I think the point could be stated more clearly by only using the bottom picture.
The snow that is scraped from the road is dirty, while the snow from the pavement is … less dirty. It might be dirt or exhaust or whatever. Roads and pavements are just filthy.
More interestingly, if you live in a place where the piles don’t thaw quickly, you can watch the road side of the snow getting progressively more dirty from the ongoing exhaust even if the road is not scraped afterwards. At the end of the season, the roadside snow piles will look charcoal black. It’s most noticable at crossings where cars run the engines at red light. Had it all been just dirt, it would look the same everywhere along the road, but it doesn’t.
I think the point could be stated more clearly by only using the bottom picture.
The snow that is scraped from the road is dirty, while the snow from the pavement is … less dirty. It might be dirt or exhaust or whatever. Roads and pavements are just filthy.
More interestingly, if you live in a place where the piles don’t thaw quickly, you can watch the road side of the snow getting progressively more dirty from the ongoing exhaust even if the road is not scraped afterwards. At the end of the season, the roadside snow piles will look charcoal black. It’s most noticable at crossings where cars run the engines at red light. Had it all been just dirt, it would look the same everywhere along the road, but it doesn’t.