a point of impact or a large object that could have caused the damage *to the window only* The rest of the car was undamaged.
The cops that came to fill out a report said that it was “blown out” from the pressure building inside the passenger compartment, and that they see this type of thing all the time. I had never heard of it.

When I closed the front door of the car, the vibration brought the window down in a shower of small glass fragments; it had been staying in place held together by friction between the pieces.
Bottom line: if you’re parked in the sun, leave one of your windows ever so slighty cracked down so that there is an air space to allow pressure to equialize with the outside.
The good news (if any) is that the window replacement people come to your house and the replacement cost is not much higher than what I paid back in the 90’s for a similar service.
===================== Bladerunner

Thank you, this is exactly the kind of information CT agents need in the field. I will pass this along to others, along with the proper way to tie your shoes, which has yet to fail me.

I do have a few questions:
Was your sun shade up?
Was there any external shade at any point in the day (tree, building, etc)?
Was your A/C vent open?
How many people did you flip off on your way through the parking lot?
Glad you are OK, and that the window shattered when you closed the door, and not while on the freeway. Still a huge pain 🙁

Thanks,
=====================Archangel

All seriousness aside, I find it difficult to believe that your vehicle is sufficiently airtight to cause that kind of damage. That would take some serious big time air pressure. Most car cabins are not fully sealed – check the bottoms and backs of the doors. Even if fully sealed, the weatherstripping is just soft rubber and cannot contain any significant pressure. Nor can the air valves in the ventilation system.
More likely I would think is thermal stress due to expansion in the sun.
Or the car was infected by ebola.
=====================Avatar

I agree with Avatar. I believe after a 6 month failure mode analysis that “Thermal Snap” will be found to be the most likely cause. This is the technical term for what Avatar was referring to when he said ‘thermal stress due to expansion in the sun’
The glass frame and gaskets of cars are supposed to be designed to prevent/minimize this phenomenon. Thermal Snap, as all CT members know, is where the expansion of a planar object such as solar panel or windshield will heat-up but the mounting sticks and does not allow it to slide along the edges to expand, until after a certain amount of build-up pressure the glass suddenly unsticks and “snaps” to its expanded size. If this occurs while you are in the car (on earth) you will hear a loud sharp snap [as was previously observed by Agent Aquaman on a mission somewhere in the deserts of the SW]. Thus the name “Thermal Snap”.
Feel free to add this to the subject posting. This posting will surely be copied for the Society of Mechanical Engineers monthly magazine or perhaps a PHD thesis at MIT.
Adventure and Integrity
=====================Agent Aquaman

Yeah, once or twice I have seen this, where a rear
window has crumbled into postage stamp sized
pieces. And I figure it’s thermal expansion, too,
happening on a hot day, and the outward
movement of the pieces just might be due to the
window being somewhat concave, and outward
is the way to relieve pressure.

I checked with Uncle Cecil at The Straight Dope,
but came up somewhat empty, hardly worth it:  http://boards.straightdope.com/sdmb/archive/index.php/t-618261.html

This seems better:
http://en.wikipedia.org/wiki/Spontaneous_glass_breakage

The Crack.

Is Back.
=====================Agent Renegade OUT.

Our police forces are well trained in the use of military-grade weaponry and crowd-pacification techniques (and I for one, am glad for this protection) but they may not have had the scientific training of CT operatives. I’ll accept the thermal expansion or “thermal snap” as a more likely solution than air pressure.
Therefore, leaving an air space will be totally useless and we are still at the mercy of the elements.

By the way, the window shades (reflective silver) were deployed when this happened, as was a secondary set of grey shades on the side. It sounds like none of this would have made a difference. Good to know. Our best solution is to park in the shade.
===================== Bladerunner

I’m tellin ya, it’s ebola.
=====================Avatar