So we're going to talk about the physics of air density and air flow in the car and how cigarette ashes are affected by this?
Ok, I'll bite.
I'll be honest and tell you that I don't smoke. But I have plenty of friends who do and had a grandfather who had a 2-pack-a-day habit.
There is one little physics experiment I remember back in my highschool days. If you have a helium filled ballon in the front of the car and you begin to accelerate the vehicle foward, the ballon will move towards the back of the vehicle. This has to do with the air molecues being pushed towards the back of the vehicle...the ballon just travels with them. The back windshield acts like a snow-plow and piles up all the air molecues against it. If there was a string attached to the ballon and the driver were to tug the ballon back to the front of the car, while they were maintaining the vehicles foward motion, the ballon would eventually drift back towards the rear.
Now cigarette ashes are pretty light. I've seen them float and get caught up in the air and they can linger there for awhile. I suspect that a few ashes are flying up into the air and get pushed back with air molecues and that is why you're finding a few back there in your hatch.
Opening your windows, front or back, probably wouldn't help. Unless the very back windshield can roll down (that's the only way the air can have an "exit"), I don't think you'll be able prevent those ashes from floating towards the back. You as a smoker, should know that ashes and smoke are just something you live with as a smoker.
And for Goodness Sake! Don't flick your ashes out the window. :evil: I've seen enough smokers flicking hot ashes out their car window, spark on the ground and don't tell me they can't start a grass fire because they can! At least you don't throw out your just-finished-cigarette out the window...I hope.
The last thing other drivers need are ashes or cigarette butts hitting their car.