A smoker walks in your direction on the sidewalk. You pass each other by and hold your breath, because of the stinky smoke. 15 feet after the smoker, you are able to breath again because the smoke has dissipated into the air.
I’m thankful that cigarette smoke fades into the outdoor air. What if it hung around like a nasty cloud? Bleh.
Or what if our atmosphere was so heavy that the smoke fell to the ground and made a line on the ground? That would be weird. We’d have contrails of smoke everywhere from smokers walking and smoking.